Thank you. That makes sense and would be the ideal method for critical measurement. I tried the Victron shunt (because I already have it), on a 230Ah Li Time, with a 2000w inverter and a space heater, with a total draw of around 145A. When the voltage dropped below the inverter threshold, I let it recover a bit, and then reduced the load, to 62A. I did that a couple of times and was able to get the total discharge measured by the shunt to 228Ah. At this point the battery was still producing, but the voltage was too low for the inverter to be effective. I could still connect lights or other 12v items to the battery without a problem. The battery did not cut out, even after discharging 228Ah. So I am guessing if I wanted to connect a smaller load and eke out the last bit of power, I would have no problem reaching 230Ah or possibly more. So for my purposes---does my battery at least meet the advertised capacity?---I feel confident that it does, with my rather unrefined testing method.
Your method basically matches my theory of "practical" capacity vs. absolute capacity, and why I am only concerned with a rough capacity test to know the battery is in good shape and not faulty or lower grade cells. The inverter is going to cutout from low voltage before the battery reaches its own low voltage cut off, since the inverter will require more voltage than simple small 12v DC loads. What is my capacity to run AC powered items on the inverter vs. capacity to run DC powered loads directly? This is the practical test. For the RV, my AC loads are actually rather short durations, or unnecessary most of the time---morning coffee, maybe a few minutes with the microwave. Rarely use the air conditioning in our area. Beyond that are only DC loads---lights, water pump, refrigerator, etc., which, by comparison sip Ah. Basic DC loads, such as lights, will continue functioning well beyond where the inverter cannot. So if the practical capacity test with an inverter is close, I know I can go well beyond it with basic 12v loads.
Agreed. The capacity testing is really just to make sure the batteries are good, and will all play nice with each other once wired together into a larger battery bank. If any of them were falling well short or having a problem charging, I would return it/replace it. The testing is to prevent me from finding out the hard way if a battery has an issue.
