Battery Switch Operation

by James
(UK)

I have always been told not to move the battery switch once the engine is running, if you do this you could blow the alternator diodes. Yet in some articles I have read it sounds as if this is ok. Could you clear this up for me.

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Aug 22, 2026
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by: John Payne

There are two issues and one is movement and the other switching off. If you switch off the switch the surge or voltage transient will destroy the alternator diodes. The next issue is switching or moving between batteries. This entirely depends on quality of the switch. Many of the cheaper battery switches have contacts that don’t slide or make correctly. When the switch is operated the contacts may momentarily break causing a transient and damage. The most common cause of accidental opening or switching off is in fact when a person wants to switch between batteries and switches it off by mistake. I have lost count of attending boats that had alternator failure from this.

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