how far can i drive on my dying battery?
August 29, 2007 6:08 PM   Subscribe

After jump-starting a dead car battery, how far can I expect to be able to drive it?

My car (a 2003 Jetta, the worst car in the history of all cars) stopped working yesterday. The battery is dead, but I was able to jump it and start it. Unable to secure a ride from the repair shop to my home, I had to leave it in the parking lot of my job, which is about 15 miles south of where I need it to be. While 15 miles is really not that far, these 15 miles happen to be through the highly congested, stop-and-go, mean surface streets of Detroit and its suburbs, and I fully expect the drive to take 45 minutes. And it is so hot here. Do you think my car will make it without shutting down? Am I better off to just have it towed?
posted by foxinthesnow to Travel & Transportation (24 answers total) 1 user marked this as a favorite
Once the engine is running, it doesn't need the battery if your alternator is working. In fact, it will charge the battery. The greatest risk will be if you stall the engine on your trip, as you won't have much charge in the battery and you probably won't be able to start the car again.
posted by cardboard at 6:13 PM on August 29, 2007

I've had cars like this- my impression is that once the car is running, you don't need to sweat the battery as long as the alternator works. I'd avoid using power-draining things like radio, A/C, defrosters, etc. if you're worried about your charge on a brief ride.

Let the car idle for 10-20 minutes before you get driving in order to let the battery juice up a little.
posted by clango at 6:15 PM on August 29, 2007

posted by clango at 6:15 PM on August 29, 2007

"... Do you think my car will make it without shutting down? Am I better off to just have it towed?"

Driving a car with a shorted battery for any length of time, in some charging systems, can damage the alternator beyond function/repair. If getting where you're going is paramount, find out what is wrong with your mule, before setting out on the journey.
posted by paulsc at 6:21 PM on August 29, 2007

If the battery is dead because the alternator isn't working, though, you may not get very far (unless the car is diesel). And from the little information here, it sounds to me like there's a fair chance that the alternator is the problem.
posted by dilettante at 6:22 PM on August 29, 2007

you should make it home. get it diagnosed, though. you might need a new battery.
posted by thinkingwoman at 6:26 PM on August 29, 2007

Is there some way for a simpleton like me to determine if the problem is the alternator vs. the battery?
posted by foxinthesnow at 6:31 PM on August 29, 2007

Does the battery light go on (especially when you rev)?

If so, it could mean the alternator isn't charging the battery. In that case, I'd call a tow.
posted by evil holiday magic at 6:33 PM on August 29, 2007

You can take the car to a PepBoys/AutoZone/AdvanceAuto and they'll tell you if it's the battery or the alternator.

My guess, if it's hot, you need to put some distilled water in the battery and you'll be fine.
posted by TomMelee at 6:41 PM on August 29, 2007

"Is there some way for a simpleton like me to determine if the problem is the alternator vs. the battery?
posted by foxinthesnow at 9:31 PM on August 29 [+] [!]

You can't determine the root of problems in a vehicle charging system, using just the charging system indicators. You can physically examine some batteries, for low electolyte levels, or shorted cells, if you know what to look for, but based on your question, you don't.

You're best bet, given the danger of the territory that you describe, that you need to cross, is to call for a tow, from a safe environs, and pay the mechanics what they want to fix your ride.
posted by paulsc at 6:42 PM on August 29, 2007

If the battery is at least three years old, replace it.
posted by flabdablet at 7:09 PM on August 29, 2007

2003 Jetta with an original, stock battery? It may just be time for a replacement. At least, the was the problem with my '02 GTI (same car, with a slightly better engine and as a hatchback). Mine consistently died if I left the car in accessory mode with the AC on for more than 5 minutes. Took it to a battery place, and sure enough, it barely had enough juice left to get the car started. Any extra drainage without the car running, and it just wasn't able to start. I never even considered the fact it might have been the alternator - damn, that would've been a really expensive fix (as is everything with these silly german cars - i still love mine, tho!)
posted by cgg at 7:14 PM on August 29, 2007

If you're able to jump start the car, and you have the ability to hit a department store of your choice - get one of these. I bet you can even beat this price by ten or twelve bucks. If your car stalls and you cannot (or in Detroit, not want to ask someone for a jump), you'll be able to get the battery started pretty easily with the device.

I had one of these when I had a Subaru sedan that would for some reason lose connection with the battery cables and the battery would die.

FWIW, this one is really cool!
posted by jimmyhutch at 7:18 PM on August 29, 2007

Yeah, I know I need a new battery. That's why I need to get it to the shop! But, no, the battery light was not on after I got it started, so we can assume that once it is started, the alternator will charge the battery and I should be able to drive with it for 45 minutes or so?
posted by foxinthesnow at 7:20 PM on August 29, 2007

If you left the lights on, or the dome light, which drained the battery while parked, then the alternator is fine and you just needed a jump to get back on your feet. No harm done. (Well actually, the battery's life was shortened by the deep discharge, but it should still work normally afterward, you just won't see as many years of service.)

But if it died of its own accord, then it's not being charged properly, and either the wiring or the alternator is at fault. Or the battery itself just plain fell apart internally. (I suspect the battery's okay, because a jump revived it, so it must be storing charge properly.) In this case, it's not reliable to drive on -- starting takes such a huge amount of energy out of the battery that even if it were fully charged to begin with, after starting the engine, it might only be able to run the fuel pump and other necessities for a few minutes before giving out. Modern cars with daytime running lights are especially bad in this regard.

After starting the car, as EHM said, look for the electrical malfunction light on the dash. If it's off, then everything's probably fine. But if it's on, then reconnect the jumper cables and let the donor vehicle charge your battery for 10 or 20 minutes, to replace some of what the start took out. Then, have someone follow you to the repair shop. If it dies again on the way, repeat the jump procedure, but again, let it continue to suckle from the donor for a while after starting, before you continue on your way.

Alternately, you could leave one of those "jump starter" packs on the passenger seat floor, with the lighter-socket cable plugged in. It'll backfeed the electrical system through that socket, hopefully providing enough current to keep the critical stuff running.

Obviously, don't use the lights, radio, wipers, or anything else you can avoid. Air conditioning isn't actually a big power drain, since it just runs a solenoid that engages the compressor. You can use the air if you want, but it does take some electrical energy.

Or, you could just call a friend who has tools, time, and a clue about cars. Changing an alternator in a Detroit gas station parking lot is a rite of passage for any shade-tree mechanic. It should take about 25 minutes to remove, 30 to get the new part from Murray's, and 20 to install the new one. After the 5 to determine the actual cause, of course.
posted by Myself at 7:28 PM on August 29, 2007

It's almost certainly the battery or the alternator which is the problem. In all my years of driving, I've never had an alternator go, but replacing batteries is as common as replacing tires. Rather than pay for a diagnostic, I'd just put in a new battery. Fully charged, it would get you 15 miles. That would cost you what - $75 or so? (and you've got a new item for what would almost certainly need replacing in a matter of months anyway.)
posted by Neiltupper at 9:04 PM on August 29, 2007

Lo-tech way I've used to tell if it's the alternator: Start the car. While it's running, disconnect a battery terminal. If the car stops running, it's your alternator. I assume this still works, anyway. I've never used it on a car built in this century.
posted by middleclasstool at 10:27 PM on August 29, 2007

middleclasstool, that's a good test with a DC generator and nonelectronic ignition, but since alternators put out AC and then rectify it, they rely on the battery's capacitance to smooth out the rectified waveform. The computer is pretty tolerant of bad electrical noise, but if the trough of that rectified ripple drops below its threshold, it'll crash.

The trouble with the portable jumpstarter is that if you buy one now, and your car battery is toast, and you replace the car battery, then the jumpstarter is getting old right around the same time your car battery gets old again. I bet one of your friends already owns one, anyway. (I seem to accumulate them, and own four, in various states of decrepitude.)
posted by Myself at 12:58 AM on August 30, 2007

If you have a voltmeter/multimeter available, check the voltage (while the car is running) from the battery's + terminal to an exposed metal surface under the hood. If the alternator is working correctly, it should be somewhat higher than 12v. If the alternator is not working or not connected, the voltage should be 12v or less.
posted by ArgentCorvid at 4:50 AM on August 30, 2007

The battery in my '99 Jetta literally blew up one night. Smoke and everything. I was short on money so I got a starter kit like what is above and used that for about a week to start the car in the morning and after work. I got to thinking of it as though I had an automatic kill switch because without the starter the car couldn't start and I was taking it inside everyday to charge.

New battery in place I didn't have any more problems aside from blowing lights constantly, but with these cars whose to say what is wrong with them. You already know it's a POS. I got rid of mine shortly there after.
posted by jwells at 5:21 AM on August 30, 2007

If you know you need a new battery, and can't install it yourself, walmart (if they have an automotive dept where you can buy tires, etc.) will usually do this. Call first to confirm that they will install the battery, the automotive dept is open, and that they have the right size of battery for your car. Some auto parts stores (Pep Boys???) might do something similar, and are open later than most auto shops.
posted by yohko at 9:01 AM on August 30, 2007

Was it very hot recently where you are? My 2003 Mazda's battery blew up mysteriously during the last heat wave. The only evidence was a very slight hairline crack in the bottom of the battery and a bit of acid on the battery tray. These things happen.
posted by Geckwoistmeinauto at 12:10 PM on August 30, 2007

Listen to Myself. Ignore Paulsc.
posted by klangklangston at 1:49 PM on August 30, 2007

Jwells, it sounds like your voltage regulator had failed and the now-unregulated alternator was putting out much too high a voltage, which boiled/detonated the battery and burned out lots of bulbs. If your mechanic didn't figure this out in 4 seconds, you need a new mechanic.

Foxinthesnow, do we get an after-action report? How'd it go? What was the root cause? What was the financial damage?
posted by Myself at 9:53 AM on August 31, 2007

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