Change in voltmeter reading with change in battery

[Snagglefoot]

I changed out a suspect AGM battery that would go dead without warning. I recall the voltmeter in the dash read between 14 and 15 V and a digital tester plugged into the power port read 14.7 when going down the highway.

After putting in a flooded maintenance free battery the dash voltmeter reads 14 on the voltmeter, backed up by 14.0 on the tester. Is this an AGM thing?

 

[f355spider]

I changed out a suspect AGM battery that would go dead without warning. I recall the voltmeter in the dash read between 14 and 15 V and a digital tester plugged into the power port read 14.7 when going down the highway.

After putting in a flooded maintenance free battery the dash voltmeter reads 14 on the voltmeter, backed up by 14.0 on the tester. Is this an AGM thing?

What is the year and model of car or truck?
One line searches claim the lower resistance of an AGM battery may cause a "smart" alternator charging system to sense and increase the voltage and conversely on a higher resistance flooded battery, lower charging voltage.

 

[Propflux01]

AGM batteries typically take a small amount of higher voltage to charge properly than a standard flooded battery.. If your vehicle came with an AGM, and you replaced it with a standard flooded battery, your vehicle is probably adjusting for this, because it may be detecting a different battery resistance.

 

[Snagglefoot]

Thanks. It’s just a 2008 Chevy suburban, so it came with an ordinary flooded battery. I put in an AGM afterward. The alternator is an AC Delco that was the trucks 2nd alternator.

 

[D1dad]

Hmm. My wife’s car got an agm after her efb died under warranty. No change in how it charges. An efb is basically a heavy duty flooded and in no way an agm. There’s always the chance, that by coincidence, that your alternator is getting weak. Something to keep an eye on. I’d love to have an alternator that simply kicks out 14 amps continuously, as opposed to the so called “smart charging system” that destroys batteries for the sake of a thimble of gas a day.

 

[bullwinkle]

Thanks. It’s just a 2008 Chevy suburban, so it came with an ordinary flooded battery. I put in an AGM afterward. The alternator is an AC Delco that was the trucks 2nd alternator.

I drive our backup 2019 Express 2500 and it has the wildest swings I've seen from a charging system-15V one time, then normal at ~14, then 12, then 15+ again. I'm assured it's normal, but the 2011 Express (both LQ4 6.0s) sits right on 14 all the time! (both running new 800 CCA floodeds)

 

[jetman]

on the 07+ we have this regulating system (Regulated Voltage Control or RVC) that basically controls the alternator
Alternator voltage gets regulation (smart charger style) by:
There are detailed explanations on how this system works in the shop manual and the GM Upfitter's manuals. Look up the key words EPM (Electric Power Management) and RVC (Regulated Voltage Control).

 

[SC Maintenance]

14.7V continuously sounds like full output - no voltage regulation. Possibly the voltage regulator is fubar and what killed your old battery? The regulator should modulate this value down as the battery gets closer and closer to fully charged.

I monitor mine always in real time and in general when charging its usually just a bit over 14V - and float - ie battery charged but alternator actually running the car is around 13.6V. When I did have the "smart charge" system enabled it would go off completely at times - ie I would see battery voltage at like 12.7V.

You could check the specs in your manual - it should show what the alternator is capable of. 14.7V is a really common max setpoint as anything more will boil a FLA battery. For example here are the specs on one of mine.

 

[D1dad]

14.7V continuously sounds like full output - no voltage regulation. Possibly the voltage regulator is fubar and what killed your old battery? The regulator should modulate this value down as the battery gets closer and closer to fully charged.

I monitor mine always in real time and in general when charging its usually just a bit over 14V - and float - ie battery charged but alternator actually running the car is around 13.6V. When I did have the "smart charge" system enabled it would go off completely at times - ie I would see battery voltage at like 12.7V.

You could check the specs in your manual - it should show what the alternator is capable of. 14.7V is a really common max setpoint as anything more will boil a FLA battery. For example here are the specs on one of mine.

View attachment 321540

How did you disable it on your Nissans? I’ve watched videos but also seen where it throws a code. I monitor mine as well and actually witnessed my amperage set @12.7-12.9 through most of Alabama, north to south. That’s a long time barely charging with my wife working on her MacBook and 2 other passengers charging their phones.

 

[SC Maintenance]

How did you disable it on your Nissans? I’ve watched videos but also seen where it throws a code. I monitor mine as well and actually witnessed my amperage set @12.7-12.9 through most of Alabama, north to south. That’s a long time barely charging with my wife working on her MacBook and 2 other passengers charging their phones.

On mine I just uplugged the current sensor by the battery and wrapped both ends in silicon tape so they don't short. Other guys have cut the wire near the fuse panel under hood. I got a code one time - current sensor or something. Never a CEL. I cleared it and its never come back - on two models same thing.

Reminds me I need to unplug the new to me one also.

 

[Snagglefoot]

I noticed the voltage went up slightly and is constant at 14.3 V at the power port. You can notice the dash voltmeter is slightly higher than exactly 14. Thanks for all the posts.

 

[Nessism1]

Likely related to the batteries state of charge, the vehicle's charging algorithm, and maybe battery resistance.

For older motorcycles, I buy Shindengen SH775 R/R units, clean them up and test on my bike, then resell for a small profit. These units are SERIES type, which shut off excessive current when proper charging is reached, instead of shunting it back to the stator, causing overheat failure.
I've noticed that these units will output 14.0 VDC when my battery is discharged a little, and 14.7 when the battery is fully charged. Would have thought the opposite, but no. Anyway, charging systems will do what the programming tells them to do.