Second opinion, please (Car filter)
February 9, 2021 3:18 PM   Subscribe

Previously. My friend says it's ok to drive a car on a battery when the alternator is bad, because "you can charge a battery up again." I said, "I read driving a car on a bad alternator drains the battery."

"Oh, so you're saying you're afraid to drive it?" I was asked.

Everything I have read in the last 24 hours says (in effect) "the alternator drains the battery and can only be driven 5-20 miles" Personally, I am unwilling to take the chance of breaking down in 5 degree weather. Since I pay $200/year for AAA and have never used it, why not? What's wrong with being cautious in freezing weather?

What say ye, metafites?
posted by intrepid_simpleton to Travel & Transportation (34 answers total)
Get a tow.
posted by doomsey at 3:22 PM on February 9, 2021 [7 favorites]

AAA is slightly different in every region, so I suppose one reason not to get the tow is if you are worried of maxing out your number of tows or number of towed miles. Or if you want to go to a shop that's outside their free tow (mine is like 5 miles I think?)

Personally in your shoes with your concerns I would get the tow, if it is free.
posted by muddgirl at 3:37 PM on February 9, 2021 [2 favorites]

I have done this before. It's OK, but it's not reliable - the battery will run down eventually, and the engine will stop.

I used to keep a spare battery in the car, and rotate them with another car for charging.

Caveat: This was in a 70s-made car - so no fancy electronics etc which may be unhappy if you completely drain the battery. I was too broke for the repair, and too broke for towing at the time.

So: you're both right. I wouldn't do it in bad weather, or when you needed to get somewhere on time.
posted by pompomtom at 3:37 PM on February 9, 2021

Seconding that the safe answer is to get towed.

A closer inspection of your battery should show some various labels/numbers on it: CCA, CA, AH and RC

According to multiple sources via Google, RC on a car battery is the "number of minutes a fully charged battery at 80°F/27°C will discharge 25 amps until the battery drops below 10.5 volts."

So whatever your RC number is, assume it's going to be less in cold weather. How much less.... that's beyond the limit of my Google and electrical engineering skills, (also I didn't know if you were saying 5C or 5F, which is a pretty big difference).
posted by tiamat at 3:39 PM on February 9, 2021

Response by poster: tiamat: I had to look this up. I didn't know either, but a quick check of shows we speak Fahrenheit here.
posted by intrepid_simpleton at 3:46 PM on February 9, 2021

As someone who has had a engine die in the middle of a highway interchange because the alternator died and the battery stopped recharging, I would strongly recommend against driving on a bad alternator. If the light is on, pay attention to it.

A mile downhill to your local service station to get the alternator fixed? I might be willing to do that. Definitely not too much further than that, and my opinion of it in below-freezing temperatures is even lower than doing it in the middle of the summer.
posted by thecaddy at 3:51 PM on February 9, 2021 [13 favorites]

Many years ago, my car died in the middle of a traffic jam because the alternator wasn't charging as well as it should. It wasn't even bad enough that any warning lights came on before the engine stopped. It was in cold, snowy weather, so the extra load from the defroster and windshield wipers certainly ran the battery down faster than would otherwise have been the case. I would not knowingly drive a car with a bad alternator any farther from home than I felt like walking when it died, which it certainly will eventually.
posted by FishBike at 3:54 PM on February 9, 2021 [1 favorite]

IMHO, this is an easy calculation. Electronics in today's vehicle are complex. Batteries aren't cheap, either. The risk of doing harm to either by driving even a short distance with a known-to-be-bad alternator may be low, but it is non-zero. Even if everything were operating properly, the risk of getting stranded in winter weather is similarly non-zero.

OTOH, the risk of calling AAA is zero. Call the tow.
posted by peakcomm at 3:56 PM on February 9, 2021

This is something that you can do. It is not something that you should do unless you are comfortable being stranded. The main current draws on a car battery are: starter motor, lighting, HVAC, and stereo. If you drive it during the day (lights off), and with all HVAC, stereo and accessories turned off you can probably get a few miles before you start having trouble, but this all depends on the size of yr battery, it's charge level, ambient temperature (5F is damn cold for a battery), and any number of external factors that make the proposition into a complete craps shoot.

If you had the inverse problem (bad battery, good alternator) once you got the engine running the alternator would generate enough electricity to run the vehicle, so you could drive it as long as you wanted so long as you keep the engine running. I wonder if your friend has their reasoning backwards.

All this to say, just call AAA and get your $$$ worth from them.
posted by dudemanlives at 3:58 PM on February 9, 2021 [2 favorites]

Having driven a lot of vehicles with failed alternators, I can assure you that having a battery fail at the wrong movement can place you in easily fatal situations. The problem is not that the battery no longer has charge, it's that being at the wheel of a vehicle that suddenly has no power can be very dangerous and predicting exactly when it might fail is somewhere between tricky and impossible, depending on your vehicle and the accuracy of the dash instruments. Part of this is that a failing alternator can damage a healthy battery, so even if you think you know what specs you're working against, what you actually have at that point may be worse.

There's also the factor of what your time is worth (especially in the cold) compared to just using AAA to do what you've essentially prepaid to do.
posted by Candleman at 4:01 PM on February 9, 2021 [6 favorites]

Candleman brings up an important point. When the engine stops, so does your power steering hydraulics and power brake vacuum assist which are both powered by the engine. Your steering and brakes will still work but you will have to provide all the muscle. If you've never had to wrestle a vehicle without power steering/brakes then you will be in for a rude awakening at highway speeds. 0/10 would not recommend.
posted by dudemanlives at 4:06 PM on February 9, 2021 [3 favorites]

Having my alternator die mid-drive sucked. I would not recommend gambling with this. A car is a piece of heavy machinery and it can kill people if it stops functioning when you're on the road. Plus, your friend sounds like they're kind of being a jerk if they're poking fun at you for wanting to do the exact right thing in this situation, which is to call AAA.
posted by k8lin at 4:07 PM on February 9, 2021 [5 favorites]

5F is bloody cold. AAA isn't always the fastest either. And there's going to be more than one person on the road needing a jump/tow/etc. If you're going to go out, make sure you're prepared. This is not something I'd personally do. I recall waiting hours for a winter jump in the past and it was no where near as cold.

/Cleveland native
posted by kathrynm at 4:26 PM on February 9, 2021 [1 favorite]

"Oh, so you're saying you're afraid to drive it?" I was asked.

"No I am making a pragmatic choice of not being stranded, potentially, in traffic or far from a place where I could reasonably get the car towed"

Nthing what everyone else is saying. The car could drive a short way depending on how much juice your battery has. I'd risk it if I was driving a few miles to an in-town mechanic during the day. I wouldn't otherwise. I know people who treat usign AAA like it's "losing" some sort of skills test with regards to their cars. I know other people who view having AAA as one of a number of tools one can use to deal with their cars. I fall into the latter camp. This is what AAA is for. It's too cold to risk getting stuck somewhere (or worse) unless it's a dire emergency.
posted by jessamyn at 4:33 PM on February 9, 2021 [5 favorites]

If the alternator is good, you can drive a car without a battery at all indefinitely, presuming you don't stall it. The reverse is not true. For one thing, your power brakes, power steering, and lights can fail more or less without warning. That alone is a big safety issue. The other is that car batteries, for reasons of cost and weight, are not made to ever discharge deeply. You really do permanently damage a car battery when it is completely discharged. "Deep cycle" or marine batteries are made to do that, but you'll pay a lot more for the same nominal power. Maybe you buy your batteries from a place with a good warranty and don't care that much, but it is wasteful.
posted by wnissen at 4:37 PM on February 9, 2021 [1 favorite]

I've done this before, when my harmonic balancer blew up and stopped turning the alternator. It drove for about 30 minutes (this was hot weather) but then various systems started failing one-by-one.

I would probably only do this again if I had a 110% charge on the battery *and* I was only going a few miles to the mechanic *and* it wasn't freezing weather.
posted by RobotVoodooPower at 4:41 PM on February 9, 2021 [1 favorite]

Take the free tow. You have already paid for it.
posted by General Malaise at 4:50 PM on February 9, 2021 [2 favorites]

My friend says it's ok to drive a car on a battery when the alternator is bad

Your friend is an idiot, particularly in the situation you have of inclement, possibly deadly, weather and also YOU HAVE THE OPTION OF AVOIDING THE PROBLEMS FOR FREE. Yes, you technically can drive a car on purely battery power, but you're essentially driving the equivalent a car with a coffee cups worth of gas in it. It is a short amount of time that the car will be usable before it will just..... stop working and you may have very little notice of when that is. Also unless you know the exact power drain of your car, you have no way of knowing how big that coffee cup of gas is. Would you put only a coffee cup of gas into a car and then drive it? Of course not.

I have driven a car that had this issue. We had time to notice the lights were dimming and then all power from the engine went, which meant power steering went, and we didn't have enough to do much more than coast to the edge of the road. We were lucky to be able to stop in a safe place. We walked the rest of the way home, as it was summer and a nice night.

"Oh, so you're saying you're afraid to drive it?" I was asked.

No. You're just not stupid enough to take needless risks. Not just to you, but the people you are on the road with when your car stops with no lights and no way to move it.
posted by Brockles at 5:21 PM on February 9, 2021 [5 favorites]

I've actually driven a vehicle back to the shop with a dead battery and no alternator belt - using a jump box. But that was because I was poor and had no ability to get a tow, and had someone following me if it died.

In case you weren't already aware, the battery is like a dam holding back a lake. Starting the engine is like dumping a bunch of water all at once. The alternator tops the battery back off once the engine is running. It also supplies the power needed to run the computers, and lights, and defroster, and blower fan, and...oh, and when it's cold out, the effective drain on the battery by all those things is doubled, roughly.

If your alternator is toast but your battery still has enough to start the car, yeah, you *could* drive it a mile or two, IF you knew what you were doing. But you're here asking the question, so the answer is...take the tow.
posted by notsnot at 5:28 PM on February 9, 2021

I did this this summer, my mechanic was less than a mile away and my power steering went out as I pulled into the lot. I would not have felt safe driving without power steering. I would get a tow.
posted by geegollygosh at 5:29 PM on February 9, 2021 [1 favorite]

A good rule of thumb is that if someone's response to you is "What are you, chicken?" the rest of their point can be safely discarded.
posted by bleep at 5:35 PM on February 9, 2021 [18 favorites]

"You're saying you're afraid to drive it?"

Whoever said that to you is an asshole. Why take the chance of making your car's situation any worse by driving it when AAA is a service that exists that you're paying for?
posted by Pandora Kouti at 5:39 PM on February 9, 2021 [4 favorites]

Response by poster: Well, the answers just get more and more dire -- validating my initial reaction (I felt like a clucker, but now I feel pleasingly cautious). Thanks all.

(I really hate it when people try to quash your reality. It's my life of being gaslighted.)
posted by intrepid_simpleton at 6:09 PM on February 9, 2021 [3 favorites]

You're unlikely to get very far on a bad alternator, especially because it can go from "going out" to "dead" very quickly. As in, I had difficulty moving a sedan half a mile with no lights or radio once it died, even though it'd been doing 100 miles a day just fine until that day.
posted by stormyteal at 7:00 PM on February 9, 2021 [1 favorite]

Sure, yeah you can drive all right, as long as you don't come to a stop because then your car will no longer drive and you will need to get that tow you were trying to avoid. Good luck avoiding red lights and stop signs!

I have some kind of alternator jinx and have owned two different cars (same make and model, though; it was a thing) with bum alternators and then my partner's much newer car started doing the same thing when we were driving out to a rural area to see a cottage/cabin that was for sale and I recognized immediately what the issue was - it was a Sunday and we were really very much in the middle of nowhere with no cell signal. We ended up having to get a jump from a random store owner after stopping and we couldn't get it to start again. That jump got us all the way back into the city (about 80km) because it was all freeways and we never had to stop, but we arrived back in town and hit a red light about 2km from home and of course couldn't get started again. We had to push it off the road and get a tow. So, you CAN, but it's not a great idea and you will almost certainly not make it to your intended destination.
posted by urbanlenny at 8:17 PM on February 9, 2021

"Oh, so you're saying you're afraid to drive it?"

arrrg I wonder what sort of death toll this kind of thinking has? drives me nuts. We're here to support your reality. it is sensible and admirable to not take chances with a huge hurtling lump of metal. Go you!
posted by 5_13_23_42_69_666 at 9:12 PM on February 9, 2021

As someone who had a car die from a short circuit at 140 km/h (87 mph in US money), simulation of it should be obligatory in all driving courses, to head off cavalier attitudes like your friend. I managed to navigate from the left lane to the shoulder in peak traffic, but even standing on the shoulder waiting for the tow was terrifying, and that was summer. Add the cold and snow and ice - you couldn't pay me enough.
posted by I claim sanctuary at 11:53 PM on February 9, 2021 [1 favorite]

"Oh, so you're saying you're afraid to drive it?" I was asked.

"Oh fuck off. What are you, twelve?" is the line I keep tucked away in my own mind's glove compartment for use at this point in that kind of conversation.
posted by flabdablet at 12:12 AM on February 10, 2021 [3 favorites]

Response by poster: And today, with the new battery, and sitting for a few days, the voltage meter (on the alternator) reads 14.5 so the tow truck leaves. And the battery light comes on as soon as he's over the horizon...
posted by intrepid_simpleton at 8:05 AM on February 10, 2021

Not even a question: use your tows! AAA is a big ripoff if you don't use it when you need it!

If you want to drive on a bad battery, drop AAA and buy 2 each year.
posted by The_Vegetables at 8:47 AM on February 10, 2021

If you do not have AAA (or other roadside assistance) then the question becomes "Is a tow cheaper than a new battery", or can I get to the service shop before I need a new battery?" In that case, if it's close (> 5 miles), you should wing it as a tow is about $40 and a new battery closer to $100. But if you have roadside assistance, use it!
posted by The_Vegetables at 8:50 AM on February 10, 2021

FWIW, AAA vendor will probably also have a battery service, where they come out and replace your battery, provided they have your model in stock.
posted by kschang at 11:37 AM on February 10, 2021

Have you called AAA again? Tell them the car has an intermittent charging problem and firmly request a tow. Maybe take a photo showing the warning light on as added corroboration or something to show the tow truck driver. If they don't step up, tell them you don't feel safe driving it and threaten to cancel your membership. (BTW, the Better World Club is less expensive, less politically dubious and worked OK for me the one time I had to use it.)
posted by Larry David Syndrome at 6:44 PM on February 10, 2021 [1 favorite]

We did this once (dead alternator but supposedly fully charged battery) and didn't get nearly as far as I thought we would. Just a couple of miles, really.

We had another car along and had to stop and jump-charge the battery for about 20 minutes (twice, if I recall) just to get a few miles across town.

And that was in warm weather.

And . . . discharging car batteries that deeply, especially repeatedly, is really bad for them.

All reasons to just get it towed. As Larry David Syndrome said, tell them it has an intermittent electrical problem and it definitely needs to be towed, not driven, to the mechanic. The fact that the charge light turns off for a little while now and again tells you absolutely nothing right now, except that something--likely the alternator--is flaky and needs to be fixed.
posted by flug at 8:25 PM on February 10, 2021

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