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LML Duramax charging at 15.3 volts #5262214 11/08/19 10:41 PM
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BlakeB Offline OP
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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?

Re: LML Duramax charging at 15.3 volts [Re: BlakeB] #5262219 11/08/19 10:47 PM
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zrxkawboy Offline
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I wonder if the new batteries had been on the shelf a while and need to charge up a bit.


"Think of all the Ford owners who will someday want an automobile." John Dodge

Re: LML Duramax charging at 15.3 volts [Re: BlakeB] #5262220 11/08/19 10:49 PM
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zrxkawboy Offline
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Also, what was the ambient temp? That will make a difference.


"Think of all the Ford owners who will someday want an automobile." John Dodge

Re: LML Duramax charging at 15.3 volts [Re: zrxkawboy] #5262222 11/08/19 10:52 PM
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BlakeB Offline OP
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Originally Posted by zrxkawboy
Also, what was the ambient temp? That will make a difference.

23°F here currently

Re: LML Duramax charging at 15.3 volts [Re: BlakeB] #5262236 11/08/19 11:31 PM
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zrxkawboy Offline
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Colder temps typically equal higher voltage.


"Think of all the Ford owners who will someday want an automobile." John Dodge

Re: LML Duramax charging at 15.3 volts [Re: BlakeB] #5262380 11/09/19 08:37 AM
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^^^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...


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Re: LML Duramax charging at 15.3 volts [Re: BlakeB] #5262411 11/09/19 09:19 AM
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BlakeB Offline OP
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Thanks for the help guys.

Re: LML Duramax charging at 15.3 volts [Re: BlakeB] #5262578 11/09/19 01:59 PM
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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.


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Re: LML Duramax charging at 15.3 volts [Re: BlakeB] #5262585 11/09/19 02:15 PM
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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.


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Re: LML Duramax charging at 15.3 volts [Re: BlakeB] #5262730 11/09/19 06:16 PM
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try checking all the grounds. body, computer, ect

Re: LML Duramax charging at 15.3 volts [Re: Gasbuggy] #5262733 11/09/19 06:19 PM
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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.

Re: LML Duramax charging at 15.3 volts [Re: zrxkawboy] #5263071 11/10/19 07:31 AM
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willbur Offline
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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

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