Some LiFePO4 Battery Testing Results Manual and Automated

On Sunday, April 21, 2019 at 1:56:13 AM UTC-4, 2G wrote:
On Wednesday, April 17, 2019 at 6:58:40 PM UTC-7, John DeRosa OHM Ω http://aviation.derosaweb.net wrote:
Every year I test my two LiFePO4 batteries to gauge how long they will last during a flight. I decided to open the field a bit and also tested some soaring friend's batteries.

You can read about my simple cheap manual process (later automated - see below) here http://aviation.derosaweb.net/#batterytest which was also detailed in Soaring (Feb 2012). Yeah, I know there are automated testers on the market but I want to create something cheap that everyone can make.

My battery go/no-go is how long it will take for the battery to drop to 12.0Vdc with a continuous 12Ω (~1A) resistive load. Here is what I found;

2013 Bioenno Model BLF-1209T- 6.5H
2015 Stark Model SP-12V9-EF - 5.5h
2015 Bioenno Model HN12V9AHF- 7.0H
2017 Bioenno model BLF-1209WS - 9.0H

Full details of my results can be found here http://aviation.derosaweb.net/batter...s_04.08.19.pdf

Anyone else tested their batteries in this same way? What are your results?

Automation - Manually gathering data every 10 minutes for 6-9 hours on four different batteries is tedious at best. I watched a lot of movies. So I decided to automate the process with an cheap $10 Arduino Duo. The Arduino software takes a reading at different points of time based on the current voltage. Every one minute at the beginning and end of the test, and every 10 minutes in the middle.

It still takes 6-9 hours to run the test but recording the voltages is fully automated so running the test is a start-then-walk-away-and-come-back-later-for-the-findings kind of deal. Luckily with the LiFePO4 batteries when they get below ~11.0Vdc the BMS basically shuts the battery off. The program detects any voltage drop below 8Vdc and terminates itself.

If anyone is interested in duplicating my Arduino test rig, drop me a line and I will share the details. My Arduino code can be found at http://aviation.derosaweb.net/batterytest/arduino.

John OHM Ω

It would be helpful to modify your spreadsheet to calculate watt-hours. Comparing batteries by amp-hour capacity was ok when using the same chemistry, but is not misleading when comparing SLA to LFP.

There is a marketing ploy among the LFP manufacturers called "equivalent SLA capacity." It turns out that they are using very high discharge rates (10C) to come up with this so-called "equivalent capacity." When WH capacities are compared at a more typical glider situation, 1C, the LFP has about a 7% WH advantage at the same AH rating.

The big advantage of LFP batteries is there slower aging characteristic. SLA batteries can drop precipitously after 2-3 years of use, far beyond any datasheets I have read. They *should* be good for 400-500 discharge cycles.. This would be at least 10 years of typical, non-commercial glider flying. Only once have I seen SLA batteries last more than 4 years, and most are really shot by then (50% capacity).

I encourage more sharing of battery testing of this sort. It will help us get a better handle on this annoying technology which even the big boys hate (just ask Boeing).

Tom

Yeah, batteries, a love-hate relationship, can't live without them, but they suck. One thing about capacities: if you fully discharge an SLA repeatedly it shortens its life (as measured in years). Thus a "9AH" SLA is really about 5AH, unless you don't care if you have to replace it in less than 2 years. (They are cheap enough that you may not care.) OTOH the LFP battery you can bring right down to where it shuts itself off and no harm done. So if it is honestly rated, a 9AH LFP truly has usable 9AH, perhaps 8AH after a few years. Just don't touch any LFP rated as "xxAH SLA equivalent", those are designed for starting motors (e.g., for motorcycles) and they are "equivalent" in cranking power (peak current) but not real AH (low current for hours).

Moreover, the voltage of the LFP stays higher (say well above 12V) for a much higher portion of its discharge cycle, then plummets. That means less warning about when it will run out, but meanwhile your radio transmissions will be good. An SLA gradually declines in voltage as it discharges, and some radios (and some other devices) may not work as well on, say, 11.7V.

Albeit most modern glider-oriented devices (e.g., varios) are designed to work normally down to 10V or so. Devices with internal switching power supplies draw more current when the supply voltage is lower, roughly a constant power draw (watts). E.g., I measured the current draw of a Portable PowerFLARM and it was roughly between 100 and 200 milliamps, depending on the supply voltage, the higher the voltage the LOWER the current (very unlike a light bulb). That's why an LFP, with its somewhat higher voltage, yields a longer run time than an SLA for the same amp-hours discharged.