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Old February 2nd 17, 02:20 AM posted to rec.aviation.soaring
jfitch
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Default Test results LiFePO4 glider batteries after 6 years

On Sunday, January 29, 2017 at 11:41:07 AM UTC-8, SF wrote:
I started using K2 LiFeP04 batteries in my glider in 2010. I have had excellent results with these batteries. The electronics, especially the radio, enjoy staying above 12V, and I've never run out of power inflight with them.

This winter after taking some of the instruments out to have them calibrated & upgraded, I took one of these batteries with me to the shop to power everything up to see if all the magic smoke remained in the equipment after it was reinstalled. Imagine my surprise when nothing came on, and I found the battery at 10.5 volts.

This battery was charged after its last usage 1-1/2 months ago and left in a cabinet. Since these batteries are not supposed to self discharge that fast I decided to charge, and test them using two batteries in parallel on a 2.04A load. My six year old 9.6AH K2's were down to 5.4 Ah each, 10.8 Ah total.

I purchased two new Stark Power SP-12V9-EP LiFeP04 batteries and wired them in parallel on the same 2.04A load, and got 8.67 Ah each, 17.34 Ah total..

The tests were run until the voltage under load dropped to 11.5 V, not the 10.5V level where the battery management board turns off the output. There are probably some additional Ah's left in there after 11.5 V but not much, these batteries drop off pretty fast after 12.5V. The stark batteries are listed at 9Ah each.

The LiFePO4 batteries are advertised as having a cycle life of 2,000 cycles. Over the six years they were in use, I probably put less than 400 cycles on these. So obviously they also degrade with age and not just the number of charge/discharge cycles.

I still recommend these over the old sealed lead acid batteries. I also advocate annual testing to see if they still have the capacity you need, rather than waiting until they surprise you in flight.

SF


Cell balancing in Li battery packs is either passive or active (I guess the modern terms are dissipative and non-dissipative). Either strategy can have several topologies. The cheap battery packs we buy are almost sure to be passive. They attempt to achieve balance by shunting a parallel power dissipating resistor across the high cells. It can only do this near the top of the charge cycle, and the capacity of the shunting element is small (a few milliamps) or it will burn up. That means you need to be at the top of the charge cycle for long period to complete the process. The hope is that the low cells will continue to charge while the high cells are prevented from overcharge by the shunting resistor.

One problem with a $20 charger is that it is likely to be a simple 14.6V voltage source, and some of them shut down when they hit that voltage (LiFePO4 cells should not be "float" charged, generally). If the charger shuts off or cuts back its voltage, there may not be sufficient voltage or time to charge the weak cells. There are further limitations to the technique, as they are balancing based on voltage and not capacity. These are just the cost limitations that we are dealing with. Make sure your charger maintains the 14.6 voltage on the battery for the period you think it is supposed to be balancing.

Expensive batteries will have active cell balancing, this is DC-DC convertors for each cell that take energy from strong cells and transfer it to weak ones, both during charge and discharge. Expensive battery chargers will also monitor each cell, and actively charge weak cells more. The price point for these features is higher than most glider pilots (who are cheap *******s) are willing to pay.

Maybe someone with more knowledge will chime in, but that is my understanding from looking at LiFePo4 batteries for my boat - we are talking there about $10K and up battery packs with passive, active, and charger regulated cell balancers, sometimes all at once.