There are two indicators of battery condition, the "specific gravity", or strength of the acid concentration, and load amperage, the current the battery can produce under starting load. A totally discharged battery would have the same specific gravity as plain water, 1.000. A fully-charged battery will have a specific gravity of 1.270 or so, (1.270 times as heavy as an equal volume of water). Buy a good-quality battery hydrometer, one that reads out in numbers, not a series of floating balls. Put the battery on a slow charge, about 10 amps or less, and check the specific gravity frequently. When the S.G. remains the same for several checks, (probably overnight) the battery has taken as much charge as it can. Further charging will simply boil the water out of it. It should top out about 1.265 or higher, and all cells should be approximately equal. One low cell means the battery is junk.
Load test- - - - -If you have access to a battery load tester, put a current load on the battery of 1/2 the cold cranking amp rating for 15 seconds, then read the voltage at the terminals with the load still applied. 12 volt batteries should read a minimum of 9.6 volts, 6 volt batteries should maintain 4.5 volts minimum after a 15-second load test. A pretty good load test can be done with the starter. Crank the engine for 15 seconds, with high beam headlights and any other high-draw accessory such as the cigar lighter also in use, and check battery voltage at the end of the 15-second test. 12 volt batteries- - -10 volts or above, while still cranking. 6 volt systems, 4.5 volts or more while cranking. Short boost-charging sessions are almost worthless, particularly with 6-volt batteries.
Jerry
