Overcharging Battery

[Tom Wood]

Whenever my boat is docked for about a week with the battery charger plugged into shore power, one of the two batteries will systematically dry up, forcing me to add more distilled water. What could be causing this?

Tom Wood responds:

I would first suspect the charger. A small voltmeter, preferably digital, should be used to test the voltage output of the charger at the battery terminals—it should read between 14.1 and 14.3 for wet cells in the first, bulk-charging phase. This test can only be accomplished if the batteries are used (without charging) for a short time. The measurement is then taken over the first 10 or 15 minutes that the charger is turned back on. This voltage should step down to a float charge of about 13 volts when the charging cycle is near the end. If the voltage is too high, or does not taper down, you have an overcharging problem.

The second place I would look is the battery. It may be damaged or defective, and this may only be apparent with a load test at a battery dealer. If the charger is working OK, but the battery has a shorted cell or other internal resistance, the charger energy is simply blown off.