12 volt vs 6 volt

[Doublegrat]

I've got room for 2 batteries on my sailboat. I'd like to know the difference in 2 12 volts hooked together to double the amp hours or 2 golf cart (6Volt) hooked together to double the volts? Will the golf cart batteries last longer? Which is the better way to go?

TIA,

John

[Nausicaa]

Putting in a pair of big sixes in series to give 12v will give you more available amps, but a pair of 12s will allow you to separate engine from house, unless you are only asking about a house bank. Do an energy audit (add up all your current draw in amp-hours) for a typical 24 hour period. Compare that to the amp-hour availability from the various set-ups and see which will suit you better. Remember, you don't want to routinely use more than 75% of your battery's rating, or you will greatly shorten its life (some would say 85%!).

[RobinScottJohnson]

Don't you have to wire them together in a certain way to double the voltage? I think if you just connected the 6 volt positive leads to your switchboard, and then grounded them both it would still be six volts.. I know you have to do something else, for instance, to turn two 12s into a 24 volt system. Personally, I use two 12 volt deep cycles. One I keep charging for three days, until the other one that isn't on the charger is used to keep things running on the sailboat. When the working battery gets down to around 12volts I switch the leads over to the one that was charging, and then put the charging leads on the one that was run down. Usually the 12V batteries are up around 18 volts when I switch them to power things... I've never had a problem doing this, and one of the routine things I do in the morning when cruising is check the voltage of both batteries.. Right before I check the weather and start my days log.

Take care..
Robin

[Nausicaa]

If you let your battery get down to 12volts, you are seriously shortening its life. The number of charge/discharge cycles it will endure will be be severely reduced. I suggest that you do some serious reading on the topic (there is a wealth of info available on the web). To make 12v out of two sixes is easy: you just connect one battery's neg cable to the ground bus, its positive to the other battery's negative, and the then the second one's positive to the positive bus. A six volt battery has three cells, a 12v has six cells. Joining the positive of one to the neg of the other is electrically the same as having a 12v battery with the same connection made internally.

[RobinScottJohnson]

Wait, you know I am referring to the Deep Cycle batteries I have. I knew I couldn't be wrong completely about how I wire them as they last a couple of years, but because you planted that seed of doubt in my mind I searched deep cycle. According to the first reference they're designed to be run down and charged over and over again, have thicker plates, etc.

I don't have in inboard engine in my sailboat, so only have the deep cycles, not the starting class of batteries.

Robin
http://i171.photobucket.com/albums/u304/Robinsvoyage/batterymount8secured-1.jpg

[RobinScottJohnson]

thanks, Naus, for explaining how to do that, my father had told me years ago but I'd forgotten.

Robin