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