Are you saying the car goes unused for months? If so, you do not have a problem. Due to the many computers with memory circuits, there is a constant drain on the battery. Unless specified otherwise by the manufacturer, 35 milliamps, (.035 amps), is the maximum allowable current draw with the ignition switch off. At that rate Chrysler guarantees a good battery will start the engine after sitting for three weeks. Cadillac is one of the notable exceptions. They allow up to 50 milliamps on most of their models so the battery can be expected to stay charged less than three weeks. You don't need a new battery every time this happens. In fact, disconnecting it or letting it run dead is a real bad idea in the last few years. GM, Volkswagen, and BMW are by far the worst manufacturers at designing in tricks that require expensive trips to the dealer to have numerous computers unlocked. If that hasn't happened to you, you're fortunate.
Instead of replacing the battery, use a small trickle charger to keep the battery charged. Some use little solar panels and I've heard they work fine. I have a Dodge Dynasty and Grand Caravan that both sit for months without being driven. One has a smaller battery cable to disconnect to prevent the battery from running down. That is what they use between the time the car is manufactured and the time a hauling company drives it onto their truck for delivery to a dealer. Newer vehicles typically have a fuse to remove. Those vehicles can be driven, and they are delivered, with that fuse removed. Only minimal safety systems like brake lights and turn signals will work. On some cars that fuse may be called the "ignition off-draw", (IOD) fuse. On some cars it is the two air bag fuses and they are in yellow handles that just get pushed in.
Wednesday, January 16th, 2013 AT 8:30 PM