If the battery has cell covers, checking the acid with a a hydrometer is still a good idea. It's really the only way to know if the charge is complete. Battery acid has a specific gravity of about 1.265 which means the liquid weighs 1.265 times as much as water. Straight water always has a S.G. of 1.000. As the battery discharges, the gravity goes down as the acid is absorbed by the cell plates. Recharging drives the acid back out of the plates, and increases the weight of the liquid.
I start a recharge by checking and recording the S.G. of the acid in each cell, and recheck about once an hour during the charging cycle. When I get two consecutive readings without an increase, the battery is charged. Further charging just evaporates the water; it doesn't help the state of charge. Hydrometers aren't expensive- - - -a good one with a floating numbered vial and a temperature correction thermometer costs about $10.00 at most FLAPS. Stay away from the el-cheapo ones with a bunch of floating balls. To be absolutely accurate, the battery acid should be measured at a temperature of 80 degrees F., or a correction factor should be used. The thermometer shows how much to add or subtract from the base reading.
Jerry
