Proper load testing method for LiFePO4 batteries.

[Mattb4]

For lead acid, deep cycle batteries are rated based on time (20hr is a common period) and amps delivered over that time scale. So if you wish to test a LA capacity you would takes its AH rating and divide by time to get amps and run your test to see if it can deliver that with not reaching low voltage shutdown. I see several methods that people are using to test LiFePO4 that makes me wonder if there is any universally agreed method for coming up with a capacity.

High amp draw tests to get the testing done quick would create the voltage slump situation under load leading to early shutoff.
Low amp tests are based on what current amount? Should it be constant current, constant load or and what is considered cutoff voltage?

When I have run the few tests that I have done I chose constant load since that seemed to me to be a good way to see if the battery would perform in a real world type setting.

 

[Daddy Tanuki]

My cottage/cabin? max use with a/c at max cold, and two 1800 watt kotatsu heaters, all lights, fridge & chest freezer, shop all lights, drill press & a hair dryer to simulate a microwave load, only barely cracked 40 amps of battery draw.
so I test mine at 40 amp draw until they hit LBCO of 3.0 volts. (this is individual cells of course, not a pack).

I have no way of drawing 40 amps on a pack except for the above mentioned stress test with solar shut off and measuring with a shunt. (I have four x 1800 watt hair dryers I bought used to use as a load as well.

 

[Vigo]

I think it's mostly based on patience. Less amps = less voltage drop (which gets converted to heat) inside the battery and across all the wiring of your test setup, so technically the slowest test is the most 'accurate', BUT not the most realistic because most people don't use their actual batteries at .01C all the time, etc. So i think it's a line in the sand.. a slower test loses less to the efficiencies of the test rig, but might also be slower than any actual real-life usage scenario.

 

[wattmatters]

For lead acid, deep cycle batteries are rated based on time (20hr is a common period) and amps delivered over that time scale. So if you wish to test a LA capacity you would takes its AH rating and divide by time to get amps and run your test to see if it can deliver that with not reaching low voltage shutdown.

Except you likely wouldn't want to take your lead acid down that low just to test them.

LiFePO₄ manufacturers usually state the test conditions for their cells. e.g. looking at mine it states:

Standard charge:
Constant Current 0.5C₅ A
Constant Voltage 3.6 V
0.02C₅ A cut-off

Standard discharge:
Constant current 0.5C₅ A
End voltage 2.5 V

It is recommended to use newly produced battery packs and new cells for related tests. Unless specified, testing and measurement shall be done under temperature of 20±5°C and relative humidity of 45~75%.