Alternator overcharging

I have a 2006 Buick LaCrosse with the 3.8 L motor. Last week I found out that the alternator was overcharging (charging at around 15.5-15.7 volts.) The car has 72000 miles on it. I replaced the alternator with one that I got from a salvage yard (to save money) but it is still over charging at the same voltage. I recently got a new battery (within the last 2 months) I find it hard to believe that I would have 2 alternators in a row doing the exact same thing although it is possible. Could there be somthing wrong with the battery? I really don’t want to spend $200 on a new alternnator if the battery might be the issue. Any ideas would be appreciated. Thanks.

How cold was it when your charging voltage was 15v+? If the temperature was near ZERO and you made the test soon after starting the engine there is/was no problem. Check the charging voltage after driving several miles.

It has been between 50 and 60 degrees here for the last week. The car has an option in the message center that shows battery voltage. When I start the car, it shows 14.6 to 14.8 volts. As I drive it, the voltage goes up steadily. I put a multimeter on it after coming back from driving it and it was showing 15.8 volts while running at idle.

That does bear some attention then @stormchaser88. 15v has been my threshold for overcharging for years and because your battery is new and assuming it is taking a full charge I wonder if there is a weak ground somewhere between the alternator and battery. But then a weak ground would likely be somewhat obvious when cranking the engine and you didn’t mention that as a problem.

@“Rod Knox” . You are correct. Cranking is not a problem at all. It starts right up. I will check the ground connections. Thanks for the tip.

The alternator isn’t overcharging.

Seeing 15.5-15.7 volts is normal on the new GM charging systems.


May I ask, what led you to replace the alternator in the first place?

I would have the charging system tested by a shop with a decent scan tool to monitor the desired voltage vs. the actual voltage. If the car is charging at 15.5 volts when it’s supposed to then there is no trouble at this time. If there is a discrepancy between what the alternator is told to do and what it is doing then we can look further.

@ Tester. Thanks for the article. I never realized that.

@ asemaster. I try to check things like this every once in a while and just saw that it was charging at over 15 volts. After reading the article that Tester sent me the link to, it makes more sense that the alternator is actually doing what it’s supposed to do. Thanks.

Sorry to hear you may have replaced 2 alternators for nothing. Though it would be nice to have the system checked just to verify that it is working properly.

I had the reverse of your scenario here a few weeks ago. One of the guys replaced the battery in a 2010 Infiniti. When he was done he checked the system voltage at idle, and then reported that the car needed an alternator, it was only charging at 12.7 volts. I said hang on, let’s check it out. Hooked up a scan tool, saw that the desired voltage was indeed 12.7 volts. And as we added electrical loads or increased engine speed the voltage climbed up to whatever was commanded by the car. The charging system was working just as designed.

Someone check me on this, but the battery voltage while charging is the no load voltage (12.6 volts at room temp) plus the IR drop of the charging current over the internal resistance of the battery. so if you are charging at 50 amps and the Ri is 0.04 ohms, then the drop is 2 volts and the battery voltage is 14.6 volts. If the battery is fully charged and there is little load on the system, the current will be low and the voltage close to 12.6 volts.

Is there some worthwile benefit to the micromanagement of the charging system? Is battery life doubled?

If the load increases the voltage should drop.

“Is there some worthwile benefit to the micromanagement of the charging system?”

It returns the battery to full charge faster, which extends battery life, especially if there a lot of short trips.
It can also save a little gas by lowering the voltage (and alternator load) when the battery is filled.

Newer cars have so much electric gadgetry I guess the engineers decided they needed a smarter system to keep the battery charged. Otherwise when the engine was first started in the AM and you put on the AC, the drag on the alternator would stall the engine at the next stoplight.