February 25, 2004

1:01 PM

Automotive question:
The battery doesn't charge and we subsequently have to manually recharge it at the garage, hopefully before it conks out on us on the road. Driving at night poses problems, since the lights obviously draw off of the battery - it's a heavy drain, trust me. Here's the question: we know it's not the battery (two brand new batteries, same problem), the alternator is showing good numbers (apparently meaning that it's charging, I'm told), all belts are running properly and are tight, the mechanics found no loose connections; what could be the problem?
posted by ashbury to (14 comments total)

Either a short that is draining power despite a strong alternator or you have a bad voltage regulator. Do your headlights surge at night?
posted by machaus at 1:40 PM on February 25, 2004

Alternator.
posted by mrbill at 1:45 PM on February 25, 2004

nevermind, I'm an idiot.
posted by mrbill at 1:46 PM on February 25, 2004

To start to test the theory of Machaus (a short somewhere), pull every fuse that you can that doesn't impair your ability to drive safely, drive the car, and see if there's any improvement.

On the other hand, you'd think a fuse would have blown by now if there was a short.

And who claims that the "alternator is showing good numbers"?
posted by trharlan at 2:05 PM on February 25, 2004

The most likely culprit would be your alternator and I would get a second opinion - "showing good numbers" does not sound like a professional diagnosis to me. If the battery is running flat while you are driving, it almost has to be the alternator or something associated with it (most likely the regulator) and a proper test from a proper mechanic or auto electrician will pick this up in seconds.

An alternative, if the battery is going flat overnight, for example, is that there is something draining the battery even when the ignition is off. The most common culprit here is the light in the boot (if your car has one), as you will not see it on - a bit like trying to work out if the light in the fridge really goes off when you close the door. It is easy to check this - get in the boot and have someone close the lid to see if the light goes off.
posted by dg at 2:19 PM on February 25, 2004

machaus, the lights definitely flicker, if that's what you mean by "surge".

trharlan and dg, the mechanix at the shop told me that the alternator was fine and said something along the line of "it's showing good numbers". (What I hear and what's actually being said isn't always the same thing, so maybe this isn't what the guy said. Either way, they discounted the alternator as being the problem.)
posted by ashbury at 2:24 PM on February 25, 2004

I'm not trusting this 'good numbers' thing. A simple test is to disconnect the positive cable from the battery while the car is running, if the alternator is good the car will continue humming along without the battery.

If the car continues to run, as others have said, an unknown drain on the battery is a likely culprit.
posted by cedar at 2:41 PM on February 25, 2004

I would say alternator as well, if the car was over 15 years old. However, in newer cars, it's a bit complicated at times. I had a Nissan Pathfinder once that did the same thing. I replaced the battery, the alternator, and even the battery cables and voltage regulator out of sheer desperation. Then, my father and I darn near re-wired the entire car one weekend. Drove it home, and it died on my halfway there.

I charged the battery, drove it to a car lot, and traded it in. At that point, it seemed the cheapest option.

Sometimes at night, I wake up wondering if a pregnant women ended up buying the car from the lot. I'm so ashamed.
posted by bradth27 at 2:47 PM on February 25, 2004

yes, flickering lights is a classic voltage regulator symptom.
posted by machaus at 3:33 PM on February 25, 2004

A simple test is to disconnect the positive cable from the battery while the car is running, if the alternator is good the car will continue humming along without the battery.

Whoa!

I think you may want to take some safety precautions before doing this.
posted by trharlan at 4:23 PM on February 25, 2004

Other battery drains can be: improperly installed Lo-jack or anti-theft systems, badly installed radios, basically anything that was set up to take power from your battery and set up wrong.

Have you always had the car and it just started doing this? Have you installed anything else recently, before the troubles started?

The fuse approach is a good one. Also, alternators are not *that* expenseive. Replacing it just to see might not be the worst thing you did, and the peace of mind might be worth the added cost. I have also had, in the past, weirdly loose ground wires [from the battery to the body of the car someplace I think] that caused the car to flat out not start in dead-battery-seeming ways that might also be worth examining.
posted by jessamyn at 5:25 PM on February 25, 2004

"I think you may want to take some safety precautions before doing this."

Nah, it's pretty safe providing you don't ground out the cable or stick it in your mouth -- it's just for a moment, it's not like your going to be driving around like that.
posted by cedar at 6:31 PM on February 25, 2004

Thanks folks, you've given me some ideas to work with, and pass along to the mechanics, not that I should have to give them ideas. They make such a killing . . .
posted by ashbury at 7:11 PM on February 25, 2004

I second the opinion that suggested a second opinion.

They make such a killing . . .
Check your next repair bill, and add up the cost of the parts vs. the cost of the mechanic's time...someone's making a killing, but it's not the greasemonkeys.

Puddy: "We don't care for that term."
posted by stupidsexyFlanders at 7:07 AM on February 26, 2004