Dear Tom and Ray:

I'm anxious to ask you guys a car question that has stumped the Chevy dealer. My sister has a 2003 Chevy Impala whose battery seems to have continuous corrosion, either on the tray that the battery sits on, or on the connector cables. The car will also periodically not start up because of this problem. The car has less than 15,000 miles on it, and the dealer said the original battery was split, so they replaced it. Well, now it's doing the same thing all over again, and the new battery is only a couple of months old. Everyone is stumped. Can you help? Thanks. -- Cathy

TOM: Your sister's car is overcharging, Cathy.

RAY: Which is what my brother and I do to our customers at the garage.

TOM: The alternator makes electricity to charge the battery. But the amount of voltage the alternator makes is directly related to the speed of the engine. The faster you go, the more juice the alternator can put out.

RAY: So, to keep it from sending too much voltage to the battery, the alternator has something called a voltage regulator. My guess is that her voltage regulator is no good, and it's allowing the battery to get overcharged -- at least some of the time.

TOM: When the battery gets pelted with too much voltage, it can outgas hydrogen, boil over its acid, corrode things around it and even split open its plastic casing. Sound familiar?

RAY: So tell your sister to ask the dealership to thoroughly inspect the charging system. And even if it tests OK, have them replace the alternator, because that's almost certainly the problem. Since this is covered by warranty, Cathy, the battery should be the only thing that gets overcharged in this transaction.