that's strange..is the battery new? or is it still the original?

basically with batteries in general, you get what you pay for, so splashing out and buying the more expensive battery now will save you in the long run after several of the cheap ones die....deep cycle batteries are usually for marine and 4x4 use, but can be put in a passenger vehicle if need be, and the car requires one..although they aren't cheap either.

Is something electrical running and draining the battery? or is it just old?

Hope that helped a bit, although I'm not one to advise the best type of battery specifically what to buy, maybe consult an auto store? Repco?

Cheers

Brett