AGM vs. OEM Flooded Lead Acid battery Issue?

[Outdoor 74]

Hi folks,

I recently replaced my ~5 yr old OEM ACDelco lead acid flooded 'Gold' Battery 94RPG (720CCA) in my 2015 Sierra 4WD 5.3L with an Oreilly's Superstart Platinum AGM with 800 CCA. (part # 94RPLT).  The OEM battery was working fine, however wanted to be on the safe side and replace since getting old.  With the new AGM battery, the voltage on startup and when running for a minute is right around 14V (according to dash gauge).  However after a few more minutes of driving it drops noticeably to I'm guessing around 12V or so.  With the OEM battery the voltage gauge would always be pegged right around 14V.  I went back to the Oreilly's where I bought the AGM battery and had them test it, and meter said it was 'good'.  When I take a voltmeter up to the battery myself it reads ~ 12.56V, which according to online data means only ~75% charged despite driving several hours.  Has anyone experienced something similar or knows what could be going on?  When driving everything is fine, and the gauge is steady, but just much lower than with OEM battery.

[HeySkippyDog]

You're probably watching your volt meter more closely since you know you changed the battery.

The OEM battery behaves the same. That "drop" is normal, the charging system in these trucks is actually pretty sophisticated. It's demand-based.

I put a Duralast AGM in my 2014 when it needed replacing, with similar specs to yours. Worked just fine.

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[Outdoor 74]

Thanks SkippyDog, I definitely am watching the meter more closely so I hope you're right.  One other thing I didn't mention earlier is it also seems to hesitate just a bit longer before starting when compared to when the OEM battery was in there..maybe it's just my imagination, but it seems a little different.  What do you think about the relatively low (for AGM) voltage reading I got?

[Outdoor 74]

Just want to add that the battery I bought was dated at 11/19 (November 2019) so wasn't sitting around a long time in my opinion.

[HeySkippyDog]

Like I said, the charging system is demand based. I still have my OEM battery in my '17 and it does the drop to 12VDC all the time. So did my 2014, with both OEM and AGM.

It's normal, doing what it has to do. Remember that 12V with the engine off is normal, you only get to 14 with the alternator running, and like I said, the alternator only pushes to the battery as needed.

It's been posted here a couple times from new owners wondering what's going on with their electrical system. All normal.

Make sure you turn in your old battery for the core charge refund.

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[Outdoor 74]

Thanks again, this is really helpful information and re-assuring.  Haven't yet turned in my old battery for the core refund..weighing whether having a good backup 12VDC power supply outweighs the $18 for the refund

[HeySkippyDog]

Thanks again, this is really helpful information and re-assuring.  Haven't yet turned in my old battery for the core refund..weighing whether having a good backup 12VDC power supply outweighs the $18 for the refund [emoji4]

I keep one around for a backup to my CB base station for power outages when I can't use my AC/DC converter. In the sticks of Florida, we still use it extensively.

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[BlaineBug]

Just a tip, check your ground connections between the battery and chassis.  Specifically where the ground cable meets the frame and/or body.  There is the possibility that there is a layer of undercoating or corrosion between the connections.

[Outdoor 74]

Thanks, but silly question since I am relative newb at this stuff.. is the ground system part of the 'bus bar' (for lack of better term) assembly on the top of the battery?  In other words, there isn't a direct connection, as in another wire aside from the main one, from the (-) terminal post of the battery to the ground, correct?  The reason I ask is I had this autosaver system installed at the dealer when I bought my truck new (to prevent rust corrosion but later found out it really is a ~$800 gimmick) that had a thin wire going to the (-) terminal post of my battery to what I assume was a ground, with the grounded end tucked inside some unreachable plastic/stripping area at the bottom of windshield.  I snipped it off, so cannot replace it.  I would hate to think this is relevant.  

[HeySkippyDog]

Anything they added to "autosave" is irrelevant. Snip away.

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[Outdoor 74]

I completely removed this autosaver system when I installed my new AGM battery since it was a small drain on the battery all the time (small flashing LED) all the time.  JFYI don't ever buy this..turns out it really only works if car is completely submerged in water (as in if your car becomes a boat!).  I didn't realize this until a year or so later.

[Outdoor 74]

5 minutes ago, HeySkippyDog said:

Anything they added to "autosave" is irrelevant. Snip away.

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Thanks SkippyDog, you couldn't be more correct!  I originally thought was a rust treatment paint or something along those lines, not an electronic corrosion preventer..nothing like learning the hard way.

[BlaineBug]

47 minutes ago, Outdoor 74 said:

Thanks, but silly question since I am relative newb at this stuff.. is the ground system part of the 'bus bar' (for lack of better term) assembly on the top of the battery?  In other words, there isn't a direct connection, as in another wire aside from the main one, from the (-) terminal post of the battery to the ground, correct?  The reason I ask is I had this autosaver system installed at the dealer when I bought my truck new (to prevent rust corrosion but later found out it really is a ~$800 gimmick) that had a thin wire going to the (-) terminal post of my battery to what I assume was a ground, with the grounded end tucked inside some unreachable plastic/stripping area at the bottom of windshield.  I snipped it off, so cannot replace it.  I would hate to think this is relevant.  

The ground is the terminal with black wires, closest to the firewall.  There should be a clamp with two wires coming out of it.  One goes to the chassis and the other to the motor.  Check where they terminate for corrosion or bad connections.

[BlaineBug]

Here is some more information for a variety of grounds you could check if you have some time.  It's worthwhile.  Any connection issue has the potential to make an appearance with lower than normal voltage, so, check it out.  Although you didn't have any other symptoms, I still think it's worthwhile and free to research on your truck in your garage.

TSB 18-NA-161.pdf PIT5562.pdf

[Outdoor 74]

12 hours ago, BlaineBug said:

Here is some more information for a variety of grounds you could check if you have some time.  It's worthwhile.  Any connection issue has the potential to make an appearance with lower than normal voltage, so, check it out.  Although you didn't have any other symptoms, I still think it's worthwhile and free to research on your truck in your garage.

TSB 18-NA-161.pdf 2.32 MB · 7 downloads PIT5562.pdf 77.09 kB · 7 downloads

BlaineBug, thanks very much.  This is also extremely helpful.  I will check some of those things, including the posts sticking up above the clamps, just to be sure.  I did have the truck recently fully inspected when I had major service done, so I would hope dealer would have caught anything amiss, but who knows.