Saturday, May 18, 2013

Lithium Iron Phosphate (LiFEPO4) batteries revisited - Equalization of cells

On August 31, 2012 I posted about problems that I'd been experiencing with three "12 Volt" LiFePO4 batteries that I'd bought in 2010 - see:


I was alarmed by the fact that after just 2 years, all three of these packs had appeared to degrade badly:  Of the three, two of them had seen their original 6+ amp-hour capacity diminished to 2 amp hours or less (the third was at about half capacity) and their capacity was decreasing every time I checked them.

In response to this post I had several comments/suggestions (you may find them at the end of the original post, linked above) about what might explain the problems that I was having, including:
  • Temperature variations.  Very cold temperatures will dramatically reduce the available cell capacity, but these batteries were all tested at similar (room-like) temperatures.
  • Perhaps I'd charged the cells to too-high a voltage and damaged them?  I'd checked and found that the internal circuits prevented the cells from "seeing" voltages in excess to those noted in the manufacturer's recommendations.  Considering the rather light usage (only a few dozen cycles) it seemed unlikely that they would suffer such damage so quickly even if they were abused a bit.
  • Self discharge?  Perhaps the protection circuits inside the battery were causing the packs to bleed down?  Maybe the cells themselves were losing charge on their own?
The first two of these points - while valid - just didn't seem to apply in this case.  Related to the third point, however, I'd checked out and found that while the built-in protection circuits of the battery pack did continuously draw current, it was very small in comparison with the normal self-discharge rate of the packs, but the poster did bring up some interesting points about the possibility of imbalance within the battery pack itself.  In my reply, I somewhat dismissed this as a possibility but in retrospect, probably a bit too quickly.

More recently I finally got the time and inclination to disassemble one of my LiFePO4 packs and do a bit of looking.

Analysis:

First, I charged the pack by connecting it to a 14.6 volt, current-limited (500 mA) source.  After the current dropped to nearly zero (overnight) I disconnected it from the charger and let it sit for a week or so to let the cells "settle out" and attain a steady-state voltage.  Upon dis-assembly to the point that I could measure the voltage across each individual cell I found that they were fairly close - within 0.1-0.15 volts or so - not radically different, but still a bit more than I would have expected.

I then put the battery pack on my West Mountain Radio CBA-II battery tester and did a capacity test which subjected it to a constant 2 amp load while graphing and integrating the results.  As expected, this pack, which had measured at about 6 amp-hours when it was new, now measured less than 2 amp-hours.

Upon measuring the voltages across the cells in this discharged state the precise nature of the problem became clearer:  Several of the cells were in the 2.8-3.1 volt area but one cell was at about 2.4 volts and it had no doubt been lower (probably down to 2.0 volts) when it was under the load of the tester!

Did I have a bad cell?

This was my first thought, but before I was about to condemn anything, I needed to do a bit more testing.

I charged the pack again and when complete - but still while on the charging voltage - I checked the cell voltages and saw that at least one of them was at about 3.9 volts - just at the voltage where the protection circuit would disconnect the charging current for the entire string - and this meant that charging would no longer proceed.  I then set an adjustable voltage supply to precisely 3.65 volts and connected it across each cell, in turn, for several hours (until the charging current dropped off) to assure that every cell was fully charged.

Interestingly, the cell that I had thought might be bad (the one at about 2.4 volts) took a charge - and then kept taking a charge for several hours before the current tapered off.  In contrast, the cell that had been at the highest voltage immediately reached full charge as indicated by the current dropping off quite soon after I'd applied it.

Cell imbalance and how it can cause apparent loss of capacity:

This behavior implied a problem that was different from what I'd expected:  Gross cell imbalance.

One vexing problem with any system of series-connected cells (e.g. a battery) is that since not all cells are exactly the same, one will inevitably discharge sooner than the others.

On "primary" batteries, this isn't too much of a problem since being non-rechargeable, a few percent of difference between cells won't have too much effect over the pack as a whole.  On a "secondary" battery - the types that can be recharged - this can be an issue, especially if the problem accumulates over a number of charges.

For good, old-fashioned lead-acid cells, the penalty of slightly overcharging isn't great:  Gasses are produced and one can trap them (as in a VRLA - Valve-Regulated Lead Acid) so that they can be reabsorbed if not generated in excess, you can convert them back to water (e.g. catalytic recombination as is done with "Hydro Caps") or simply add water as needed.  In a normal "float" service where the cells are held at a "full charge" state they constantly pull a small amount of current and if this is maintained for a fairly long period of time, cells/batteries that weren't fully charged will gradually equalize... for the most part.  In other words, we can - to a reasonable extent - prevent cell inequality from occurring by purposely allowing lead-acid batteries to be "gently" overcharged and the chemistry allows this without severe deleterious effects.

Perhaps the most notable of deleterious side-effect of cell imbalance is that experienced in NiCd packs where the premature discharge of one cell results in its effectively being charged backwards and a chemical process occurs in which dendrites (metallic hairs) grow, reducing its capacity and in severe cases, shorts the cell and/or causes it to self-discharge very quickly.  In this case, this cell - already weakened due to dendritic damage - will be the first to discharge the next time it is used and this will happen again, causing even more degradation!

In the case of a Lithium pack, this unintentional "reverse charge" doesn't occur as there are (supposed to be!) circuits that prevent any of the cells in the pack from being discharged too deeply by disconnecting the entire battery from the load - something that (in Lithium chemistry) can cause permanent damage.  This same circuit also does something else:  If, while charging, the voltage on one or more cells exceeds a particular threshold (typically 4.2 volts for most conventional Lithium-Ion cells) the entire battery is disconnected.

For the LiFePO4 battery, the chemistry is a bit different.  While the "low voltage disconnect" of these cells is comparable to that of some conventional Li-Ion chemistries (2.0 volts) the high-voltage disconnect on the particular LiFePO4 cells that these packs contain is 3.95 volts maximum.

Comments:
  • The nominal "full charge" voltage for these particular LiFePO4 cells is normally 3.65 volts.  As is typical for any rechargeable cell, the voltage on a cell immediately decreases when the charging source is disconnected.
  • Unlike lead-acid cells, if we attempt to even "gently" overcharge any lithium-ion type of cell to force an equalization, the chemistry of the cell is such that we will damage it in attempting to do so!  At the very least this can greatly reduce the lifetime of the cell and in the worst case, cause a catastrophic failure such as leakage, cell rupture or even fire.

What seemed to be happening to these LiFePO4 battery packs was this:
  • When the battery pack runs down:  One cell reaches 2.0 volts, the internal circuit disconnects the load.  Other cells have probably not reached 2.0 volts and still have remaining charge.
  • When the battery pack is charged:  One cell (but not the same one as above) reaches 3.95 volts and the internal circuit disconnects the charging current.  Inevitably, the other cells that did not reach this voltage and were thus not as fully-charged.
  • When the battery is discharged the next time.  The cell(s) that didn't get fully charged the last time will likely be the one(s) that discharge first:  When that happens, the load is disconnected, but the other cells will still have charge on them.
  • When the battery is charged again.  Those cells that had the most charge will reach the maximum voltage first, leaving the more-discharged cells with a less-then-full charge. 
  • This process repeats.   The disparity between those cells that are fully charged and those that were not fully charged will increase.  Because the overall capacity of the pack is dictated by the weakest cell - in this case, the one that has the least amount of charge - the apparent capacity of the battery pack will gradually decrease.
Figure 1:
One of the LiFePO4 battery packs when nearly new (black) and after 2 years (red)
there is very clearly a loss of available capacity!
Click on the graph for a larger version.


Figure 1 shows the degradation of the capacity of one of these battery packs.  The black trace shows the capacity of the battery when it was fairly new while the red trace shows this same battery pack's capacity just 2 years later.
It should be noted that the load test current in the graphs on this page were 2 amps, much higher than the "20 hour rate" at which battery manufacturers typically rate batteries.  Because of this, the tested capacity was somewhat lower than the pack's official 6+ amp-hour rating.
In each case you can see that near the end of the discharge curve, the battery voltage starts to droop - and then is abruptly cut off by the protection circuit when one or more cells drops below 2.0 volts.  What I should have noticed was that the overall voltage before the curve started to get steep was actually lower when the pack was "good" than it was when its capacity had diminished.  What this indicated was that when the battery pack was newer and performing better (black trace) all of the cells' voltage were dropping more nearly in unison while on the red trace, we don't see that gradual drop to nearly the same extent implying that, perhaps, just one cell was dropping out sooner.

Equalizing the cells - the result:

As mentioned above, I individually charged the cells using a regulated, current-limited power supply that was set to 3.65 volts and then individually tested each cell's amp-hour capacity.  I was pleased to note that every cell seemed to be fairly close to its rated specifications and, as I now suspected, the cells that had been noted to have discharged first when the pack as a whole was tested were, in fact, those that had slightly lower capacity than the others by 5% or so.

As described above, I recharged the entire pack and then topped each cell off, individually, to 3.65 volts, and then re-tested the entire pack again:
Figure 2:
Same graph as Figure 1, but with the green trace showing the capacity after equalizing.
The black trace is that of the battery when it was new.
The red trace was that after 2 years, showing the degradation.
The green trace is the capacity after equalizing the cells in the battery.
Click on the graph for a larger version.


Figure 2 shows the result of that test.  As can be seen, the capacity of the battery pack after having equalized the cells (the green trace) is slightly better than it was when it was brand new!

Clearly, there was a problem to be solved:  How do I keep this - and the other - LiFePO4 battery packs "equalized" and still have them be practical and safe to use.  Here are the points to consider:
  • I did not want to remove the battery's built-in protection circuit as it takes very few cycles of gross over/under voltage before one permanently wrecks Lithium-Ion cells.  While LiFePO4 cells are considered to be (more or less) "intrinsically safe" when compared with conventional Lithium-Ion cells, that just means that they are less like to catch fire/explode if mistreated, not that they won't degrade quickly!
  • A practical method of charging the battery pack and maintaining equilibrium of the cells within.  What this meant was that I was not going to disassemble the pack every time I wanted to charge it so that I could get at each cell!
  • Charging/equalizing should be relatively simple and convenient.  This goes with the above and it would imply a simple box that I could connect to the battery while charging, when I wanted to equalize the cells.
One thought was to attempting building a circuit into each LiFePO4 battery pack.  The problem with this idea was that there was not very much room in any of the batteries in which to fit a circuit - much less a way to dissipate the heat that was likely to be produced when the action of the circuity prevented overcharge of specific cells!  Additionally, any circuit that was left connected full-time would have to be designed to minimally load the cells and run them down when not being charged.  Needless to say, it would also be time-consuming to build a circuit for each battery and with the lack of room, there would be the likely need to repackage each battery to accommodate the circuitry!

Instead, I decided to modify each pack, bringing out connections from each individual cell so that the voltages could be monitored and controlled during "normal" charging of the pack.  To do this, I added a short cable with a 5-pin DIN connector which provided the points needed to which I could connect an external box with which the cells could be equalized when charging. What this also meant was that I needed to construct only one external device to monitor and control the voltage on three LiFePO4 battery packs that I had!

A charge monitoring and control circuit:

Figure 3:
Example of the charge monitor/control circuit.  This example shows a 2-cell circuit for simplicity, but one
could scale this design up to accommodate practically any number of cells.
The "X" in the leads marks where the connector used to attach the charge equalizer to the battery would go.
With the values shown (R103/R104) the shunt voltage is about 3.65 volts/cell which is appropriate for these
LiFePO4 cells:  This voltage would need to be adjusted if other types of cells were to be used. For more
than 2 cells, R1/LED1 would be connected across the entire pack (all cells)  to provide a reminder/ indication
that the device was still connected.
Click on the diagram for a larger version.
Figure 3 shows the circuit that was constructed to aid in properly charging and equalizing the voltages of the cells within the battery.  Based on the readily-available TL431, this is a simple shunt regulator that, if the voltage across any cell gets to be too high, starts to conduct the current around that cell by clamping the voltage to the maximum level.  In this way, if a cell reaches full charge before the others the shunt regulator will allow the other cells to continue to charge and, eventually, all cells will reach voltage equilibrium:  For our purposes, this is the best we can do in assuring that all cells are charged equally!

Please note that the diagram in Figure 3 represents what would be required for just a 2-cell (nominally "6 volt") LiFePO4 battery:  One would simply replicate the circuits as needed for as many cells as the battery pack contained, connecting them in series as shown in the diagram.  For the discussion below, we'll generally refer to the parts in the circuit connected to Cell 1 which are numbered with parts designations in the "100's".

How it works:

As noted above, wires are connected to the cells inside the battery pack and brought out through a cable that is separate from the main power connector.  I used a 5-pin female DIN connector as it had precisely the number of connections needed:  Battery - (minus) and the + (positive) terminals of each of the four individual cells.

You will note that in the diagram there are devices marked F101 and F201 which are 1 amp, self-resetting thermal fuses located at the point where the wires connect to the cells (or protection circuit) inside the pack.  These are extremely important as an accidental short-circuit of one of the wires emerging from the pack could result in burns or a fire without its presence as the cells will easily supply enough current to burn the wires open!  Adding these devices is a cheap way to make the modification safe and should a short occur, simply fixing the problem (removing the short) will cause the fuse to automatically reset when it cools down - more on these devices later.

The heart of the circuit is U101, a TL431 shunt regulator - a part that may be found in nearly every PC power supply.  This device is essentially a programmable Zener diode:  If the "adjust" terminal is more than 2.495 volts (nominal) above the anode terminal, it will conduct via the cathode.  The values of R103 and R104 are chosen so that if more than 3.65 volts appears between the "top" of R103 and the bottom of R104 (which is also connected to the anode of U101) more than 2.495 volts will appear on its adjust terminal and the device will turn on.

When U101 turns on it will start to "pull down" on the base of Q101, a PNP power transistor, which will conduct, amplifying the effects of U101 and start drawing current through R101  (R102 keeps Q101 turned off otherwise.)  Were it not for the presence of R101, a 0.25 ohm resistor, the "knee" of this conduction would be extremely steep, but this resistor gives the circuit a bit of elasticity (a bit of range over which the current will go from nearly zero to maximum) and also assure that the circuit will not oscillate.  With the parts in the diagram a current of about 200 milliamps through the shunt regulator - plus the finite gain of U101 - will result in a clamping voltage on the cell of around 3.75 volts, not including the effects of the resistance of the wire/connector that runs to each cell within the battery.

As a convenient indicator that the U101 circuit is actively shunting current from the cell, Q102 detects when U101 is "pulling down" on the base of Q101 and turns Q102 on, causing LED101 to illuminate.  Q102 and associated components (LED101, R105, R106) are optional if you don't need an indication of a particular cell reaching full charge.

Finally, there is also a simple circuit consisting of R1 and LED1 that is across the entire battery.  The sole purpose of this is that of an indicator to show that this circuit is connected to a battery and serve as a reminder that it should be disconnected when charging is complete!

Comments on parts and construction:

As noted above, by cascading multiple copies of this circuit one may apply them to as many cells as needed - whether it is just a pair of cells, or many more than that.  Because my LiFePO4 packs were "12 volt" types with 4 cells in series, I cascaded 4 of the circuits together, building it on a small piece of perforated prototype board as shown in Figure 4.

The type of prototyping board that I used has a small copper ring around each hole making construction much easier, more rugged and more durable. 

I do not currently have plans to produce a circuit board for this project, but let me know if you do as it might be of interest to others.
Figure 4:
A 4-cell Cell equalizer circuit built on perforated
prototype board.
Click on the image for a larger version.

The TIP32A PNP power transistors are of the TO-220 type (with metal tabs) and were chosen simply because I had a lot of them on hand, but practically any PNP power transistor could have been used - but I would not recommend Darlington types due to possible emitter-base voltage limitations.  As can be seen in Figure 4, pieces of copper, bent to fit within the box and to increase surface area - were soldered to the tabs of these transistors to improve power dissipation.

With the heat sinks shown and with the box put together, the unit can easily withstand 200 milliamps of shunt current on each of the four sections without causing excessive thermal stress of the components inside from the 3-4 watts total heat generated.  If much more shunt current capacity is desired, larger heatsinks and/or better ventilation - or perhaps connecting the transistor's tabs to a metal box (making sure that their heat sinks were electrically insulated) - would be recommended along with the decrease of the value of R101.  Practically speaking, by proper choice of the transistor, the value of R101 and heat sinking one could design this sort of circuit to accommodate practically any amount of shunt current!

In the lid of the box I drilled some holes and used cyanoacrylate ("Super" tm) glue to to hold the LEDs in place temporarily (overnight) and then used clear epoxy to more-permanently secure them into position afterwards.  Because this shunt regulator was intended for 4 cells, there is a red LED for each of the shunt regulators to indicate its "full charge" status plus a fifth, bright blue LED as an indicator to show that the unit is, in fact, connected to a battery a serves as a reminder to disconnect it when charging is complete.

Figure 5:
An external view of the cell equalizer unit built to accommodate
up to 4 cells.  There are separate indicators that show which
cell(s) are at full charge - plus a "power" indicator.
Click on the image for a larger version.

Figure 5 gives a view of the completed shunt regulator showing the short cable and the male DIN connector that is connected to the mating connector on the battery.  As the label indicates, the five LEDs on the front show the full-charge status and whether or not it is connected to power.  It is worth noting that when they were installed, the LEDs weren't fully pushed to protrude through the front panel.  If they had pushed all of the way through the hole, they would have stuck out too far and would have risked being accidentally broken loose and pushed back into the box during handling.




Comments: 
  • A "simpler" way to do this:  In theory, one could parallel a fairly low-value resistance across each cell - say, a 22 ohm, 1 watt device - and achieve equalization with this value of resistance equating to about 165 milliamps of current at 3.65 voltsEventually - after, perhaps 36-48 hours for a pack of this size - the cells would gradually equalize their charges - and the lower the resistance, the more quickly equalization will occur.  If this is done, resistors with the appropriate power rating should be used and they should all be of equal value and, of course, they should be disconnected immediately after charge has been completed, just before the charging voltage is disconnected.  Failure to remove these resistors after the charging voltage has been removed will surely run down (and likely ruin!) the battery!
  • For other than LiFEPO4 cells:  For different types of cell chemistry one would choose the values of R103/R104 as appropriate (see the data sheet for the TL431 for more information.)  Note, however, that this circuit will simply not work for voltages much less than 3.0-3.2 volts!  If, for example, one wanted to regulate "2 volt" lead-acid cells you could use this sort of circuit directly, but multiple regulators could be placed across pairs of cells (and the shunt voltage set to approximately 4.55 volts) in an interleaved manner and achieve the same effect.
  • Make sure that all circuits are equal:  Once constructed, the clamping voltage of the individual cells' circuits should be checked at the same current, say 200 mA and if necessary, adjust the value of R103 or R104 to make all of them equal:  All cells being within 50 millivolts (0.05 volts) of the target voltage should be more than adequate.  It's worth noting that normal variations in the resistor values (particularly if 5% tolerance parts are used) as well as variations among TL431's can cause voltages to differ slightly.
  • Where to find the TL431:  As mentioned above, the TL431 can be found in nearly every PC-type power supply made.  If you have some scrap power supplies laying around, you may be able to find enough of these devices for your project!  The devices that I used are in TO-92 packages and look just like a transistor.  Different manufacturers have their own prefixes, but if you see a part that looks like a transistor and ends in "431", it's probably a TL431!  If you don't want to dig around in old power supplies, these chips are readily available from many distributors at well under US$1.00 each.

Modifying the battery packs:

Because of the high currents involved, there is some risk of burns/fire should connections be accidentally shorted.
Do not perform this sort of modification unless you know exactly what you are doing!

One of the more critical aspects of this projects was safely modifying the battery pack.  As noted above, the wires connecting to the outboard charge equalizer have current limiting on them in the form of (approximately) 1 amp, self-resetting thermal ("PPTC") fuses that are located inside the battery's case.  

For a Wikipedia article on these PPTC devices, click on this link.

Again, the fuses are necessary safety components to prevent burns/fire should the added connection(s) be accidentally shorted out!
Some builders may not be familiar with these self-resetting fuses so a few words of explanation are in order.

Typically resembling small, yellow disk-ceramic capacitors, these devices are actually specially-designed thermistors:  If the current through them exceeds the design threshold, they will get hot (but not too hot!) and their resistance will skyrocket.  When in this "tripped" state, enough current will still flow through them (typically 5%-15% of the rated trip current) to keep them "warm" and in the "tripped" condition with a high resistance.  Once the fault has been removed, they will cool down and reset themselves as their internal resistance returns to normal.
Figure 6:
This shows the modification to one of two identical packs.
This battery - housed in a water bottle for bicycle use -
was really two separated batteries, each with its own protection
circuit, the outputs of the two connected together.  These packs
were rewired in "2P4S" (series-parallel) format, still with the
protection circuits in parallel.
The four yellow self-resetting thermal fuses are visible.  Some
pieces of cardboard were added to protect the plastic cover
of the cells from the heat of these fuses (somewhat over
100 C) if they are tripped, which could damage their thermal
shrink wrap.
Click on the image for a larger version.

These devices are readily available from electronics suppliers like Digi-Key, Mouser and Jameco (to name but a few) and aren't all that much more expensive than normal "one-time" fuses.  Available in current ratings from below 100 mA to 10's of amps, the 1-amp rating noted is a suggestion:  The devices that I used were actually rated for 1.1 amps of "holding" current, but anything in the range of 0.6-1.5 amps would have been just fine in this application as that current range will keep the wire from burning up in the event of a short, yet be enough to handle the expected shunt current.

As mentioned, when "blown", these self-resetting fuses do not completely stop the current flow, but reduce it to a "safe" level.  What this means is that should the wire be shorted out indefinitely, the affected cell(s) will eventually be discharged (and the cell damaged/ruined!) but you aren't likely to set anything on fire - and this is particularly important since our connection to the cells must bypass the protection circuits within the battery pack!

Of course, if you wish you can substitute "normal" (one-time) fuses.  While these have the disadvantage of being usable only once, they do have the advantage that if a wire shorts and the fuse blows, current completely stops and you won't likely run down - and possibly damage/ruin - a cell.

Figure 7:
This shows the modification to a more normal "2P4S" battery
pack.  Some of the self-resetting thermal fuses are just
barely visible around the edges, connected directly to the
protection circuit board.  Using a voltmeter, the connections
on the protection board to the cells were identified and marked.
Note that the wire connections are insulated with
heat-shrinkable tubing.
Click on the image for a larger version.
Of the three LiFePO4 packs that I modified, two of them were built into plastic water bottles, the original intent being to provide power for bicycle lighting and being placed in the water bottle holder.  These packs actually contained two individual battery packs (a "4S+ 4S" configuration) - each with its own protection circuit.  Rather than connect two equalizer circuits to the pack (one for each "sub-battery") I simply connected the cells of the packs in series-parallel - as is the normal practice - leaving in place each of the protection circuits.  The end result is that there now one series-parallel sets of cells and two parallel protection circuits.

The other pack (Figure 7) was constructed in the traditional way:  Series-parallel connected cells (four pairs in series in a "2P4S" configuration) to get the desired voltage and capacity and as such, the modification was much easier to accomplish.

To find the connections to the individual cells, the battery packs were (carefully) disassembled and the locations of the individual wires traced.  Using a voltmeter, find the "bottom" cell of the pack (usually marked "B-" and using it as a reference, locate and mark each subsequent cell by the successive increase in voltage.  Often, these other cells are marked "B1", "B2", etc. with the "top" cell marked with "B+".  Note that it is not necessary to put a fuse on one of the "end" leads (the B+ or the B-) - but I usually omit the fuse at the B- lead:  It is safe to do this as any possible combination of wires being shorted will always involve at least one fuse in the leads!

Note:  On the battery packs, female DIN connectors are used to prevent accidental contact with the voltages that are always present on these pins.
 

Using the Shunt Regulator/Equalizer:

Figure 8:
The three modified LiFePO4 packs, the extra wire/connector
being clearly visible.
Click on the image for a larger version.
When I charge my LiFePO4 packs I set a current-limited voltage-regulated supply at 14.60-14.70 volts which works out to 3.65-3.675 volts per cell - just enough to trigger the shunt regulator on each cell were the voltages to divide equally.  When I connect the battery to the supply, I also connect the cell equalizer/shunt regulator and then walk away for a while.  At a maximum charge current of 1 amp, I typically allow 12-16 hours for the pack to fully charge if it is already reasonably well-equalized.

Through experimentation, I determined the precise voltage at which all four shunt circuits would conduct at 150-200 milliamps - which turns out to be about 14.72 volts for the device that I'd built.  When the current from the power supply settles down this amount for a few hours I know that the voltages on the cells are equalized and that the battery should be removed from the charging voltage.  At a setting of 14.60 volts overall, a well-equalized pack is obviated by all four red LEDs equally glowing very dimly once full-charge has been achieved.

Using it for the first time or on a battery that is badly unequalized:

The first time I used the device on a battery pack that was known to be badly out of equilibrium for which I had NOT already charged each cell individually, it took 24-36 hours for all of the cells to come to equilibrium.  What happened was that once the "most charged" cell achieved its target voltage, the shunt regulator took over for that/those cell(s) and the others were charged by the "excess" current being passed around the fully charged cell(s).

Because this shunt current was only on the order of 100-200 milliamps, it took quite a while for these other cells - some of which were still at only 20-25% charge - to be fully "topped off" at this rather low "charge" current.

Subsequent charges:

One does not need to use this device every time a battery is charged.

It takes several cycles (and time!) for the cells within the battery to "drift" apart and fall out of equilibrium so one could probably get away with using it only every 6-12 charges (or 3-6 months, whichever is sooner) and still maintain good charge capacity on the battery pack.

When done:

Once the charging has been completed, the battery should be disconnected from the power supply used to charge it and the equalizer circuit should be disconnected immediately.

This equalizer circuit, when the voltage(s) on the cell(s) are below the clamping threshold, doesn't draw much current - somewhere in the 3-5 milliamp range - so it is not likely to discharge a pack of this size very quickly.  If this were were left on a battery connected across a small solar panel, for example, its contribution to the discharge of the battery would be negligible.

Final analysis of the problem:

One of the ways to most-quickly "kill" any Lithium-Ion based cell is to maintain it at a full charge voltage all of the time:  Best longevity is generally obtained if it is kept it in a relative state of discharge (30%-60%) and charging it fully only and just before you need it as keeping it in a state of "partial charge" reduces the rate of inevitable degradation that occurs over time due to the intrinsic chemical decomposition.  One must also be aware that allowing the voltage on any cell of a lithium-ion battery to drop below 2.0 volts can also damage  or make it unusable:  On some packs, if a cell voltage drops too low the protection circuit will actually prevent charging.

Remember that this discharge can occur "naturally", due to the current leakage in the cells themselves as well as the very slight, constant drain of the battery protection circuit so it is recommended that one check - and at least partially "top off" the cells/batteries every 6 months or so.  It should also be noted that this self-discharge rate accelerates rapidly with increasing temperature and can be alarmingly severe at higher temperatures such as those above 90F (32 C):  Under those conditions - should it be necessary to store them that way (avoid it if you can!) - they should be checked more often!

With this in mind, I generally keep my Lithium-Ion power packs in partially-charged state - or at least never leaving it on a charger for long periods - and have had good luck over the past 15 years or so that I have been applying that principal, typically getting at least 5 years out of the "new" pack before its capacity drops below about 60% of its design.

So was the intent when I got these new LiFePO4 packs:  I would top them off occasionally - usually just before using them - and when I was done, I'd typically leave them in their partially-charged state.

Again, I'm reluctant to put these batteries on a charger, set to 14.6 volts (3.65 volts/cell) and just walk away.  In theory, if I did do that they would eventually reach equilibrium as the fully-charged cells, having gotten above their maximum voltage threshold would occasionally self-discharge just below it and allow charge current to be applied across all the cells in the pack, if even occasionally, for a brief moment.  Again, doing this will inevitably reduce the overall lifetime of the battery, but perhaps this is an acceptable tradeoff for you.

What the "protection" circuits current found in batteries should really have is a more intelligent charge management capability in that if one cell was at a higher potential than one or more of the others, current would be shunted around it, allowing the other cells to equalize.  Alas, that's not the way they work, hence this need/purpose of this project!

Note:  In one of the comments below, the existence of such a circuit is discussed briefly, but I don't have direct experience with them.

Why don't other LiIon battery types experience this "inequality" problem more readily?  Part of this has to do with the fact that many of them (e.g. batteries in laptops) are typically "stored" in the device and kept at full-charge voltage all of the time and will eventually equalize.

Again, lead-acid (e.g. car) batteries along with NiCd and NiMH cells are less-often subject to this problem (inequality related to charge/discharge rather than cell degradation) mostly because they can be subjected to a degree of abuse (overcharging) without worrying too much about damage, and this allows all of the cells to equalize.  Since you can't do this to Lithium-ion cells and expect them to last a long time, this "purposeful overcharge" technique isn't really an option!

(Note:  There are equalization techniques applied to various types of batteries - namely lead-acid - but those will not be covered here.)

Results:

How do the batteries test now, after more than 2 years of ownership, before equalization, new and after equalization?  The graph in Figure 9 tells the story:


Figure 9:
Graph showing the capacities of the batteries over time, before and after equalization with the
dates of the "mid-point" readings shown in parentheses as well as indications of
readings taken when the battery was "new" and "equalized."
  Discharge current for all tests was 2 amps.  This is higher than the more typical "20 hour"
rate used for determining the capacity of batteries.  This higher discharge current reduces the
measured capacity of the batteries in comparison with the manufacturer's methods.  A graph point
for the "Metal (5/12)" data was not available, but drawn-in based on notes.
Click on the graph for a larger version.

As you can see in Figure 9 there was significant degradation of capacity as the cells drifted apart from each other, rapidly decaying in just a few months (5/12 to 9/12).  Packs #1 and #2 are identical, originally consisting of a pair of LiFePO4 packs connected in parallel at the outputs of the protection board (one of these was depicted in Figure 6) while the "Metal" pack had just one protection board with the cells originally connected in series-parallel format.

All three of these packs were used approximately equally, but it's interesting to note that the metal-cased pack seems to have degraded far less:  Was this due to differences in usage, or because of the way the cells were configured?  It could be that the "metal" pack with its "2P4S" configuration (rather than the original "4S+4S" format of the water bottle batteries) the rate-of-change of their respective states of charge from the pairs of cells in parallel caused a slower rate of degradation.

It's also interesting to note that in the case of the metal pack and water bottle #2, the capacity after equalization was actually higher than it was when new, indicating that when it was "new", the cells were already drifting apart and that the cells' capacities probably haven't degraded much in the 2+ years since they were made!

What do I think about LiFePO4 cells now?

I'm more than willing to concede that, perhaps, there is something to this longevity of LiFePO4 cells after all.  As can be seen above, once the packs were equalized the cells seem none the worse for wear after several years have passed, along with a degree of abuse!

What about this (apparent) propensity of the cells to drift apart?  It might be interesting to have another pack that is left "on charge" for long periods of time to see if it will equalize itself and/or if it degrades at an accelerated rate.  It could also be that newer protection circuits and cells are more likely to maintain a closer charge state than these (now) older packs.  Perhaps other brands of batteries would be less susceptible to this problem.

Would I get more LiFePO4 batteries in the future?

Yes, now that I know that I should be on the lookout for a problem like this!


Final comments:

Since I purchased these battery packs from Batteryspace.com, they have started offering "equalization boards" for LiFePO4 battery packs designed to address this very problem although there was no indication that these specific battery packs described on this page (which appeared to be still sold at the time of posting) contain such circuitry.  While I have not used these specific equalizer boards, comments that appear elsewhere on the web indicate that these types of boards do the job for which they are intended.

If you get a LiFePO4 battery (2 cells or more in series - that is, more than 3.6 volts or so output) I would strongly recommend that you make certain that it includes a charge equalizer - or be sure to add one yourself!

If you already have some LiFePO4 battery packs and are not absolutely certain if they have equalizers, you really should check it out - and if they don't, add them!


Update on battery longevity (June, 2016):

I recently re-tested the three batteries depicted above and found that their capacity ranged between 4.8 and 5.4 amps-hours - this for batteries that were at least six years old.  Based on their capacity when they were new, they have lost somewhere around 20% of their original capacity in that time.

While I'm a bit skeptical that they will make it to the 20 year mark it is worth noting that practically any lead-acid battery of this same age would have since been relegated to the recycler!

[End]

This page stolen from ka7oei.blogspot.com

Sunday, May 12, 2013

50 years since operation "Red Line"

It's hard to believe - especially, I am sure, for those who were involved - that it's been over 50 years since Operation Red Line.

For more details visit the Operation Red Line web site:  
 
http://www.modulatedlight.com/eos/Operation_Red_Line.html


What was Operation Red Line?



Back in early 1963 - just a few months after the invention of the visible Helium-Neon Gas Laser - a group of (mostly) amateur (ham) radio operators that happened to work at EOS (Electro-Optical Systems, later affiliated with Xerox/Parc) in Pasadena, California took on the challenge of doing something that would be both fun and challenging:  Go for the distance record for laser-beam communications.

A 1963 Laser
Figure 1:
On the workbench, the EOS Helium-Neon laser tube.  The Viking II
transmitter - the source of the RF excitation and modulation - may
be seen to the right of the laser.
Click on the image for a larger version.

At this time you couldn't just go out and buy a Helium-Neon laser tube, so they managed to cobble it and the optics together with the blessings of the senior management and a bit of help from their own well-equipped glassblowing and optics shops.

One of the vexing problems with early gas lasers - or practically any gas-discharge tube - was that of contamination/wear of the electrodes used to excite the gas mixture within the laser tube, so rather than mess with trying to engineer a solution to yet another problem, they chose electrode-less RF excitation.

But first, they had to get it to work!

The RF source itself was pretty easy:  Being ham operators, they already had access to a 100-watt class AM transmitters so Bob Legg, W6QYY, offered the use of his Johnson Viking II transmitter.  They chose a 10 meter frequency of 28.620 MHz, because it was a rather high frequency and would make the matching a bit easier, but it would also be more likely to couple into the tube and properly excite the gas - plus there was less chance that the 10 meter band would be open and the unintentional RF radiating from the coupling system would be heard halfway across the world!

In this case the idea was easy, but the implementation was more difficult:  How does one properly excite the gas inside the tube so that it lases?  Electrodes on the glass in the form of self-adhesive copper tape were an obvious choice, but it turned out to be a bit of a challenge to achieve uniform excitation until someone struck on the idea of interleaving elements, with every other element connected together, fed 180 degrees out of phase with the other set via a balanced-wire output from the transmitter coupling.

Finally, lasing!
Figure 2:
The "business" end of the laser, in the lab.  A pair of semi-silvered,
confocal mirrors were used to allow the laser to work.  The laser
assembly was put into a large piece of metal channel so that it
could be transported and maintain a degree of mechanical/optical
alignment.  A "10 power" telescope was used to collimate the
beam to approximately 2 inches (5 cm) diameter to minimize
atmospheric scintillation.
Click on the image for a larger version.

Even though around 100 watts of RF was being input to the matching network, the EOS folks were able to measure only about 125 microwatts of light emerging from the laser itself.  In speaking with those that were there at the time, they believed that with additional work they could have gotten more power out of the laser, but they felt compelled to get it out of the lab as soon as possible since crunching the numbers indicated that 0.125 milliwatts should be more than enough power to cover any distance for which they were likely to find a line-of-sight path!

The RF alone wasn't usually enough to "strike" the tube and cause the gasses within to ionize, but they had on hand a device that they referred to as a "zapper".  About the size of a high-power soldering iron, this gadget was used in the neon light and vacuum tube industry to test for gas within the device being tested by outputting a very high voltage, low-current arc that can ionize the gas within, through the glass via capacitance -  much like the trigger electrode of a xenon strobe tube:  This device is actually visible in Figure 1, sitting on the upper shelf on the far right, to the left of what looks like a water glass.

The mechanical adjustments for the laser were also very finicky.  At each end was a partially-silvered confocal mirror on a micrometer mount and it was very easy for slight temperature variations and flexure of the laser assembly to knock these out of alignment and prevent lasing.  As it turned out, both mirrors were identical so laser light was actually emitted from both the "front" and "back" of the laser, but this "wasted" laser energy from the back was useful in tweaking the laser while it was in operation since this could be done without blocking the light going to the distant end.

Finding a long, line-of-sight path:

Meanwhile, other members of the group were busy poring over maps to find two locations that were line-of-sight with each other and offered some hope of being accessible by vehicles that would be available for use and eventually, a pair of sites emerged as candidates worth checking out on the ground.  The west-ish end of the path, near the Grassy Hollow campground in the San Gabriel mountains and another site in the Panamint range, next to Death Valley and east-ish of the (almost) ghost town of Ballarat.

Getting to the Grassy Hollow site didn't appear to be too much of a problem:  Easy access from good roads near an established campground.  The Panamint site was more of a challenge, but making contact with the sole inhabitant of Ballarat - an old miner who called himself "Seldom Seen Slim" - and a few others that had mining claims in the area - proved fruitful and they were able to get permission to travel across these claims as well as get directions to some of the higher-elevation roads.  After a bit more scouting about they found a location along a road at which there was enough flat-ish area for both vehicles and setting up the gear and it offered a good view back toward Grassy Hollow.

The calculated distance?  A bit over 118 miles according to the maps.

Detecting a distant laser:
Figure 3:
In the lab, detecting the purposely-attenuated laser light using a 12.5-inch
reflector telescope.  This was one of many tests "in the lab" before the
gear was taken into the field.
Click on the image for a larger version.

At this point, one may start to wonder about the "other" end of the laser path.

It wouldn't do just to shine a laser from point "A" to point "B", but it was necessary to be able to communicate - if only one way - over the beam electronically, preferably via voice.  To do this, a means of detection of the (likely) weak beam was required.

Fortunately, one of their number, Parks Squyres, owned a large reflector telescope, a Herring-Cave 12.5 inch (approx. 32 cm) Newtonian.  To it they fitted an external box containing a movable front-surface mirror to direct the gathered light either to an eyepiece or an electronic light detector known as an S-20 "Photomultiplier" tube.  This type of tube, invented in the 1930's, is used today as it is still unsurpassed in its sensitivity to light and it was the natural choice at the time since there were no other types of electronic light detectors that came anywhere close to being sensitive enough for this task!

For testing they took advantage of the fact that the EOS building was laid out such that, using mirrors, they were able to send laser light - severely attenuated to attempt the simulation of atmospheric effects - up one hall and down the other, a distance spanning well over 100 feet.  After a bit of tweaking they were able to get good results transmitting voice from end-to-end of this simulated path, giving them confidence that the entire system would have a reasonable chance of working!

Modulating the laser was actually the "easy" part.  Originally, they had intended to take advantage of the polarized light of the laser itself (its design included a pair of "Brewster's Windows") and use a "Kerr Cell" - a device that can be used to electronically rotate the polarization at extremely high speeds - and using the two together it would theoretically be possible to convey many channels of voice.  In the interest of time and simplicity, however, they used the same Viking II transmitter that was providing the RF excitation to amplitude-modulate the light since it was, after all, an AM transmitter!    Because amplitude modulation (AM) RF was used for modulating the laser, some care had to be taken to avoid excessive excursions to "0%" modulation or else the laser would "go out" and require re-striking with the "zapper"!
Figure 4:
The laser at Grassy Hollow, set up in a tent.  Communications between this, the transmit site and the receive site
about 119 miles away was maintained using the 2 and 6 meter amateur bands.
Click on the image for a larger version.

Time was of the essence:

The EOS folks were on a bit of a "fast track" with this project because they got wind of at least two other groups making preparation to span a fairly long distance.  One of these was associated with Ryan Aircraft and other with the U.S. Military, both of whom were making rather elaborate preparations.  Since this was a purely volunteer effort using donated time, equipment and funds, there was the double pressure of making it work quickly and cheaply!

Soon after getting both the laser and receiver working they scheduled a weekend during which they would go out into the field and make a first attempt:  Friday, May 3, 1963.

Getting to the Grassy Hollow campground was fairly easy:  They just drove their cars and station wagons to the site, set up tents and then fussed with the fickle laser while the receive site group covered the greater distance to Ballarat, dropped off some of their cars in the care of Seldom-Seen Slim and then packed themselves into two four-wheel drive trucks and Bob Legg into his wife's Plymouth Valiant and they bounced their way up the steep roads into the mountains.
Figure 5:
The telescope used to receive the distant laser.  The box to
which the eyepiece was attached also had a front-silvered mirror
that allowed light to be redirected to the photomultiplier tube for
detecting the modulation (audio) on the distant laser light.
Click on the image for a larger version.

After getting to the receive site just after 3 pm on May 3rd - with Bob having to be towed the last little bit up the last, steepest part - they set about putting up tents and setting up the gear.  Soon, they were in contact, via 2 meter and 6 meter radio with Grassy Hollow over the 118+ mile path and they waited for dark.

Pointing the laser:

One of the challenges with a long-distance optical path is that at 118+ miles, even large landmarks at that distance are difficult to discern - not to mention trying to determine the precise location of the other party!  Knowing only generally where the the receive party was, the crew at Grassy Hollow needed more a more precise visual reference on which to base the aiming of the laser and the complications of doing this changed as the daylight faded, old landmarks disappeared and new ones such as city lights came into view.

Aiding this effort the receive site crew set up a very powerful Xenon strobe, but even though this was blindingly bright to those in the Panamint range at the receive site, it was stubbornly invisible from Grassy Hollow despite the use of both binocular and telescope.  Having anticipating the possible failure of the strobe a surplus military aircraft rescue flare was set off by the receive site crew and this was quickly spotted, just before the wind blew its smoke in front of the flare and blocked it from view, but at least those at Grassy Hollow now knew exactly where to look.  On a hunch that the atmosphere was blocking the dominantly green-blue Xenon flash, a humble 100 watt clear incandescent shop light was clamped into place at the focus of the Xenon strobe's reflector and this proved to be visible at the transmit site as well, especially now that they knew where to look!
Figure 6:
At the receive site, the flashlamp/reflector and the telescope.
Click on the image for a lager version.

As it got dark they began the arduous procedure of aiming the laser and something very quickly dawned on everyone:  While considerable attention had been made in the design and alignment of the laser's optics and in achieving good sensitivity of the optical receiver, no-one had really thought too seriously about the practical difficulties of aiming a very narrow beam over a distance of 118+ miles!  Using a number of improvised techniques, the laser crew managed to get the beam "close", setting the elevation with various shims and other pieces onhand, but getting both azimuth (horizontal) and elevation (vertical) dialed in proved to be a hair-pulling task.
Figure 7:
At the transmitter site, making adjustments to the laser.  The laser
itself was very "touchy", often requiring adjustment of the
mirrors at each end of the laser tube to sustain oscillation
(laser action.)
Click on the image for a larger version.

After a bit of fussing, the receive site crew was tantalized by the occasional brief, bright flash from the distant laser but it seemed as though the transmit site crew could never repeat the maneuver - plus the necessary corrections - to get the laser back and on-point!  When the receive site crew queried the Grassy Hollow folks about this on the radio it turned out that they were using two primitive tools to adjust the aiming of the laser:  A large rock tapped at the end of the metal channel in which the laser was mounted for coarse adjustments and a much smaller rock for fine-tuning!

Eventually, after much finessing and hair loss, a reasonably bright and steady beam was obtained at the receive site.

Bob Legg, in a 2008 interview, told me that after successful alignment he had "walked" the beam at the receive site and found it to be about 150 feet across at the 118 mile distance implying an overall beamwidth of approximately 0.014 degrees (about 0.25 milliradians) - a narrow beam, indeed!

Success at last!

At about 11:15 pm on the evening of May 3, 1963 the crew at the receive site was finally able to make out a voice in the weak, fading signal being detected from the distant laser.  A bit more than an hour later and after a bit more tweaking of the laser and its its aiming, signals were somewhat improved and the voice of Jack Pattison, W6POP could be heard coming across the link with reasonable clarity.




* * * * * * * *


Actual audio from a laser transmission
on the evening of May 4, 1963

Other recordings from this event may be found on the "Operation Red Line" web page - (link), near the bottom of the page.
Figure 8:
Success!  The group at the Grassy Hollow transmit site celerates
success in the late evening of May 3, 1963.
Click on the image for a larger version.

In the intervening years lasers have become commonplace, consumer commodity items with several being present in nearly every home - most notably in CD/DVD and Blu-Ray players. Although widely used on fiber-optic communications, they have found only scant use for "through-the-air" free-space optical communications

* * * * * * * *

The actual distance of the path: 

At the time the distance was measured using the approximate locations determined from the available maps and it was calculated to be "over 118 miles."

Since that time the precise GPS locations of the two sites have been determined and the calculated distance, using the "Haversine" method, is now known to be approximately 119.145 miles (191.74 km). 

Acknolwledgements:

I'd like to thank Bob Legg for supplying most of the images and many details of Operation Red Line during a 2008 interview.  I would also like to thank Parks Squyres, Ron Sharpless and Dave McGee for providing additional pictures and details to fill in some of the missing pieces.


Of course, there were many others involved in the project, each contributing their own, important part to the success of this project

Please visit the Operation Red Line web page (link) for additional attribution and details!

Links:

If you are interested, be sure to check out the links below for more information about Operation Red Line and optical, through-the-air communications in general.

[End]

This page stolen from ka7oei.blogspot.com

Monday, May 6, 2013

Not all cells are created equal! (Shunt regulation of batteries...)

If you have ever had a device (and who hasn't?) in which several cells were connected in series to make a battery, you may have noticed something an unfortunate fact:  Not all cells are created equal!

If you have been curious enough to take a voltmeter to a battery and independently measure the voltage of each cell in a string of cells, you will likely have noticed that their voltage can vary quite a bit - particularly when the battery is old and/or nearly discharged.

This mismatch is inevitable:  Not all cells were made exactly the same when they were new and can vary by capacity (in amp/hours) and/or internal resistance.  As the battery ages, there is also the fact that each of these cells is in a slightly different environment:  Some are at the ends of the string while others are in the middle, surrounded by other cells and this can affect their temperature which, in turn, affects both their longevity and performance over time.

For the most part we ignore this problem when using conventional non-rechargeable cells such as Alkalines, ceding only to the recommendation that one uses the same type and brand of cells to best assure that they will be reasonably well matched to begin with.

For rechargeable cells, it's a different story!

In that case, we often live with that which we were given.  Since it is likely that one will be (re)-using such cells for a much longer period of time than "primary" (non-rechargeable) cells, the initial match amongst the cells will inevitably diverge over time as the cells degrade at different rates, for different reasons.  For many types of cells (e.g. NiCd and NiMH) we ignore the problem at our peril - particularly with NiCds - and it is the chemistry that is oft-blamed for longevity issues rather than the way that they are applied in the first place!

There is one place where we can easily make a difference in how "equally" the cells are charged, discharged and maintained:  Lead-Acid cells.

The "UPS" Example:

One clear example of this is a typical small-large UPS (Uninterruptable Power System).  At my work, we have a 300kVa (yes, that's well over 200 kilowatts!) that uses two parallel strings of forty "12 volt" batteries (about 542 volts, nominal), for a total of 80 batteries.  Left to their own devices, the voltages on these batteries will inevitably diverge over time from the average 13.55 volts with some batteries going down to about 13.2 volts and others rising up close to 14.0 volts!

Again, this divergence is inevitable, particularly considering that the self-discharge and leakage currents that are responsible for these divergences are typically in the range of 1-20 milliamps for the (nominally) 100 amp-hour battery!  In other words, it takes VERY LITTLE difference between batteries to cause a seemingly radical change in float voltage!

This divergence actually poses a problem:
  • Much below 13.4 volts, the battery is not fully-charged and is more prone to sulfation and loss of capacity.
  • Much above 13.65 volts, the rate at which gasses are generated increases.  Even though typical UPS batteries such as VRLA (Valve-Regulated Lead-Acid) are able to contain and reabsorb gasses to an extent, continuous/excessive gas generation will likely lead to the slow loss of electrolyte and thus shorten the life of the battery.
In either of the above cases, there will be a bias toward shortened battery life and because such a system is only as strong as the weakest cell in the string, reliability and longevity can be impacted!

Even if you don't have such an extreme case as 40 batteries in series, it is common to find small-ish UPSs that use 24 (2 batteries) or 48 (4 batteries) volt systems and these, too, can suffer inequality - particularly if one uses "pulls" (e.g. aging, but still usable batteries) from larger UPS systems.

Fortunately, there is an easy solution to part of this problem:  A shunt regulator.

While we really can't do too much about the voltage of the individual 2 volt cells within a 12 volt lead-acid battery because the interconnects between cells is (usually) inaccessible, we might as well do what we can to help mitigate the problem by regulating the float voltage of the 12 volt battery overall.

Zener Shunt regulator:
Figure 1:
Zener-based shunt regulator.
Click on the image for a larger version.

The simplest system is that shown in Figure 1 which consists of just a resistor, an LED, a Zener diode and a few "ordinary" silicon diodes.

In this circuit, the Zener diode (ZD1) has a fairly sharp "knee" at 10-11 volts such that a small increase in voltage causes a very rapid increase in current.  Included is a series LED - mostly used as an indicator of current flow - that also has a sharp diode "knee" response along with a current-limiting resistor that also serves to soften the curve of the knee somewhat.  Finally, there are several ordinary diodes in series, the tap between each can be adjusted for precise adjustment of the float voltage.  While it would be possible to put many (say, 18-22) ordinary diodes in series to achieve a similar drop, this gets gets to be awkward and cumbersome and the shunt voltage of such a system can change significantly with temperature.  The system described here - while not perfect - is a reasonable compromise between performance and simplicity.

While the resolution of the 0.6 volt taps between diodes may seem to be a bit coarse, the actual effect on the float voltage is much less than this owing to the fact that the aforementioned knee isn't ultra-sharp and the amount of current required to achieve a certain amount of voltage drop isn't necessarily a linear curve, anyway!  As noted in the above diagram, small tweaks may be accomplished by tweaking the resistor value, but I rarely found this to be necessary in practice.

As you might suspect, the above circuit is going to have the effect of clamping down the voltage of a battery that would tend to float high (above 13.55-13.6 volts) but by doing so, it will (indirectly) bring up the voltage of those batteries that would tend - because of internal leakage currents - to be on the low side.

What is surprising to many people is that just a few 10's of milliamps is all that is required to equalize batteries that are in reasonable health!  The initial reaction is often one of "What good can the current through a few LEDs do to equalize the voltage on a string of 100 amp-hour batteries" but this belies the nature of the problem in the first place!  Of course, if one of the batteries is in particularly bad shape (high internal resistance or high leakage/self discharge current) then all bets are off - and this particular battery is probably at the end of its life, anyway!

Others may question the assertion of a nominal "13.55" volt setting as this "optimum" voltage tends to vary with temperature as well as whom one might ask, but it's hard to argue that this isn't, in fact, a "reasonable" voltage for the vast majority of conditions that one might encounter:  Anyway, it would be more harmful to deviate much from this voltage for long periods of time by ignoring the problem altogether!

Construction:

As can be seen from Figure 2, this circuit is quite simple:  The components are simply soldered in series with a piece of red wire and alligator clip forming the positive side.

For the negative side, a short, black jumper with a clip lead on each end is used:  One end goes to the negative terminal of the batter while the other is used to select which diode tap (and thus the shunt voltage) is to be selected.

While would could "permanently" attach the shunt regulator to the battery with a ring lug on each of its terminals, I chose not to do this since adding an extra "layer" of conductor on the battery bank of the 300 kVa UPS could contribute to added terminal resistance at the battery - a particularly important fact when it is considered that the batter current could easily exceed 100 amps!  The use of the humble spring-loaded clip lead has proven to be more than adequate and is easily removed/installed when the battery bank is maintained.  If, however, your battery bank is mobile (say, in an RV) or of modest size (say, a few thousand watts or less) then it might be more appropriate to use ring lugs as well as wire ties and electrical tape to make sure that nothing can shift and short out during transport.


Adjusting the voltages:

As noted in the diagram above, setting the precise float voltage can be a bit tricky as it can take several hours for things to settle after an adjustment.

Figure 2:
Zener shunt regulator.
Click on the image for a larger version.
It is also recommended that one actually slightly overshoot on the amount of current required to bring down the the voltage of those batteries that tend toward the "high" side and even cause those batteries that were slightly low in voltage due to higher internal leakage to come up in voltage - even rise slightly above the desired voltage.  In this case, one would also equip those "low" batteries with a shunt regulator, but one would necessarily set the shunt current commensurately lower to make the voltage equal.  In other words, it's a good idea to have at least a little shunt current across all batteries in the series string as this will help assure overall stability over time.

One should remember that the voltage on each battery should be approximately that of the average voltage of the charging system - that is, the overall float voltage divided by the number of batteries.  For example, if your UPS float charges the battery bank at 54.10 volts, you would want to set the average float voltage 13.525 volts on each battery - but, of course, it could not be higher than this!  Some UPSs have the means to adjust the precise float voltage - either with a potentiometer (sometimes unlabeled on a circuit board) or via a menu setting, and if  the average voltage turns out to be much outside the range of 13.40 to 13.65 volts, you might consider figuring out how to do this!

Observations over time:

In a number of systems - including the 300 kVa UPS - I've used this (and similar) shunt systems for over a decade and have achieved extremely good results and longevity with the battery bank.  As it happens, the 300 kVa UPS's battery bank is equipped with a system that periodically does internal resistance measurements of each of the batteries within the bank and that over time, this value of internal resistance has been correlated with the actual health of the batteries - both in terms of high-current load test capability and a longer-term amp-hour capacity test.

With good-quality 100 amp/hour VRLA batteries, it is typical to get 5-7 years of life from a battery before its internal resistance exceeds our threshold (about 5 millihoms) and it is pulled from service, at which point its capacity is typically in excess of 70 amp/hours and it will provide at least another 2-4 years in less-critical backup service.

By amassing data for each and every battery over its lifetime, I have observed a significant correlation with its float voltage and longevity:  There seems to be relatively little penalty for running the battery slightly low in voltage (e.g. 13.40 volts) but there is a statistical disadvantage to having it above 13.65 volts!

Charge/Discharge concerns:

When this system has been described, one concern often raised is that of charging batteries that may be somewhat unequal in series strings.

First of all, this is inevitable as it is simply not practical to keep all cells precisely matched and when the battery bank is being charged, it will always happen that some cells will be overcharged before others achieve full charge.  While there is nothing that we can do about this, having a degree of extra shunt current can reduce the time during which a particular battery is exposed to higher-than-desirable voltages.  Again, within the 12 volt battery, we can also do little about the state of the individual 2 volt cells within, but we can at least minimize the impact by regulating the battery, overall.

Practically speaking, it has been observed from monitoring the computer that keeps track of the state of the individual batteries that they actually achieve equilibrium quite quickly - typically within 24-36 hours - after a fairly significant discharge if equipped with such a shunt regulator system.

Other voltages:

It just happens that most of the system with which I deal use 12 volt batteries, but the shut system described could be easily adapted for 6 volt (e.g. golf cart) type batteries or, possibly, individual 2 volt cells, although the latter would require a bit more precision!

Final comments:

The Zener-based shunt system described here is about as simple as it could be in terms of construction and adjustment and still be reasonably consistent over time, under varying loads, and at different temperatures.  I did, in fact, use this system for a number of years on the 300 kVa UPS mentioned, but I gradually changed out the Zener-based shunt regulators with others based on the TL431 shunt regulator IC - but I still use the Zener-based regulators on smaller, multi-battery systems and continue to experience excellent results!

I will post an article about the TL431-based shunt regulator in the future.
 

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