LML Duramax charging at 15.3 volts

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My dads 2016 LML has been turning over a little on the slow side for a while, but this morning it wouldn't turn over at all. I jumped it with another vehicle and checked the battery voltage with it running after the jump vehicle was disconnected. It was reading in the mid to upper 14's on the multimeter. I thought ok, it needs batteries... so I take them off and go get new ones.
I ended up getting the Everstart Maxx AGM H6 batteries at WM for ~$150 each. I checked the resting voltage before I installed them, both were at 12.6-12.7 volts.
I got them installed and started the truck up, all seems well until I throw the multimeter on it and find that it is charging at 15.3 volts. I let the truck run for about 30 minutes and the voltage did drop to ~14.5 volts for a minute and then went right back to 15.3 volts.

This truck was manufactured in 8/2016 so the factory batteries were only 3 years and 3 months old. I thought it was odd for them to be dead at just over 3 years old, but now I'm wondering if the charging system/alternator is overcharging and cooking the batteries?

The truck has 48k miles and ~3,300 engine hours. I did not have the old batteries load tested before replacing them but the truck jumped easily and I did verify that the alternator was charging before I removed them so I am confident that at least one of them was bad.

I did check for codes and the only thing it's showing is a P21DD for the DEF heater. No pending codes.

It runs great and doesn't have any other symptoms of anything being wrong. I'm just concerning about it charging at 15.3 volts, that seems way too high to me. What do you guys think?
 
I wonder if the new batteries had been on the shelf a while and need to charge up a bit.
 
Originally Posted by zrxkawboy
Also, what was the ambient temp? That will make a difference.

23°F here currently
 
^^^Exactly. The variable charging setup on most GM vehicles will really pile on the volts when it's cold outside.

Normal behavior. Expect it to drop like a rock when it heats up...
 
Sounds like you have a voltage regulator going out. 15.3 is to high and will fry battery as well as do goofy things (IE limp mode) if left like this.
 
GM's system alternates voltage depending on what it determines is needed, with fuel economy in mind. My CTS-V I've seen anywhere from 15.2 to 13 flat.
 
Originally Posted by Gasbuggy
GM's system alternates voltage depending on what it determines is needed, with fuel economy in mind. My CTS-V I've seen anywhere from 15.2 to 13 flat.


Yes and the "Maintenance free" batteries GM invented need a few tenths more charging voltage for the different chemistry.
 
Originally Posted by zrxkawboy
I wonder if the new batteries had been on the shelf a while and need to charge up a bit.



I think you are right. A fully charged AGM is 12.8-12.9 volts (according to East Penn/Deka), and do not charge above 14.6
 
^^^Exactly. The variable charging setup on most GM vehicles will really pile on the volts when it's cold outside.

Normal behavior. Expect it to drop like a rock when it heats up...
I think that's right. I've been noticing less voltage dropping in the colder weather with my 2005 Silverado. Sometimes it goes to 12.5 in the summer and or long drives. Lately it has been staying higher, but then I noticed slower cranking speed...so I swapped it with a smaller but good 1 year old battery in and it cranked a lot better. Voltage is still staying in the 14s at all times never getting below or above that now...but it's been cold a bit longer now and I have had more short trips and time spent in the truck with the radio on and charging my tablet, but engine off. So I'm probably draining it down more lately.
 
Originally Posted by zrxkawboy
I wonder if the new batteries had been on the shelf a while and need to charge up a bit.



I think you are right. A fully charged AGM is 12.8-12.9 volts (according to East Penn/Deka), and do not charge above 14.6
On page 110, it says that the 4 cylinder diesel pickups come with an AGM battery. It says to limit charging voltage to 14.8 volts if using a battery charger.

Also check out the below link for a look at how the charging system on these trucks work.
 

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On page 110, it says that the 4 cylinder diesel pickups come with an AGM battery. It says to limit charging voltage to 14.8 volts if using a battery charger.

Also check out the below link for a look at how the charging system on these trucks work.
So what is your point? A gotsha, maybe.. I got my info directly from the battery tech support manager at East Penn/ Deka. Good enuff for me.
 
So what is your point? A gotsha, maybe.. I got my info directly from the battery tech support manager at East Penn/ Deka. Good enuff for me.
No point, I am just posting relevant info that I’ve ran across since I started this thread. East Penn says in the linked PDF not to exceed 15.4 volts when charging their HD AGM (group 31) batteries.
 

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AGM's typically have a bit higher voltage than wet lead acid. Depending on the battery, they'll be 12.8 to 12.9 fully charged when new, so the batteries were a little bit down when you installed them. That would also be a reason why they were being charged hard for a while.
 
No point, I am just posting relevant info that I’ve ran across since I started this thread. East Penn says in the linked PDF not to exceed 15.4 volts when charging their HD AGM (group 31) batteries.
That applies to Heavy Duty Trucks. Perhaps those AGM batteries are capable of handling 15.4 bolts. I was talking about AGM in a car, because that's what I have.
I'll call East Penn and ask them about the 14.6 vs. 15.4. The guy there is kinda cranky but knows his stuff.
 
That applies to Heavy Duty Trucks. Perhaps those AGM batteries are capable of handling 15.4 bolts. I was talking about AGM in a car, because that's what I have.
I'll call East Penn and ask them about the 14.6 vs. 15.4. The guy there is kinda cranky but knows his stuff.
Just called EP tech support about AGM. They told me 14.4-14.6.
 
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