Voltage issue: Battery or alternator?

Thread Starter

strantor

Joined Oct 3, 2010
5,543
Twice in the past week I've had to jump start my yukon. I drive it every day. The battery is from 09/2018, so not old enough to be time to replace. I noticed that the voltage meter in the dash is running higher than normal. It's typically right at 14V (13.8V per the scanner, which agrees with my DMM) which is normal for every GM vehicle I've had. According to my scanner, voltage is fluctuating between 14.9V and 15.3V. Not normal.

What might be the issue? Bad battery overworking the alternator? Or bad alternator overcharging the battery?

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P.s. what happened to the automotive subform? I thought we relaxed the automotive topics ban and created an automotive subform. Am I gaslighting myself?
 
In cold weather say -13°F/-25°C, 15V in a car is fine. What are your ambient temps like?
If your battery went quite dead and it's cold, it likely has a frozen cell so it will never take a charge or really work. It's common to take them indoors, thaw them out and charge afterwards. Don't charge a block of ice, hydrogen builds up.

If you are somewhere warm (above freezing) then your battery sounds like a near open circuit.
Then you get high ripple voltage and alternator regulators are not true RMS. A shorted alternator rectifier diode also can drain a battery when the car is off, not charge a battery due to AC but read 13.8V on some multimeters. I got burned by that once.
I would look at the water levels in the battery and for bad connections to it.
Car battery quality is a crap shoot nowadays, so 2018 doesn't mean it's still good. Bought a Costco one and had to take it back, it was a lemon.
 

bwilliams60

Joined Nov 18, 2012
1,397
Two cents. Normal charge rate is 13.5-14.5 V. If it is charging high, several things come to mind. You mentioned you were reading it on a scanner. Put a DMM on the battery and measure it on the posts. That is the reading to start with. Start with having the battery load tested as mentioned. Always the starting point. If battery is okay, check for a parasitic draw and see if it is more than 50mA. If it is, lets look at that. If not, check your charge rate at the battery. If over 14.5, you could have a broken sense wire or bad voltage regulator. High ripple voltage does not cause high charge rates. The opposite would be true. You would get roughly 66% output with one diode and no charging with two.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,543
In cold weather say -13°F/-25°C, 15V in a car is fine. What are your ambient temps like?
If your battery went quite dead and it's cold, it likely has a frozen cell so it will never take a charge or really work. It's common to take them indoors, thaw them out and charge afterwards. Don't charge a block of ice, hydrogen builds up.
It was ~40f/4C both times I had to jump it.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,543
The first thing I would do, is do, or have a load test done on the battery, this should give a good insight to its state.
Takes 5 or 10 min.
Max.
Yeah, will do. Probably Tuesday. Not leaving the house tomorrow. We are expecting historical low temperatures for gulf coast TX tomorrow and (gasp) even snow! we aren't cut out for it. Nobody here knows how to drive on icy roads, we don't have salt, it's a recipe for disaster.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,543
I lied. I did leave the house. Power went out at 2am, woke up to 50f temperature in the house. Had no gas for the generator. Had to go to town. While there, I stopped and had them check it out. Mr. Autozone said the magic gadget didn't give a conclusive answer but his suspicion is that the regulator in the alternator has failed/is failing hence the 15+V, and that probably killed one of the cells in the battery, as the gadget said it only had 66% capacity. It sounded pretty plausible to me. I bought a new alternator and battery.
 

MisterBill2

Joined Jan 23, 2018
8,723
The first system diagnostic check is to measure the battery voltage no load and the engine off. Then start the engine and the voltage should be about a volt higher, if the alternator is charging. If the alternator is still charging that much after a 15 minute drive then that battery is suspect. So the next step is to put a charger on the battery and note the current. If the battery is still taking a 5 or 6 amp charge after overnight charging then it is replacement time. If the charge has dropped to a very low value and the voltage stays about 14 volts when you disconnect the charger, then the battery is OK and the charging system is suspect.
And the good part about these tests is all you need is a voltmeter and a battery charger, even a fairly cheap one. No disconnecting the battery or removing it. No chance of damage to anything.
 
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