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Alternator failure due to weak battery?

08-21-2007, 04:40 PM
After my alternator failed, a local service station said that it was likely due to
the battery, which was not taking a full charge (which I verified, with my own
trickle charger)-- though it was still starting the car, operating all systems.

After installing the alternator, he advised me to also get a new battery, as the
alternator would be "working extra hard" on the deficient battery, and that
would void the alternator warranty.

Can anyone comment on whether this information is accurate or not?

08-21-2007, 10:41 PM
most reman alternator's come with a big warning about discharged batteries causing alternators to fail, and you should always make sure they are charged.

But IMO, this is BS. Alternators can only put out so much amps. If the battery is flat, then the alternator will be maxed out for a little bit until the battery is charged back up. After that, everythings back to normal

A damaged battery on the other hand, WON'T take amperage. Such as a dead cell. In that case, the alternator isn't working hard.

You'd need to have the battery tested for a final answer on this. If the battery is bad, then you should replace it....because the battery is bad. Not because you replaced the alternator.

If your battery is good, and you want to play it safe, charge the battery before running it with the good alternator.

Or just let the alternator charge the battery. Really....thats what alternators are designed to *do*. maintain the system voltage and charge the battery

08-22-2007, 12:06 AM
I agree with Unclebob, but it depends on the alternator. Some alternators will operate at their max amperage forever and not break a sweat, others will die in a few hours. Its quite possible that a fried battery could supply enough juice, but if the alternator is straining trying to get more voltage to the system it will overheat and die.

Its the age-old charging system problem. Starter, alternator, and battery are all linked. When one goes bad its possible for it to affect the others. Knowing how they all function is the key to knowing which is the problem.

08-23-2007, 09:13 PM
Thanks, Unclebob & Curtis.

So here's a related question- the reverse situation: If the alternator failed-- not caused by a bad battery-- and it wasn't charging the battery (which eventually would die), could that damage the battery so that it would not take a charge from a charger or a replaced alternator?

08-24-2007, 01:47 PM
depends how the alternator failed. When the rectifier diodes break down, you'll still be charging some, but you'll have a lot of AC signal in the system. This can toast a battery.

Simply running a battery down, assuming the battery is in good shape before that, shouldn't hurt it. Sometimes its the final straw that broke the camels back though

08-24-2007, 08:39 PM
Agreed, unless the battery was on its last legs a soft discharge shouldn't hurt a thing

08-25-2007, 10:30 AM
UncleBob's understanding of electricity is right on the dot. The alternator is a constant voltage generator. It will vary the current to maintain proper voltage. Either way, the alternator supplies 14.4 volts on most cars. The battery supplies 12.75 volts. So the battery is charging as long as the car's running. Now unless your battery is discharging completely within minutes, it will not draw a lot of amps from your alternator anyways. Either way man, if your alternator dies again, go put in a brand new battery, and take the car back to the shop. Tell them it died and your battery is fine :P. How the hell will they know?

08-25-2007, 08:41 PM
To Uncle Bob, Chris, and INF

Thanks for all the info. At this point, it is difficult to know whether the initial problem was the alternator, the battery, or both, as I had not checked to see if the alternator was putting out the required 14 volts. I have now replaced both. The battery was 3 years old, the alternator just over a year-- unfortunately both out of warranty.

To review, with some more details, I had to be towed when the battery ran down, and the repair shop I had it towed to said that it was the alternator. I don't know if they bench checked it or not, but when I had it replaced a year earlier, the symptoms were the same-- the power went out slowly-- first the AC compressor stopped working, the fan slowed down, then the ignition was irregular, and I finally sputtered to a stop. As I had to wait a long time for the tow, after a while the battery regained some charge-- enough to start the engine-- but then everything died again after a few minutes, so the alternator was not charging it (otherwise, I would have called off the tow).

At the same time, I had looked under the hood a few days before all this happened, and noticed the charge indicator on the battery was red; when I tested the cells with one of those small battery testers, 4 of the 6 cells were only "fair," so it seems that the battery might have been on the way out (and I didn't do anything about it at the time). Then, if the alternator was of a type that Curtis indicates could be affected by a bad battery, that might explain why it failed shortly thereafter. Or if the alternator diodes failed, and it was putting out AC, it could have caused the battery to fail. Chicken and egg.

After the new alternator was put in, the battery did not take the charge, and then one of the cells was completely dead, so I replaced teh battery. When the alternator was replaced a year earlier, the battery had not gone bad at all.

What I have learned is that next time I lose battery power, I need to check the alternator output voltage-- or check it periodically. But there is now no way to check the state of the battery-- it is "maintenance free," and doesn't have an indicator window,which surprised me (at least it has a 5 year warranty).

I now have another problem with the same car (a 1998 Windstar 3.8) which I will post separately.

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