battery cyclers
I use an ACE abacus to cycle my batteries down and see how they are doing. If i had done it more I wouldn't have lost a plane to battery failure in flight. Yes I checked it out just before the flight with a voltmeter and it showed a full charge but trouble was waiting for me on that flight. I also have a quick charge that my friend in Australia designed and it will show you right then if a battery is starting to go bad way before anything else will.