A small solar panel goes a long way to maintaining the battery's charge over the winter. Even with a bilge pump connected.
There are flexible panels that you can put just about anywhere. Or get a rigid panel so you can permanently install it eventually. Then it'll serve you in the summer too. Hey, the winter is a great time to install it, for perhaps the same amount of effort as checking (and worrying about) your batteries all winter.
Note that with a small solar panel you'll only need a diode to connect it to your battery bank; you won't need a charge controller.
Last edited by Bene505; 12-01-2008 at 10:14 AM.