Can a bad battery damage an alternator?

[Supernaut]

I have a 2004 Pontiac Sunfire with 345,000km (214,000mi) which I have had to abandon tonight and take a cab home. It was the classic signs of a dead alternator, voltage drops, won't start and eventually dead. The battery light came on intermittently so I parked at my local Canadian Tire and tried to see how long it would run. After about 10 min it was dead.

The obvious thing I need to do is go back and replace the alternator. However I knew since last summer this battery is on its final days at its original and showing signs of weakness last summer.

My questions as follows

Since this car is well used and I probably don't plan to have it for long, If I just change out the alternator can I run on that old battery for a day or so and maybe get a used one from a wreaking yard? Or if the battery has totally failed would that make it impossible to complete the electric circuit and run the engine? I could just get a new battery from Canadian Tire while I get the alternator from there, but I'm just wondering if there was a way to save a few bucks on this.

Though I highly doubt it, could there be a chance that this is just battery failure and the alternator may still be ok?

 

[Mursal]

Check battery connections
Jump start using jump leads and see if it runs the same way from the other battery? If it makes no difference, its probably the alternator or wiring going to it/loose drive belt.
If on the other hand, it runs OK, then you just need a battery.

 

[Supernaut]

If it was just the battery, shouldn't the alternator have been able to keep the engine running assuming the alternator was ok? Or would a bad battery break the circuit and prevent that from happening?

 

[Peter.N.]

I would have thought that the alternator should have kept the electrics running unless the revs went very low, but at tickover it should be OK. The condition of the battery won't make any difference to the new alternator.

Peter

 

[Supernaut]

I would have thought that the alternator should have kept the electrics running unless the revs went very low, but at tickover it should be OK. The condition of the battery won't make any difference to the new alternator.

Peter

That's exactly how I thought it should go. Which in my case then the alternator should be almost certainly fried.

 

[alan333]

The battery light is sign of the alternator not charging properly, so I'd say your diagnosis of a faulty alternator is probably correct. To test it start the car and put a voltmeter across the battery - it should read about 14 volts (or a bit more) when engine running.

Also, I once had a Peugeot car with a duff alternator and it would only run until the battery went flat. When jump started it ran fine while connected to the other car but died very shortly after disconnecting the jump leads ie when the battery went flat again. So yours might run with a flat battery, and it might not.

If your old battery is starting the car daily then you shouldn't need to replace it due to the alternator fault, but one day it will let you down. One from a breakers is fine as long as it's good.

 

[Supernaut]

I have the problem all squared away. I bummed a ride to the Canadian tire in town from the nice old lady who lives next door (I live 30km out in the country) where my car was abandoned and the store conveniently had a new battery and alternator in stock for me. I had my tow service, which was well worth the $75 for the year which gives 5 tows, bring me and my car home so I could fix it in the driveway.

The old battery was not leaking but at a glance it seemed to have been somewhat bulged out on the ends which suggests it may have froze. For ****s and giggles I charged up the old battery to see what would happen. It charged to 13.4 volts and had dropped to 12.5 by the next day.

The alternator wasn't too bad to change out as the GM "J-body" cars have a large engine bay for a small car, and the ecotec engine and connected systems as a unit seems pretty well organized.

With a new (rebuilt) alternator and battery that's one less thing to worry about. Maybe I'll even get another 100k out of the old bugger... not that I exactly want to keep it around that long.