I’m in the middle of a discharge capacity test and am using resting cell voltages as my outcome variable, measured against the output through a shunt. The shunt (Victron BMV700) reads instantaneous watts, but cumulative ah, which is unfortunate. I’m trying to simulate realistic conditions so i have a small fridge running non-stop and a dehumidifier running in about 25-30a bursts for an hour, and then off for an hour, then taking my voltage read at the end of the ‘rest’ hour. Finally, I’m graphing it against a standard LiFePO4 discharge curve. Lots of holes in the methodology, but it’s just to give me a better confidence in the capacity. So far, so good. I’ll repeat it with different loads and overlay the results. Any suggestions to make it more accurate an assessment?
Discharge capacity testing
- Thread starter Braddo
- Start date
It depends on what you want to know. To understand real run time with devices that cycle, it may be better to hook up the fridge or dehumidifier, and just let it go till a cell hits 3 volts (or whatever you want to define as the lowest acceptable voltage before cutting off the loads) and see how long it takes, then look at AH used. Recharge and repeat for each device you are interested in. That way you have data with real running conditions. Treat cumulative AH as a gas gauge. As you do more of these tests, you'll get a good picture of how many AH the battery delivers before depleting too low and how many AH the devices use over a given time. I was surprised to see my fridge stay on it's defrost cycle for nearly an hour. I know I can run the fridge for 16 hours before needing to recharge the battery, more if I could disable the defrost.
For raw capacity testing of the battery itself, a heater or fan of some sort provides a nice, constant load and you can run the battery down to the last mAH. It may be good to do a test till the BMS or inverter shuts off to see how long you can really go if the conditions demand it and note AH consumed. Now you have an idea of how much "emergency" buffer you have and you have verified that the BMS and inverter cut offs actually work.
For raw capacity testing of the battery itself, a heater or fan of some sort provides a nice, constant load and you can run the battery down to the last mAH. It may be good to do a test till the BMS or inverter shuts off to see how long you can really go if the conditions demand it and note AH consumed. Now you have an idea of how much "emergency" buffer you have and you have verified that the BMS and inverter cut offs actually work.
jasonhc73
Cat herder, and dog toy tosser.
Get a bi-directional meter.
ELSTER WATTHOUR METER (KWH), A1RL+, FM2S, 200A, 120V-480V, BI-DIRECTIONAL | eBay
Model: A1RL+, Type: Bi-Directional, Form: 2S, Class: 200, Volts: 120V to 480V Range, Wire: 3, Cycle: 60 Hz, Test Amp: 30, Capacity: 200 Amp, 4 lugs.
www.ebay.com
Turd Furguson
it's a funny name.
- Joined
- Jan 27, 2020
- Messages
- 64
did you perform an initial top/bottom balance when you received the cells?
c-rating on discharge will move the curve up/down relative to voltage. If a low discharge, like in your case, expect the voltage to sit a bit higher and crash around the bottom 10%.
c-rating on discharge will move the curve up/down relative to voltage. If a low discharge, like in your case, expect the voltage to sit a bit higher and crash around the bottom 10%.
