A light bulb staying on, like a glove box light for example, would kill the battery in a day or two so we can rule that out. You don't have an electronically-tuned radio with a memory circuit, so that isn't it. Those are found on newer vehicles that also have numerous computers that draw a constant standby current. Unless the manufacturer specifies otherwise, 35 milliamps, (.035 amps), is all that is allowed, and at that rate a good, a fully-charged battery will still have enough charge to crank the engine fast enough to start after sitting for three weeks.
For any vehicle the age of yours to lose a charge over a few months, the most common suspect is self-discharge over the top of the case. You can find this with an inexpensive digital voltmeter. Put one probe on one of the battery's posts, then drag the other one over the top of the case. It's not unusual to find one or two volts, but if you find 12 volts, clean the top with a mixture of water and baking soda, or, at a minimum, wipe the surface clean.
When acid gets sprayed from the vents and condenses on the top of the case, you will usually find that white corrosion around the posts too. Both of those are signs the battery can be expected to fail in about the next six months. Cleaning the case and posts is not a solution to a failing battery. They are simply indicators of the inevitable.
You can verify this by disconnecting the negative battery cable. If the battery is still run down after sitting for a few months, it is due to self-discharge. If it only runs down when the cables are connected, we'll have to discuss performing a battery drain test. That can also be done with the inexpensive digital volt - ohm - milliamp meter.
Monday, February 23rd, 2015 AT 5:33 PM