CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Car Battery Dying

My battery has been dying intermittently. Twice after a week long vacation I have come home to a dead battery. No lights were on in the car and no clear source of a leak. It charged up but would die again after 12 to 24 hours of sitting in the driveway. After the 1st episode mechanics said the alternator and all voltage testing was normal so they just replaced the battery. After the 2nd incident I took my car to a new mechanic and they are saying again the alternator is fine but noticed the battery is shorting (alternator output amperage too high and voltage too low - they put in the shop battery and all returned to normal) and they say I need ANOTHER new battery. I have a hard time believing this but no one can figure out why my car battery keeps dying???

How old is that battery? They don’t last forever. Also if a battery is allowed to drain too low even a new battery can be damaged.

The battery can be load tested 

Based on what you have told us, I would say you need a new battery. 

Sorry I misread your message.  Please ignore

You have to assume that you got good batteries installed both instances.
If new batteries are truly dying to the point of needing replacement that quickly, I wonder whether your alternator is killing them. It is not that common of a thing to happen, but I’ve seen it.

Sounds like there is a substantial piece of info missing, ie. mechanic(s) may have misdirected you or I don’t see something that i should. In your shoes, I would connect an ammeter to the battery (preferably) inductive with the key OUT of the ignition and see if there really is current draw. If all the facts line up, there should be parasitic draw. If so, start pulling fuses from the unswitched systems like headlights, one at a time, until draw is found. This could be a very hazardous situation if you have a chafed wire or other partial short.

I have 2 cars that need to be run at least once a month or they likely won’t start due to a battery being rundown. This isn’t uncommon on modern cars. So, after your vacation the car started but if you didn’t use it much the battery didn’t recharge much and was still rundown.

Modern batteries don’t take a charge as fast as old one’s used to. The old batteries (where you added water) took a lot of charge quickly. Newer (no maintenance) batteries take longer to accept a full charge, which means more driving than a quick trip to the market.

Still, in this case your new battery could simply be defective. It happens. Or, your charging system may not be up to snuff.

I resolved my battery problems with a jumpbox for times when I waited too long to drive the car. Then I have a battery tender charger, and a standard charger if needed. A jumper box comes in real handy and makes a low battery problem a minor inconveniece instead of a major event.

i would check and make sure that the vanity light in the visor or the glove box light is not staying on have had it happen to me look at night while its dark

Seconding what jsims said, and adding trunk light and under-hood light (if any, of course). Everybody else here is right, too, but sometimes it’s just a lightswitch that doesn’t turn off. Easier and cheaper to fix even than replacing the battery. But note that if a simple problem like that has been draining the battery, then you might have to replace the battery, too.

A good mechanic could quickly determine if something in the car’s electrical system was draining the battery. They’d put an amp-meter in the battery circuit when the ignition was turned off and all the doors were closed (so the dome lamps weren’t working, etc.)

If that test showed everything normal, then it would have to be either the alternator or the battery that was bad. Since they say it isn’t the alternator, it has to be bad luck, you’ve got a series of bad batteries.

A bad battery, after you charge it, it will self-discharge even if it is out of the car, sitting on your driveway.

I guess if I had question whether a new battery was actually good or not, I’d charge it up, remove it from the car and take it over to Sears auto (or most any big-box auto repair place), and have them do a load test on it.

If the alternator checks out, I have had several mysterious battery drains. On the one car the battery tested good but the battery would be intermittantly dead, I happened into the garage one night and noticed the interior lights on. Traced the problem to the door handle thinking it was being pulled on and turning the interior lights on and draining the battery. Cut the wire and all was fine. On another car, I traced it to the electonic control module for the level control. It sat under the car and would short out and discharge the battery. Also had a trunk not completely close at the airport and discharged the battery due to the light on. It sometimes take some investigative work to find some of these since they are not necessarily on all the time while being tested.