Join Date: Aug 2006
Location: Arlington, VA
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The "industry standard" for testing battery capacity these days is a sophisticated -- and pricey -- computerized device which measures internal resistance. The Micronics series are very popular, but they cost about $600 for a medium-range one.
I have used these for extensive battery tests over a couple of years, and they're pretty good if you use them right. But, they're not 100% accurate.
The best way to load-test a battery is to put a constant load on it calculated to be sufficient to deplete it fully in 20 hours, based on it's 20-hour rating. For example, for a 225AH battery bank such as two Trojan T-105's in series, you would use a load of just over 11 amps.
As well, it's good to put a little energy-monitoring device like a Watts Up in-line, to actually measure energy delivered (in watt-hours). That way, you can see how closely the battery's actual capacity compares to it's rated capacity.
Everything else is a "next-best".