I just got done playing around with trying to get an idea of the capacity of my 304ah 12 volt battery.
I have one of the cheap constant-current/constant-voltage power supplies that can provide about 10 amps. The battery bms is a 200A JDB smart bms. I also have this
hall effect DROK charge/discharge meter, which is bi-directional. I also have this
Kaweets DC clamp meter.
I wanted to get an idea of how the meters that i have compare, considering they are all on the bargain-bin side of test equipment. I decided to compare them while charging because the power supply is less likely to have a lot of high frequency noise or pulsation as compared to an inverter. So I connected everything and started charging with the power supply display saying that it was delivering 10.3 amps. At the same time, the JBD BMS reported a charge current of 10.2 amps, which is acceptably close. However, the DROK was showing 10.6 amps of current, and this was backed up by the Kaweets clamp meter was showing 10.55 amps. So I have four meters, and still a split decision on what to believe.
One of the uses for this battery is for power backup for my deep freeze. So I put it on the inverter as a load. I thought I would see higher current draw, but it turns out to be pretty efficient, and only pulled about 10 amps from the battery also. However, this did let me confirm that the same reading differential exists between the JBD bms, the DROK meter, and the Kaweets meter while discharge as while charging.
So considering it is a nominal 304ah battery, this is a 12ah difference in results on a full discharge. Not the end of the world, but it could be the difference between thinking you got what you paid for or not. Mine came out between 310ah and 322ah at very low C rate. I need to conduct another test where I keep the C rate closer to 0.2.
Also,
@DThames has a good point about setting the BMS nominal capacity higher than the actual capacity for testing. The JBD will only read from 0 to the nominal capacity I set, so unless I set it higher, I can't read the full potential capacity of the battery.