Replacing 8D battery with 2 batteries

rogerbum

All,

My Tomcat came with 2 lead-acid 8D batteries. One for house, one for start. The house battery appears to be dead. I'm tempted to replace it with two group 31 (comparable amp hours) or group 27 (somewhat lower amp hours). I'd wire these in parallel with short lengths of wire with a carrying capacity that is greater than 2x the max amps available from each battery. The main motivation for replacing with 2 smaller batteries is to get the weight down to something I can manage on my own. The 8D's weigh around 150 pounds where as the group 31's are about 1/2 that weight. Anyone see any particular issue with going this route?

journey on · Posted: Mon May 21, 2012 2:25 pm Post subject:

8D's are big batteries. Personally I don't think that you need that size battery (and 2 of them?) on a 26' boat.

I'd get 2 ea 6 V golf cart batteries to replace that 8D and wire them up in series. That'd give you a more than adequate amount of power and you'd avoid the trouble that comes with wiring batteries in parallel. Also, it'd be lighter than that 8D (given) and lighter than 2 31's.

If you wire 2 lead acid batteries in parallel, you run the risk of self discharge through the weaker one. At the very least start out with new batteries, and if the problem occurs put a switch between them.

Boris

thataway · Posted: Mon May 21, 2012 8:46 pm Post subject:

I agree that 8 D's are not the best choice for a Tom Cat. But I have put group 31's in parallel a number of times--(and thousands of ocean crossing miles where the entire function of the boat hinged on thier working--no issues).

Go for the two group 31's in each bank. As long as they are new and well maintained there sould be no issue. If there is, t hen you cna separate them. To be ultra safe you could isolate and use VSR between the two batteries, and use one at a time.... but I don' think that is necessary.

Robert H. Wilkinson · Posted: Tue May 22, 2012 9:44 pm Post subject: replacing 8D battery

I would agree with Boris - If you connect 12 volt batteries in parallel and 1 developes a problem it will cannibalize the good battery and they will self discharge. This problem is very rare with 6 volt batteries connected in series. It is also a better system for charging. If you do put a switch between two 12 volts connected in parallel and use them 1 at a time - then when you go to charge them you end up with 2 batteries at different levels of charge.
Just make sure any 2 batteries you connect together are the same capacity and the same age.
Regards, Rob