[Richard]

Where do you have the low battery cutoff set?

An easy and good baseline trial to run, is to fully charge the batteries, turn off the magnum inverter, reset the amp hour counter, and let the inverter run the house loads until you trip the LBCO. You will generate a usable capacity number for your bank. Sometime in the future if you are curious as to how the bank is withstanding the test of time, you can rerun the test under the same conditions and compare.

I wouldn’t fret if I didn’t get the nameplate capacity, since in all likelihood you will have the LBCO set before the batteries approach a dangerous discharge state, or the internal BMS disconnects them.