Upgrading from 1 battery to 2 batteries

[jteabo]

I am upgrading my boat from only 1 battery to 2 batteries, a house battery and a starter battery. My 1991 Mercruiser Alpha 1, 4.3L, 55 amp alternator has #6 gauge wire leading from the battery to the starter and ground. My new starter battery is a 800 CCA 24 group and I am not sure if I need to change the + and - leads to #4 gauge. Will the #6 gauge stock wire be ok with a stronger battery?

[msmith10]

The "strength" of the battery doesn't dictate the wire size. It only provides an indication of how much current the battery can provide over a period of time. The wire size is dictated by the amount of current that will be flowing through the wire out of the battery. That is, the load, in amperes, that whatever is using the electricity will draw. Since this is your starter battery, the biggest load would be the starter motor. Add to this anything else connected to that battery. You then need to adjust for the length of the run (the longer the wire, the larger the diameter must be to avoid voltage drop). Once you figure this (from tables) you can determine the size wire you need, irrespective of the battery capacity.
With an Atomic 4, you should be using a 4 gauge wire from the battery to the starter. 6 gauge is smaller than recommended, which could lead to overheating the wire (not safe). If your motor came from the factory with 6 gauge cable, that should be OK unless you have anything else connected to that battery which could draw power at the same time as the starter. Changing the battery "strength" does not in and of itself change the recommended wire size.

[rigspelt]

My 1991 Mercruiser Alpha 1, 4.3L

Check out the cranking amps required by the starter on a Merc 4.3 -- that will help to determine wire and battery size. I found this with google, for example: http://www.perfprotech.com/store/art...fications.aspx