Which is killing which? The battery or the alternator?

I have a 2003 VW Jetta with a turbo engine. Last Thursday at a stoplight the radio stopped working and when I turned the corner all the warning lights came on. I was able to get to my house a few blocks away and turn the car off. It would not turn back on. I took the battery to a local auto parts store to be checked, and the clerk advised me to leave it overnight to charge. After reinstalling the battery I drove back to the store the next morning to have the alternator checked. The clerk said it was fine and I drove home happy. Not entirely trusting, I drove a friend’s car for the next two days (as I had some out of town traveling to do and was afraid to get stranded) and did not try to start the Jetta until Sunday morning, at which time it was deader than a doornail. A friend gave me the “dummies” version of how an alternator and battery work together, but how do I know which is my problem and how much is it gonna cost me to make it right? I have a long history of being taken advantage of by mechanics and want to go into this as well-educated as possible. Thanks for any advice!

How old is the battery? A bad battery could do this, or a bad alternator could also do this. Did the store test the battery after it was charged?

When you drove home was the battery warning light on? If yes, suspect the alternator. If not, you have a bad battery.

If your battery is more than five years old, replace it. That may be all you need. It certainly is due. Don’t give up on the alternator, not just yet.

I believe that battery was new in 2009. When I picked up the battery after it charged overnight, the clerk tested it and pronounced it healed. When I drove home from AutoZone no warning lights were on.

There is one more possibility. Both the battery and alternator are in good shape but some accessory in the car is draining the power even when everything is shut off.

You can determine if this is the case by fully charging the battery and then disconnecting it. If the next morning it is still fully charged, then you have a parasitic drain. If it is discharged then the battery is bad. (We still cannot say anything about the alternator.)

There are ways to find a parasitic drain. It requires an ammeter and it can be time-consuming. Let us know what you intend to do.

I think you have a diode in the alternator that is in the process of breaking down. You need to get it tested separately.

I agree with Steve. does anything stay on in the car that shouldn’t radio etc.? Some other simple culprits have been glove box, and trunk lights along with doors that keep trying to lock,but can’t.

I can’t recall any rogue accessories. The only thing out of the ordinary is that my switchblade keyfob hasn’t been working consistently (sometimes locks all doors, sometimes none; sometimes only unlocks front doors), but I figure that’s a battery problem with the remote. So Steve, I would take it to store and have it charged and then tested another day, without installing it in the car?

Yes, that is the possibility I alluded to. Many of us have our own battery chargers at home for just such occasions.

Or you can have it charged at the shop and tested in your car (along with the alternator) and drive straight home. Then disconnect it. If everything is OK, then your car should start the next morning when you reconnect the battery. In that case your electrical problems are caused by an electrical drain somewhere.

I’m not going to completely disagree with Steve, if it wasn’t for one little fact, I would be completely onboard with his diagnosis. And that is, you had trouble when the car was running. If the alternator was working at 100% (for the rpm she is operating at), she should not have had those problems. If there was a parasitic leak bad enough to drain down the battery while driving, there should be smoke involved.

Steve’s last suggestion is a very good one, but it only tests the battery. If it works as he described, there would be a parasitic drain, but that drain could be in the alternator and it could be due to a diode that has a low inverse resistance, the earliest stage of breakdown.

It is even possible that it could be both, a parasitic drain from the remote system coupled with a marginal alternator.

Where we are now. Last night I was going to take the battery out and take it to be recharged but I noticed the thing attached to one of the terminals was loose. I pushed it back down and instead of no action, when I tried to start the car the clock and indicator lights would flash. I went to the store and bought a nut that fits the screw and tightened the connection. I jumped the battery and drove the Jetta for a few miles, thinking all my problems were solved. A few hours later I started it just to make sure it wasn’t a fluke and it started fine. This morning…nothing but flashing lights and clicks. Does this help the diagnosis any?

If there was no problem while driving, then the parasitic drain is the most probable cause.

Is there anything electrical in the car that is not working, like a power window or power door locks etc. Often something that is not working will be the cause of the drain.

Otherwise check for the simple things. Go out at night and look all around the car for a light on. Possibly the trunk light or glovebox light isn’t going out.

“The only thing out of the ordinary is that my switchblade keyfob hasn’t been working consistently (sometimes locks all doors, sometimes none; sometimes only unlocks front doors), but I figure that’s a battery problem with the remote. Try a new battery for the remte,”

if nothing changes after a new remote battery see if there is a fuse for the doorlocks you can pull without adversely affecting anything else to see if that is the problem.

Are the battery cables good, no corrosion? Are they tight on the battery? Is the ground good and clean to the engine? How about the connectors on the alternator, tight and clean?

It sounds like you found a loose connection alright but it does appear there is a parasitic drain on the battery as others have already stated and is most likely due to a problem in the alternator. The problem may have occured when the warning lights flashed at you. I suggest you take the vehicle in to a shop and have them test the system for an excessive current drain while the car is parked. Normal current draw on the battery should be less than 30 milliamps while the car is parked and systems have gone into the sleep mode. A good shop will know what to do when making the test. You could tell them you suspect the alternator is causing the trouble if they find there is a drain. I think a new alternator will fix you up. If the battery is more than three years old have the shop do a load test on it to check the condition of it. When changing the alternator it can be wise to replace the battery also since they tie together.

Took it to a shop and told my tale, asking them to check the electric, alternator and battery. Called an hour later and said they replaced the battery. Here’s hoping that’s the fix!

If they didn’t check the current draw while the car is parked to see if there is an excessive current draw on the battery you may be back in the same boat again and find a dead battery in the morning. I suggest you make sure they checked that out before you take it back.