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post #1 of Old 09-27-2013 Thread Starter
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Location: Bremerton, Washington
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How big of a starting battery do I really need?

My current battery setup involves three identical group 27's (flooded), two as a house bank, and one as a starting battery, with an ACR to charge both. It occurs to me that my investment in the starting battery (both in terms of dollars, and in space) is going largely to waste, since very little of the capacity is needed to start the engine.

If I purchased a smaller starting battery, I could add my current starting battery into the house bank, which would substantially increase my capacity. But to do that, the new starting battery would need to be pretty small, because I don't have much spare room in the battery compartment (starboard lazarette) (I'm envisioning glassing a small battery shelf partway up the inside of the hull).

So what's the smallest battery I could get away with? (Engine is a Yanmar 2QM15. No glow plugs, so a bit of cranking is needed, particularly in the winter). I've seen comments on SN about using a small lawn mower battery as a starting battery -- would a battery that small really be adequate? Is there a way to figure out what I really need, short of trial and error?

(I don't have a clamp on ammeter, but I'm planning to install a battery monitor in the near future -- might be able to do some experiments with that).

1981 Spirit 28 "Bonnie Lass"
Bremerton, Washington

Last edited by Rusty123; 09-27-2013 at 11:49 AM.
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