Dear Car Talk:
We have a 2004 GMC pickup, which we rarely use. We disagree as to the best way to keep the battery charged: Just start the truck occasionally, and let it sit in the driveway and run for a little while every week, or actually take it out on the road, and drive for 20 miles or so? Or put it on the charger every week or so? Also, if the battery goes dead, will that mess up the computer for emissions testing, just as if we had to replace the battery?
The answer to your last question is yes. If the battery goes completely dead, the computer will lose its stored emissions data. Then you'll have to drive it for a few days to create new data before you can pass an emissions test. Not to mention that you'll have to restore all of your radio presets.
In terms of maintaining a car you don't drive much, the worst thing you can do is start it up and just let it run in the driveway for a few minutes. That's bad for two reasons. First, you're not really recharging the battery. So you're using the battery to start the truck, and then leaving it weaker than before you ran the car.
Second, when the engine runs, the combustion of gasoline and air creates water vapor. But if you don't run the car long enough to really heat things up, you allow the water vapor to sit there and cause rust.
So I'd make two suggestions: One, start it up and drive it for a day every few months. Not only will you charge the battery and evaporate any water vapor you create, but you'll also exercise some moving parts of the truck that should move once in a while. Things like brake calipers and transmission solenoids can stick or rust if they're left for a long time. Belts can dry up and crack.
My second suggestion is to get a trickle charger. For $30, you can get a battery charger that "trickles" a small amount of electricity to your battery as it's needed. The advantage of a trickle charger over a conventional charger is that a trickle charger never lets your battery go dead, so the truck can be started whenever you do want to use it. And a trickle charger doesn't need to be monitored. You just hook it up, plug it in, and your battery is always ready to go.
And then, with all that extra time you have from not starting your truck every couple of weeks, you can ponder the question of why you're keeping it in the first place.