It's called an hydrometer. In the old days (ah-hem) every mechanic had one in his (few hers back then) toolbox. With the advent of "maintenance free (hah!)" batteries, this tool has sort of disappeared, but it's a great tool nevertheless.

Lately, on an overnight, I switched on battery # 2 to get my coffee maker going in the morning. To my amazement (and disgust) the voltage available went DOWN when the extra battery was switched in. To make a long story short, testing revealed a dead cell in this battery, which won't hold a charge. The charger will keep it at 13 volts or so but, when the charger is off-line the battery drops to 10 volts in a few hours. Another one bites the dust! My trusthydrometerer instantly (more or less) told me WHAT was wrong with it, so the guessing game ended.

Great tool, and not that expensive. I recommend one for YOUR toolbox

Jeff

Jeff,

Good tip on the hydrometer. I keep one in the engine room parts box. Along with that I keep a bottle of distilled water and a turkey baster in the dock box or on board if traveling. The "maintenance free" batts, as you mentioned, are a crock unless the batts are physically sealed, IE. VRLA (valve-regulated lead–acid) etc.
If you have a properly functioning battery charger the battery water should not boil off except from normal cycling but in any case visual inspection of all cells should show water just over the top of the lead plates. Be careful not to overfill above the splash guards.

Note, the acid residue on or in the hydrometer, baster etc will damage upholstery & carpet and clothing. I learned real fast one day when my jeans began falling apart while sitting in my college class years ago.

-Karl