I read an article many years ago that said to use a hydrometer to test the specific gravity of your batteries as they are charging and only charge them up to 1.265. Best advice I know of for battery longevity. The only one I know of that you can do this with is the one Prestone makes - it's a flat clear plastic housing with a rubber hose on the bottom and a squeeze bulb on top. Just periodically test each cell until the pointer is on 1.265. If you have a cell that is getting weak, it will be obvious from your test readings. This will give you a little advance warning that your battery needs to be replaced.