You can limp by this winter with one, and if it's only going to be there "to have a battery", you can probably get by with a really cheap one. If you out a second, bad battery in there, hook up the positive terminal to the bad battery, but not the negative terminal. This will keep the positive terminal from dangling, and grounding, and shorting out.
Then, next spring, you can get a pair of good group 24 or group 27 deep cycle batteries to put in the there.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.