AGM won't fully charge from alternator?

[SC Maintenance]

A couple weeks ago I bought a 24F AGM from Walmart - Clarios build. I tested it in Walmart, and my little tester said it was in good shape. I brought it home and put it on my 1A Noco trickle charger in AGM mode. It took about 24 hours to go green even though my tester said it was pretty much charged when I bought it.

I have tested the battery a couple times since, and it seems like my truck's alternator is not fully charging it (2011 Nissan Frontier). It says it has lots of amps - but the voltage is a little low - like around 12.45, and it says its only 80% charged. I presume not a high enough voltage? I do have the smart charger disabled, so its running old school an the alternators voltage regulator - which worked fine for my old lead acid.

Is this normal or not normal for an AGM. My lead acid's - even when bad - would stay fully charged mostly.

I will do a parasitic test, but I did one not long ago when my 4 year old lead acid was getting week and there is literally nothing when everything goes to sleep. Maybe 10mA. So I doubt that's any problem.

So I am wondering if I might have been better off sticking to lead acid in my case - I never had this problem in the past. Never had an issue starting the truck though, and lots of guys on the Nissan forums run these without issue or so they say.

 

[wpod]

Did an AGM battery come as oem?

 

[SC Maintenance]

Did an AGM battery come as oem?

No

 

[LenZee]

No

Run it.

 

[joekingcorvette]

I have an AGM battery in my Corvette and F-150. If I drive the vehicles it takes a while usually overnight for the trickle chargers to show green. In other words the alternator's do not keep the battery as charged up as a trickle charger will. I wouldn't worry about it and I believe your alternator is fine.

 

[ripcord]

What is the voltage at the battery with the car idling? What is it when the engine is rev'ed up? Ideally, you should see 14+ volts.

I believe Nissan's by that era have ECM controlled charging and it under charges the battery on purpose to save fuel. I think there is a mod you can do to force the alternator to charge all the time if you google it.

 

[CKN]

There are smart alternators these days-many will not charge a battery to 100%.

 

[JavierH19]

It's not a problem if it doesn't charge it to 100%. They almost never do anyway.

 

[JHZR2]

12.45 flags something else wrong. While AGM can show higher open circuit voltages when fully charged, 12.45 is really low in all but the coldest conditions.

 

[SC Maintenance]

I believe Nissan's by that era have ECM controlled charging and it under charges the battery on purpose to save fuel. I think there is a mod you can do to force the alternator to charge all the time if you google it.

There are smart alternators these days-many will not charge a battery to 100%.

As mentioned in my OP, my smart charger is defeated - by me - common mod on these and its really not all that "smart" truthfully. Its running completely off the alternators internal voltage regulator old school - has been for years.

The charge voltage varies between 14.2 and 13.2V while driving.

 

[The Critic]

12.45 flags something else wrong. While AGM can show higher open circuit voltages when fully charged, 12.45 is really low in all but the coldest conditions.

Normal at idle for some of the Nissan smart charging systems.

 

[SubLGT]

.......The charge voltage varies between 14.2 and 13.2V while driving.

At what temperature?
Immediately after a start at 30F (battery temperature), charging voltage should ideally be around 14.8 -15.00V, then taper downwards as you drive the vehicle.

 

[mr_boring]

There are smart alternators these days-many will not charge a battery to 100%.

Ford calls it a BMS - battery management system. I turn that off with ForScan and still put mine on a charger at least once a month, preferably more.

 

[ripcord]

13.2v to 14.2v is a bit on the low side but 12.45 volts is pretty discharged for a regularly driven vehicle. You may have a parasitic draw on the battery. I would test for that.

 

[SC Maintenance]

At what temperature?
Immediately after a start at 30F (battery temperature), charging voltage should ideally be around 14.8 -15.00V, then taper downwards as you drive the vehicle.

14.2V may be a little low, but anything over 14.6V will boil a battery. AGM's are worse, because they are sealed, so you definitely don't want to over-voltage them.

Current temps here range from freezing to 60F.

 

[SubLGT]

.........but anything over 14.6V will boil a battery...

Not at a battery temperature of 30F.
Does your voltage regulator have temperature compensation?

 

[ripcord]

That is outright bad information. 14.2V may be a little low, but anything over 14.6V will boil a battery. AGM's are worse, because they are sealed, so you definately don't want to over-voltage them.

This is true at 25C. At freezing, you need to add .75v to the top end charge voltage to be the equivalent to 14.6v. (i.e. 14.6 + .75 = 15.35v)

 

[mr_boring]

My Ford has a parasitic draw in my console shifter sensor (I'm 99% sure that's where it is) but if I engage PARK rather forcefully the battery stays charged. 2-3 times last year mine drew down overnight. One night I camped out in my truck (last fall) and noticed that the cluster only went super dim, it didn't shut all the way off. I slammed doors, I popped the hood, I checked the console and glove box light - nothing. I finally decided that I'd drive it a bit to top it up as I was only going to rack out about 4 hours and when I parked it again the cluster went right off. I was hot until I delved into it a bit.

My girls Navigator is so stupid loaded that her battery draws down in about 3 days. The more electronics with Keep Alive, the faster it drains. And it's all normal.

 

[meep]

With BMS enabled, my ford charges at higher voltage (14.0 - 14.8) during downhills and deceleration, and around 12.6 and 12.4 at cruise, temperature dependent. The oem setting targets 80% state of charge, as set by the factory.

I played with it for a while and eventually defeated it for a standard float voltage of 14.0-14.4.

So, 80% SOC is not unheard of.

Note - I’ve lost somewhere between 0.6-1 mpg since disabling BMS.

 

[ripcord]

With BMS enabled, my ford charges at higher voltage (14.0 - 14.8) during downhills and deceleration, and around 12.6 and 12.4 at cruise, temperature dependent. The oem setting targets 80% state of charge, as set by the factory.

I played with it for a while and eventually defeated it for a standard float voltage of 14.0-14.4.

So, 80% SOC is not unheard of.

Note - I’ve lost somewhere between 0.6-1 mpg since disabling BMS.

Wow, thanks for letting us know your MPG experience. I had no idea it could be so significant.