For an update about what turned out to be happening with these batteries - and one possible solution - see the May 18, 2013 post, "Lithium Iron Phosphate batteries revisited - Equalization of cells" - linkAbout
Why switch from SLAs?
|The three LiFePO4 battery packs in question.|
LiFePO4 packs seemed to be attractive for the following reasons:
- LiFePO4's were lighter than the same-capacity SLAs - roughly 1/2-2/3 as much weight.
- Claimed 1000-2000 charge durability for LiFePO4's versus 100-200 or so for SLAs.
- Claimed 10 year lifetime for the LiFePO4's versus 3-5 years for SLAs and conventional Lithium-Ion packs (even the polymer types.)
- With all of the above, the relatively high initial cost ($95) of the LiFePO4 batteries that would last 10 years seemed to be reasonably comparable to $15-$30 (when new) on a "per-year" basis with typical 7 amp-hour Lead-Acid packs - and the lighter weight was a plus!
So, over the period of several months, I ordered three of these 6.2 amp-hour LiFePO4 packs that put out about 13-ish volts over their discharge cycle - slightly higher than SLAs, but still well within the realm of what typical "12 volt" gear will accommodate.
As I typically do with newly-acquired batteries I checked the amp-hour capacity of each of the three battery packs shortly after arrival using my West Mountain Radio Computerized Battery Analyzer at 700 milliamps and found that they were reasonably close to the advertised capacity - that is, around 5.8 amp hours: Typically such batteries are rated at the "20 hour" rate which would have been about 310 milliamps and the higher rate that I used would reduce the measurement by 10-20% so I was pleased with the results.
At about the same time I acquired some 2 year-old 12 volt, 7 amp-hour lead-acid batteries that had been pulled from UPS service on a routine basis and these were found to have about 6.2-6.5 amp-hour capacity at the same 700 milliamp rate.
In the intervening years I used these batteries (both LiFePO4 and SLA) about equally, running radio equipment and the like and earlier this year I suddenly realized that something was amiss: The LiFePO4 packs were dropping out far earlier than they should have.
A bit of explanation here:
All rechargeable lithium-ion packs have built-in circuitry to protect against excess over-discharge, the reason being that if you run a lithium battery down too far an irreversible chemical change occurs and they cannot be safely recharged ever again. For this reason when a lithium pack runs down too far it will suddenly drop off, the internal circuit disconnecting the battery to protect it.
Lead Acid packs, on the other hand, do not do this: Their voltage slowly drops down and their effective internal resistance goes up and one eventually realizes that the equipment being powered is no longer working correctly.
As it turns out both Lithium-Ion and Lead-Acid packs are charged in similar ways. One simply connects a power supply of voltage appropriate for the type of battery pack and let it charge. Both types of batteries, when discharged, will pull more charge current but this will gradually drop off as the battery approaches full charge and for this reason it's typical for these power supplies to be current-limited as well as be fixed voltage.
A major difference between how one treats Lithium-Ion (including LiFePO4) and Lead-Acid (SLA) batteries appears at the point of full charge:
- For SLAs one obtains the best lifetime by continuously maintaining them at a constant voltage - typically 13.5-13.8 volts for a "12 volt" lead acid battery
- Lithium types should not be maintained at the "full charge" voltage after full charge has been achieved.
What happens with Lithium-Ion batteries (including LiFePO4) is that if you maintain the "full charge" voltage its internal chemistry degrades much more rapidly than if you were to fully-charge the battery and then immediately disconnect the source, allowing the voltage to sink down a bit on its own.
What this means is that you will get much better longevity out of a Lithium pack if you do not keep a high-level float charge on it. In fact, the best longevity of Lithium-type rechargeable batteries can be obtained if you store them in a half-discharged state - provided that you check once in a while to verify that their self-discharge hasn't caused their voltage to go so low that they become damaged from that!
* * *
That is how I treated the LiFePO4 battieries: I would attach the pack to a 1-amp, regulated 14.2 volt power supply for 12-18 hours and then disconnect it and then place it on the shelf, possibly topping it off briefly just before using it. The Lead-Acid batteries, on the other hand, are left connected to a 13.6 volt power supply and allowed to sit there all of the time when not being used.
I was, therefore, chagrined when after just two years the now 4 year-old SLAs were outlasting my LiFePO4 packs.
This observation spawned some further testing, so I put the LiFePO4 packs back on my battery tester I was further distressed to note that those that had originally tested out as having 5.8-6 amp hour capacity were now, at the very most, in the 1.5-2 amp-hour range while the much older SLAs were still in the 5.0+ amp-hour range.
In the time since I did the testing for this entry, the LiFePO4 packs have continued to degrade at about the same, alarming rate while the old Lead-Acid cells are still holding in, degrading much more slowly.
So, what's the deal? Why are the 4+ year old SLAs still in better shape than the 2 year old LiFePO4 packs?
I really don't know. I've attempted to correspond with the sellers of the LiFePO4 batteries (batteryspace.com) to find out their "take" on this observation, but I've not heard back from them - too bad since I've had reasonable luck with their customer service in the past...
Perhaps they got a batch of "bad" cells - but since the three LiFePO4 packs were actually purchased several months apart it would seem to me that it's more a problem with manufacture/chemistry of the cells themselves.
What to do?
At the moment I'm sticking with the old, heavy SLAs since I'm now understandably "gun shy" when it comes to LiFePO4s since the former do seem to be fairly predictable in their longevity and performance - at least when treated properly!
For an update about what turned out to be happening with these batteries - and one possible solution - see the May 18, 2013 post, "Lithium Iron Phosphate batteries revisited - Equalization of cells" - link
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!
This page stolen from ka7oei.blogspot.com