Originally Posted by Gordeno
I read someplace that a simple battery load tester was not reliable on deep cell batteries.
$50 400 watt AC inverter, outlet strip, incandescent light bulbs. Or, any other house or external load you can apply and control. Digital voltmeter, and a clock.
Step one, charge the bank. It must be 100% charged as evidenced by the charger completing the requisite charge cycles, verifying specific gravity or manually. Even better is to run an equalize cycle if the charger has that option.
Connect the inverter/load to the batteries. There should be no other loads connected.
I pick a combination of light bulb wattage that gets me the appropriate current draw from the batteries as drawn by the inverter. So if you have 220 Ah batteries, divide that by 20 (for the 20 hour rate), = 11 amps.
Turn the inverter/load on and start the clock. Run the test until the batteries reach 10.5V. How many hours the test runs is your net capacity. New/good batteries will run for 20 hours, usually a bit more. 15 hours would be 75%, 10 hours 50%, etc. I consider "good" to be greater than 80%. Less than 80% and it's battery shopping time. 50% or less is failed.
Cold batteries will exhibit less capacity so factor that into the result. Some battery data sheets will provide that derating but others don't. Just so you're not surprised if your batteries come up short when they're cold (<77F). Below is a Trojan derating chart.
As a bonus, what I do is with this freshly dead battery bank I plug in the RV and let the converter bring them back up. Gives me an idea how long that takes and verifies the converter goes through the proper cycles.