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Reply To: Engine and service battery linking

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David Babsky

I haven’t “hot-wired” that relay to switch in the “house” batteries (..and some of the charge from those would then go to charging up the engine-start battery, thus dropping, I think, the overall voltage before you even turned the starter key!)

What I HAVE done is to put in another switch – obtainable from almost any chandlers – which can switch between the engine-start battery (just a single one on our boat) and the “house” batts (three in parallel on our boat) so that EITHER of those can be used to start the engine. This means that if the starter-batt is flat (terrible situation, as completely flat lead-acid batts never, apparently, and in my experience, never recover their original capacity or “staying power”) then the house batts can be switched in, and there’s no consequent voltage drop as a result of their trying to boost the flat starter battery.

As soon as the engine starts – which it does, with the power of three “house” batts – and the relay switches on, then the engine-start batt gets quickly brought back up to power, but without draining the “house” batts, except during that engine start.