2002 BMW 325 Battery and Alternator Problems

[ET16]

A friend of mine has an '02 325I. He was on the highway yesterday and his car konked out. AAA came and they thought it was the battery. He had the car towed to his mechanic, and the mechanic said that the car needed an alternator. The mechanic put it an alternator and then also said that the battery was bad.

I'm thinking that there is a chance that my friend paid for an alternator that he didn't need, as I've rarely seen both battery and alternator and bad at the same time. Any way to know?

 

[crw]

As I understand it, a bad battery can cause the alternator to work too hard. When it eventually dies, they both need replaced.

 

[ET16]

In this case the car had been sitting for about year. It was jump started the day before to get it going.

 

[AandPDan]

Then he probably killed the alternator.

The alternator is not designed to recharge a dead battery. It can overheat and destroy the diodes/windings.

The battery may have also had a fault that increased the load on the alternator.

A battery should be charged on a charger, not by the alternator.

 

[ComfyShorts]

Originally Posted By: AandPDan

The alternator is not designed to recharge a dead battery. It can overheat and destroy the diodes/windings.


x2. He most likely could have avoided entire thing by letting the battery charge prior to driving.

 

[JimPghPA]

Originally Posted By: ComfyShorts
Originally Posted By: AandPDan

The alternator is not designed to recharge a dead battery. It can overheat and destroy the diodes/windings.


x2. He most likely could have avoided entire thing by letting the battery charge prior to driving.


Yea, modern cars realy demand a lot from the alternator. Add the load of a dead battery, and you can fry the alternator.

IF the battery would have held a charge then charging the battery up before using it would of save the alternator from being damaged.

IF the battery would not hold a charge then the alternator would have still been damaged. Unless you put a new battery in before using it.

 

[Kruse]

Originally Posted By: ET16
In this case the car had been sitting for about year. It was jump started the day before to get it going.


This is the same reason a lot of aftermarket alternators fail. Somebody needs a jump start and the assume it's the alternator. (And by now, the alternator does have a problem) So they buy a rebuilt alternator and it quickly fails. They take the alternator back to the store, tell the parts man that they sell junk, and then have the next alternator fail. About now, either the car owner realizes or the parts man recommends that the battery needs to be replaced. By this time, the (3rd replacement) alternator usually lives a long and normal life.
Of course, the owner will tell all of his buddies not to buy a Brand X alternator because you need to buy three of them before you get a good one.

 

[fdcg27]

The alternator and the battery share an intimate relationship.
A bad battery, with shorted cells due to sulfation, for example, can cause the alternator to fail.
A dead battery should be put on a charger, to see whether it will accept a charge. It sometimes takes some time on a charger to bring a serviceable battery back to life.
It is not good practice to jump a car that has sat in storage with the battery allowed to discharge and then to expect the alternator to bring it to a full charge.
It sounds like the battery killed the alternator in this case.
If the car actually died in use, it sounds like the alternator really was bad.

 

[ET16]

This is all very interesting. It seems as if I was wrong, though, about the time interval. The car was jumped today. The alternator must have given out only an hour or two after the jump.

 

[Vaca]

A good alternator will not be damaged by charging an uncharged battery. Sounds like they replaced the alternator by mistake. The only moving wear parts on an alternator are the bearings and brushes and neither are affected by alternator output. The only other possible issue would be heat, extreme heat, like being stuck in rush hour traffic in Death Valley during a heat wave.

 

[HTSS_TR]

Originally Posted By: Vaca
A good alternator will not be damaged by charging an uncharged battery. Sounds like they replaced the alternator by mistake. The only moving wear parts on an alternator are the bearings and brushes and neither are affected by alternator output. The only other possible issue would be heat, extreme heat, like being stuck in rush hour traffic in Death Valley during a heat wave.

I stuck in Death Valley on the way to Vegas at noon for 3 hours with ambient temperature in mid 120's in the E430, soon after that the battery died but alternator is still working well.

 

[Kruse]

Originally Posted By: Vaca
A good alternator will not be damaged by charging an uncharged battery. Sounds like they replaced the alternator by mistake. The only moving wear parts on an alternator are the bearings and brushes and neither are affected by alternator output. The only other possible issue would be heat, extreme heat, like being stuck in rush hour traffic in Death Valley during a heat wave.


The non-moving diodes and the diode trio are major failure areas, especially in the can-we-rebuild-them-any-cheaper? replacements. You start a car with a dead batter and let an alternator run at full amperage for a few hours and the risk of failure increases.

 

[mechtech2]

Jumping a car can cause alternator problems for the donor and recipient.
Bit by bit we are getting information on this car.
Maybe in a few weeks we will have enough info to tell what's going on.

 

[Vaca]

[/quote] The non-moving diodes and the diode trio are major failure areas, especially in the can-we-rebuild-them-any-cheaper? replacements. You start a car with a dead batter and let an alternator run at full amperage for a few hours and the risk of failure increases. [/quote]

Yeah, the diode bridge and the soldered joints would be stressed by extreme heat, but a good alternator is built for it, a cheaply rebuilt alternator...who knows. Also, lots of thermal cycling can create some short circuits in the stator windings, but that is usually a gradual issue, and shouldn't be an issue for a good alternator - unless a dead short directly to the stator.

Anyway, who knows what really happened as we're getting this second hand at best.

And yeah, charging a battery too fast is much worse for the battery than it is for the alternator.

 

[antonmnster]

I'd suspect sticky brushes long before anything else. Or possible damage from improperly jumping technique.

The alternator can only put out so many amps, and it's rated to do so. The alternator on this car has a duct that runs from the side of the fog light straight into the diode/rectifier. Cooling couldn't be better on this particular unit.

We tend to overload alternators at work all the time, yet the never fail. We run New Holland tractors at idle with all forward and rear lights on to the point where the idle has to be bumped up to keep the battery from discharging. We also run dynamos on our F350s so hard that the battery will actually completely discharge while the truck is still running. Never had an alt fail. (The dynamo is a 12v motor that spins a 120v alternator. We use them to run tools or, in my case, a coffee maker.)