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Old 04-10-2002
Tom Wood Tom Wood is offline
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Switching Batteries

Is it true that I shouldn't change my battery switch when the engine or dockside charger is running?

Tom Wood responds:
Thanks for your question. The answer depends on where your battery switch is at the time. I am unaware of any battery switch that would not allow you to change from "Battery 1" to "Battery 2" or to "Both" while the engine is running. However, most switches can never be turned to "Off" when the engine is running without instant failure of the diode trio inside the alternator. And it is easy to make a mistake and turn the switch the wrong way, so some sailors just have one simple ruleónever touch the
switch when the alternator is turning.

There are battery switches available with an "Alternator Field Disconnect" (AFD) circuit built in that can overcome this problem. The field wire to the alternator is led through the battery switch, and is automatically disconnected when the "Off" position is chosen. This protects against the captain and crew members accidentally switching to "Off" when they thought they were switching to "Both." These switches look outwardly the same as your current one, and only cost a few dollars more. The installation is identical except for the running of the alternator field wire to the switch.

Another alternative would be to add a battery combiner or isolating diode in the alternator's output circuit. This would eliminate the need to be concerned with the charging process at all when the engine is running.

Your battery charger question will depend on the charger. If it has one output that is wired to the common terminal of the battery switch, you would need to read the charger's manual to decide how much charging capacity it has. If it has two independent outputs already wired to each battery bank, the position of the battery switch is irrelevant. Here's hoping this answers your question.