A bit OT, but I've found 12V batteries can last a long time if not abused. In my previous house, I had a snowplow mounted on a FJ40 landcruiser. The only time I ever ran that car was when I was plowing snow or moving it to a new parking place (about once per summer). The plow motor is basically a starter motor connected to a hydraulic pump that runs a few rams for up/down and left/right. In other words, when plowing it is like you are running the starter motor over and over and over again, often for several seconds at a time. The alternator doesn't supply nearly enough current to run the plow motor so the battery has to do the job. Long story short, I always plugged the battery into a trickle charger after plowing and I usually got about 7-8 years out of one before it would die. Next to living in Phoenix or something that has to be about the hardest life a battery could have but with proper car they lasted a long time.