Can a bad battery kill a running car?

My 2001 Miata died on the freeway yesterday, and I’m confused by what the mechanic is telling me, now.

Here’s what happened. I was driving on the 101 in backed up traffic, and I started to lose power. It felt kind of like running out of gas. Within a few seconds, the engine stopped. I turned the key and could hear it crank, but it wouldn’t turn over. After getting it pushed onto the shoulder (thanks to some very kind guys who pulled over and walked out to help push it up the slight incline), I waited for a tow truck. After 10 minutes or so, I noticed that the hazard lights were no longer flashing. I tried the key in the ignition, but there was no crank. The electrical system was completely dead. I also noticed a buzzing noise from near the fusebox under the hood, although none of the fuses were blown. I eventually got it towed to a shop in the area, and after some testing, they told me that the alternator and the battery both needed to be replaced. They said that they didn’t have the alternator, so it’d be fixed the next day.

This morning, they called me and told me that the alternator was ok, that just replacing the battery had fixed the problem. I am suspicious of this. Can a bad battery actually stop a running engine? I know that you need electrical power to spark, but I was under the impression that the alternator would provide that, and that disconnecting the battery from a running car would not cause the engine to die.

The reason I’m particularly worried about this is that the car is ~70 miles away from where I live. I’m happy to not replace the alternator, but if the real problem is not the battery, I really don’t want to break down again on the drive home after I’ve begged a ride or taken the train.

ETA: The mechanic said that one of the terminals on the battery was loose and not making proper contact.

A bad battery can definitely kill a running car. Most cars these days rely on the battery for a lot of the bulk filtering, so if something catastrophic happens to the battery you’ll get very wild voltage fluctuations that could actually damage your car. In the old days you could remove the battery from a running car and it would still run. These days that is no longer true, and you run the risk of damaging the car if you try it.

A loose terminal could prevent the battery from being properly charged by the alternator, so this is possible.

I’m very suspicious of the fact that the car would still crank though. If the battery was porked bad enough to kill the car I wouldn’t expect it to be able to crank the engine over. It takes a lot more power to crank the engine than it does to keep it running, so generally if the battery has enough power to crank the engine it has more than enough to run it.

You may have an intermittent short somewhere in the car. This could kill power to the ignition circuitry and drain the battery to the point where it self destructs.

ETA: Whatever was buzzing near the fuse box is likely the real culprit here.

I called the shop and they said they’d try starting it up again later, and do another check that the battery was charging and the alternator was working. I was not too impressed with their dedication or apparent knowledge during the call, but I’m not sure what more I can do from 70 miles away. I think I’ll just upgrade my AAA membership to the 100 mile limit so that if it dies on the way driving it home, I can at least have it taken to a local mechanic who I know and trust.

From everything I see you were treated fairly. They did not try to sell you an alternator. They eventually started it and tested the output.
Yes, a battery can do exactly what yours did. Actually, a battery that dies quickly can save the alternator from burning up trying to charge a faulty battery. And you verified your battery’s sudden demise by the fact that all the electricals of consequence went dead.

For future reference. Even a battery that cranks the engine won’t necessarily run a modern vehicle. The voltage needs to be up to at least 9-10 volts or it won’t start. Too low of a voltage while running can load the alternator enough to cause a stall.

That’s true, and maybe I’m being to hard on them.

When I called, I explained my worries, as I did here, and I basically got “The machine says the battery is dead, so we replaced it. Now the machine says the battery works.” No indication of whether the symptoms I describe match the testing they did.

It’s good to know that they are consistent. Gives me more confidence that I’ll make it home without another unplanned stop.

Could be that the mechanic was just tired of answering questions from dumb customers (like me), but I like to think my $90 diagnostic fee entitles me to more expertise or explanation than I received.

Is the voltage regulator another possibility? It’s often mounted externally on Fords. Other brands have it inside the alternator housing.

I think on a Miata of that age it’s in the ECU… and if that had fried, they’d know.

So they basically have the troubleshooting skills of a well trained chimpanzee. For $90 I’d expect more than that too.

Their machine should have told them that something was wrong, so I doubt that this is the case.

There are an awful lot of semiconductor thingies inside a modern car, of which the voltage regulator is one of them. Semiconductors do on rare occasions exhibit a phenomenon called latchup, and when this happens it basically makes a dead short across the power leads. Usually you can figure out that this happened because the semiconductor that latches up lets out quite a bit of smoke and occasionally catches fire, but sometimes the device will survive. Once you remove power, the latchup condition goes away and when you hook the battery back up you may never see a problem with it ever again. So it is possible that something like that happened here.

There could also be a loose wire or something somewhere in the car. The OP may want to drive it very carefully home, then once close enough that walking home isn’t a problem, do a bunch of hard turns to the left and right and some hard stops and starts to see if anything will shake loose and short out. If the car dies again it would be a good idea to disconnect the battery before it gets killed and has to be replaced again.

The noise near the fuse box could be a clue, or it could be a relay that was just chattering due to not having enough power to stay energized. It’s worth taking a close look at it though.

I hope that the battery was all it was, but I don’t have a lot of confidence in the monkey mechanics.

It wasn’t cranking the engine over.

I don’t think the OP is using the term “crank” the way you are (maybe referring to the clicking noise you hear when turning the ignition with a nearly dead battery?.)

I drove it home (without having read this) pretty hard. If there was something loose, I probably would have shaken it out. So far, so good. Hopefully it was just the battery.

I think I am using it the right way. I mean the revving “rr-rrrrr-rrrr” that the starter makes. It was making that sound, but the engine wouldn’t start. Definitely not the clicking sound that a mostly dead battery makes.

Thanks to everyone who helped in this thread.

Alternators require field voltage to work. If there is enough juice to turn the engine over then there is enough to run the alternator. Now if the connection was bad it’s possible to lose not only the alternator but power to the injectors and ignition.

Modern batteries don’t give any warning when they fail. Years ago they would slowly fade away and you would know it when the engine turned over slowly. They were large batteries that only produced 3-400 cold cranking amps. The plates in the batteries were spaced far apart. Now batteries are smaller with many more plates. They have twice the power. The downside is that they can short out across a number of plates and take the whole battery with it. In my experience any battery over 4 years old is actively planning on leaving you at the worst possible time or place.

I have had the experience of the engine quitting at 4am on a highway. In my case the dash lights went out but everything else worked. That told me it was a fuse so I checked the engine fuse box and found a blown fuse. I then swapped out a fuse that wasn’t necessary.

Since the question has been well-answered by actual experts, I’ll throw in a relevant anecdote. Last year my girlfriend’s car wouldn’t start. We successfully jump-started it using another car, but as soon as my girlfriend turned the headlights on, the engine died. I (as an amateur non-mechanic with a little knowledge of how engines work) immediately diagnosed a bad alternator, but when we took to a mechanic, the alternator was fine and it was just the battery that was dead. This seems to fit in somewhat with the OP and subsequent explanations.

Cool. That sound is the engine turning over.

In my never-ending quest to fight ignorance. :smiley:

Wrong use of the term “turn over.” Crank = turn over. You could hear it crank (turn over), but it wouldn’t start.

Correct use of the term “turn over.”

Ignorance fought. I always thought that “turn over” meant that the engine had caught. That is, it was turning over under it’s own power.