Differential comparison is an excellent tool. A standard TWELVE VOLT ONE HUNDRED WATT LIGHT BULB from N.A.P.A. is an excellent 8.5 ampere load. It is threaded just like the old household screw in light bulb but 12-volts.
Buy a ceramic base. Wire it to battery with 14 gauge wire and two alligator clips.
Charge batteries as much as rationally possible
Discharge "bad" battery 4 hours then disconnect
Discharge any other suspected "good" battery"for four hours.
This test is simple yet demanding and exact. The parts are DIRT CHEAP. The actual testing is simple and direct.
Increase time to 8 hours discharge each battery for amplification of end result.
There will be a huge difference in end of test voltage if there is a bad battery.
See what it is... ... ... ... ... :)
