Bad idea. Car batteries are designed to produce a lot of power for a
few seconds and then be immediately recharged. They can survive being
deeply discharged maybe 5 or 6 times before they die.

Assuming you have a 25 watt anchor light burning 8 hours. That is 16
AH. Then you have 3 10 watt cabin lights burning 4 hours. That is 10
AH. Over a three night weekend that is about 75 AH. That will kill a
group 27 auto battery in short order. You need 150 AH of battery minumum.

The cheapest and longest lasting route would be a pair of $50 golf cart
batteries from Wallymart, Sam's or Costco and a $60 charger with auto
shutoff from Harbor Freight. You will eat that much in car batteries in
one season.

Jens K wrote:

As I normally daysail I do not need a battery in my boat. But when I
occasionally cruise for a few days, I would like one, but only for the
lights. I do not have other power consuming devices in my boat.

So now the question is this: could I simply move my car's battery to
the boat? Let's say for a four days cruise during summer time. Will it
still start the car afterwards? I guess there will be the pleasant
side-effect that the car will be less likely to be stolen.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com