It seems like a month or two between charges. Here is a link to what you need from Radio Shack. Get the 4th and 5th items from the bottom. Together they make your cord. 20 bucks….a good one time investment.
http://www.radioshack.com/search/index.jsp?kwCatId=&kw=12%20volt%20adapter&origkw=12%20volt%20adapter&sr=1
I do the same thing for my underwater camera that I use for ice fishing. I have the old mower battery in a 5 gallon bucket and then last year I attached the camera to the top of the bucket. The 100 foot camera cord attaches to the side of the buckets so I can wrap it up easily. It is not great but it works for me. I like having the camera up higher where I can see it better. With this method I also don’t have to worry about the battery running out when I am on the ice.