2015 Yukon SLT charging issue

[Kelleytechh]

My 2015 Yukon SLT has a goofy charging issue. Just for full background it was in for service at the dealership for AC work and they replaced the negative battery cable because they said it has high resistance they also said the battery tested good. So shortly after it came back the alternator quit charging, battery voltage dropping as I was driving but notably no gauge warning lights or warning messages. I replaced the alternator with a new AC Delco along with new belt tensioner and belt. No issues after for a while, then I was idling in a parking lot (idle at ~500 rpm) and saw the voltage was dropping to mid 11 volts. I revved the engine to 1k-1500 rpm and also drove it around the parking lot at parking lot speed and the charge would come back to mid 12v. Drove it back on the freeway and charge back up to mid to high 13v. (voltage is measured by gauge and radar detector that shows system voltage as well, both match very closely) this issue has continued. This was also with the AC and accessories on and off made really no difference. So it does not "feel" like there is an issue, no dimming lights, no warning lights or warning messages but it will still drop voltage to high 11v to very low 12v at idle but not 100% consistently. always charges while driving but sometimes erratic mid to high 12v. At startup it always jumps to mid 14v. I am kinda at a loss, I know the Yukon will call for different voltages but from what I have read not that low, any thoughts? Thanks all!

 

[Big Mama]

Most likely a failing battery if you have good connections and a good alternator. Go behind the repair shop and check all of your ground wires.

 

[Doubeleive]

I'll second that, usually this is indicative of the battery getting weak it might still work for now but not as well as it once was. (not holding a steady voltage) like a resh battery will

 

[the 18th letter]

My brand new battery does this, but my failing battery didn’t. I was told this fluctuation is actually normal for the way this charging system works. It was explained that because my old battery was failing it needed to be continuously charged where as the new one does not need to be continuously charged and at times the alternator will stop charging.

 

[Doubeleive]

My brand new battery does this, but my failing battery didn’t. I was told this fluctuation is actually normal for the way this charging system works. It was explained that because my old battery was failing it needed to be continuously charged where as the new one does not need to be continuously charged and at times the alternator will stop charging.

your brand new battery shouldn't be dropping down to 11v unless you are on a LONG drive perhaps with nothing turned on even then it would be super rare, a fresh battery should hang pretty much at 12-14 except at initial startup where it automatically gives a boost charge for the first minute and half, if anything it should be above 12v almost always.
"some" flucuation is indeed normal but a new battery should do it much less, as the battery ages and becomes more prone to discharge and heat it will be less stable.
I can only assume some peoples experience will vary but I have spent some time troubleshooting the RVC system and if the battery is not working properly then neither is the RVC, it tries but if the battery is not charging correctly then rvc just does what it wants anyway, for instance if the RVC calculates the battery is hot then it reduces the charge and the battery is getting depleted when it should be getting charged. One way to tell is to turn a few accessories on AC, Seats, rear defroster, wipers, headlights, etc and watch and see if it still sits at low voltage, it should not. It should show charging and if it does not then the battery is not working properly, it's too hot or has a bad cell or something.
I have tested the above with a scanner showing the RVC system operating modes, turning things on should make it go to charging mode if the battery is good.

 

[the 18th letter]

your brand new battery shouldn't be dropping down to 11v unless you are on a LONG drive perhaps with nothing turned on even then it would be super rare, a fresh battery should hang pretty much at 12-14 except at initial startup where it automatically gives a boost charge for the first minute and half, if anything it should be above 12v almost always.
"some" flucuation is indeed normal but a new battery should do it much less, as the battery ages and becomes more prone to discharge and heat it will be less stable.
I can only assume some peoples experience will vary but I have spent some time troubleshooting the RVC system and if the battery is not working properly then neither is the RVC, it tries but if the battery is not charging correctly then rvc just does what it wants anyway, for instance if the RVC calculates the battery is hot then it reduces the charge and the battery is getting depleted when it should be getting charged. One way to tell is to turn a few accessories on AC, Seats, rear defroster, wipers, headlights, etc and watch and see if it still sits at low voltage, it should not. It should show charging and if it does not then the battery is not working properly, it's too hot or has a bad cell or something.
I have tested the above with a scanner showing the RVC system operating modes, turning things on should make it go to charging mode if the battery is good.

Maybe my eyes can’t tell the difference between 11 and 12 volts, I am of the reading glasses age now, but it def fluctuates. No more problems starting after sitting a week or more so I’m inclined to follow my mechanics advice that my system is operating properly. Hopefully it doesn’t leave me stranded.