Originally Posted by bike2math
As others have said disconnect the negative terminal of the battery if you are concerned.
That won't solve the problem. The battery will self-discharge over time, and a lead-acid battery that has self-discharged forms sulfur ions on the plates, which leads to reduced capacity, and eventually it'll destroy the battery.
Lead-acid batteries really need to be kept charged. A few days idle at a time won't hurt anything, but leaving them to discharge for weeks at a time on a regular basis with only short charging spurts is going to destroy the battery. You want something that will continually charge them at a low rate. As I recommended above, get a cheap solar charger, they're < $20 and your problems are all solved.