Car Talk: A perpetually dead battery is the start of trouble

Q: I bought a 2002 Toyota Avalon with around 120,000 miles on it for my soon-to-be-16-year-old daughter to drive. If I let it sit for a week or more without driving it, the battery will be so low that the starter just clicks and I lose all the settings such as the date, radio presets, lock settings, compass, etc. If I jump it or put the charger on it for a few hours, it starts right up.

I got a new battery, but that didn't help. I even took the battery back to the auto parts store. It was discharged but tested fine after they recharged it. I also checked the trunk light and others to make sure they weren't on all the time.

I talked to the previous owners (family members) who said they got rid of the car because their elderly mom wasn't driving it enough anymore and got tired of it always having a dead battery when she did need to drive it. What might be causing this? I've had other cars that sat for a month and started fine.

A: I unloaded a car like that on my in-laws once. They never spoke to me again, so it was a win-win.

Most modern cars actually can't sit for a month without their batteries failing. There are too many electronic components that use power, even when the car is turned off. There are presets to maintain, an alarm system and, today, even internet connections.

In addition to these "normal" drains, it's possible you have a parasitic drain of some kind. Ask your mechanic to hook up an ammeter to see how much current your car is using when it's turned off. If it's more than 0.8 amps, that'll drain the battery pretty quickly.

If that's not the case, your problem probably is a weak charging system. Every time you run the car, the alternator charges the battery so it'll be ready to start the car next time you need it. If your alternator is weak and you don't drive much, you might never be fully charging the battery. And when you combine that with the "normal" drains, you could be out of juice in a week or so.

Shedding light on a problem

Q: I have a 1999 Honda CR-V with 188,000 miles. Sometimes the parking brake signal on the dashboard illuminates as I am driving, but the parking brake itself is definitely not on. This used to only happen when it was below 30 degrees, but now it's happening in warmer weather, too. Is this an indication of a more serious problem or just a minor glitch on a well-worn car?

A: There are only a couple of things that would make the parking brake light come on if the brake is disengaged. And neither is serious.

Your parking brake is operated by a foot pedal to the left of the brake. When you step on it to engage the parking brake, there's a little switch at the top of the lever that activates the dashboard light. Over time, that switch can get out of adjustment. And cold weather can make it act up. Or, after enough use, it can simply wear out.

The other possibility is that, even though you've fully released the parking brake pedal, the cables that apply the parking brake could be sticking. If they're not releasing completely, it won't be enough for your foot or your eye to notice, but it's enough to make the light stay on.