For what it's worth, I test my almost-1000A battery bank (4 8D AGMs) by switching on numerous high-load items and letting them run while monitoring the battery voltage during the process. I switch on the spreader lights, my few incandescent running lights, interior
lights and fans, chartplotters, computer, etc, until my ammeter reads 25A or so. At 25A, it takes 20 hours to consume 500Ah, which is a good test of the battery condition, and a reasonable load for testing the house bank.
This doesn't give me a particularly accurate battery capacity number, but it does tell me if the battery is still in good health
. I've had this set of AGMs for over ten years now and they are still going strong. Most of the time they are sitting idle with the solar panels
keeping them topped up, but every so often they get a real workout on my Hawaii