On a 2026? Scenario is that I get big voltage differences and don't understand why.

Example:
83 MPH........12.7 indicated on dash
Parked idling.....14.4
45 minutes later cruising 66 MPH..... 14.3
20 minutes later: 13.4 at 70MPH

Just doesn't look normal to me, any ideas?

 

C8 charging is controlled by the ECM and adjusts output accordingly so it will vary. There are many vehicles like this, including our 2007 Cadillac DTS.

Is there something else you are trying to understand?

Here's a previous thread...check out post #3 from RichieRichZ06

https://www.corvetteforum.com/forums...g-voltage.html

 

It's normal ... see attached PDF for detail on how it works.

Here's a quote from the attached document:

"The electrical power management system is designed to monitor and control the charging system and send diagnostic messages to alert the driver of possible problems with the battery and generator. This electrical power management system primarily utilizes existing on-board computer capability to maximize the effectiveness of the generator, to manage the load, improve battery state-of-charge and life, and minimize the system's impact on fuel economy. The electrical power management system performs 3 functions:

1. Monitor the battery voltage and estimate the battery condition
   
2. Take corrective actions by boosting idle speeds, and adjusting the regulated voltage
   
3. Perform diagnostics and driver notification
   

The battery condition is estimated during ignition/vehicle off and during ignition/vehicle on. During ignition/vehicle off the state-of-charge of the battery is determined by measuring the open-circuit voltage. The state-of-charge is a function of the acid concentration and the internal resistance of the battery, and is estimated by reading the battery open circuit voltage when the battery has been at rest for several hours.

Any time the ignition/vehicle is on, the vehicle algorithm continuously estimates battery state-of-charge based on adjusted net amp hours, battery capacity, initial state-of-charge, and calculated temperature.

While the engine is running, the battery degree of discharge is primarily determined by the integrated battery current sensor, to obtain net amp hours.

In addition, the electrical power management function is designed to perform regulated voltage control to improve battery state-of-charge, battery life, and fuel economy. This is accomplished by using knowledge of the battery state-of-charge and temperature to set the charging voltage to an optimum battery voltage level for recharging without detriment to battery life."

 

your numbers are fine think of it as demand and supply amperes wise

 

Thanks everyone! Just used to older cars where the voltage was always in the 14V when running. The PDF was very helpful! What I do find interesting is that after extensive driving yesterday I plugged in the GM battery tender last night since the car will be sitting for several days, and this morning it is still showing a yellow charging light and hasn't gone to green yet.

 

The normal voltage range with the engine running can be anywhere between 12.5v and 15.5v (as noted in the attachment in an earlier post), and will fluctuate considerably,

GM makes voltage one the available info tiles, but provides no guidance in the Owner's Manual about how to interpret the voltage reading. The owners manual does not even indicate the voltage range for normal operation. Instead, it says this about that gauge:

"Battery Voltage: Shows the current
battery voltage."
"Use the Battery Voltage info tile to determine
if your battery is low and needs to be charged."

I don't normally even select voltage as one the info tiles on my DIC. You cannot use it to tell if your "battery is low", unless you turn the ignition on in service mode with the engine off. And under that condition, the car will be drawing at least 10 amps from the battery (I measured that) because all of the systems and modules are fully awake - so you have to have some idea of what to expect for a normal battery SOC with a 10 amp draw. I have a new FLA battery in my 2021, installed 3 weeks ago. This morning, it is reading 12.1 volts on the DIC gauge with the car in service mode (engine off) and the maintainer disconnected for the measurement.

A while back, using the info in the Service Manual, I made a graphic representation of the normal voltage ranges for each of the charging modes that can be selected by the Body Control Module (BCM) - it shows the min voltage and maximum voltage for each charging mode, and the conditions for selecting that charging mode. I did it a while back, but I believe it reflects what is stated in the attachment in a previous post. I share this only to illustrate the factors that affect the voltages we see when the engine is running.

 

Andy,

Thanks!!!!! I had only put the gauge up as a couple of weeks ago I got a low battery indication and was surprised as the car had only been sitting a few days. So I charged up the battery and decided to monitor it to see if it showed anything suspicious. Now that I have been educated I'll take it off and go back to something else.

 

I did a test of resting voltage this afternoon.
2024 SR HTC Z51 - chassis battery was replaced last summer with a new GM (AC Delco?) version AGM by the dealer.
Normal voltage on the DIC is like everyone else's, all over the map depending on load and conditions.
So, I parked the car in the garage, frunk lid aloft, plugged in my little cigar lighter voltage tester, and had dinner.
About a half hour and a bit later I went to check the results after (I assume) I'm in sleep mode or whatever we're calling it.
12.8V on the meter.

FWIW?

 

I like it! That is a great way to get a relatively meaningful resting voltage measurement. I'm gonna guess that since it is showing only one decimal place, its rounding up - so your actual voltage is between 12.75 and 12.85 (subject to some measurement error). And based on the attached chart, for an AGM battery, would put the SOC between 92% and 94% (interpolating between 90 and 100), give or take - since considering measurement errors. Looks good I think.

 

Just trying to help. I will add I drove it for about an hour and a half prior to the testing, mostly around town.
I think it supports your hypotheses.

 

FWIW, since the C7 GM like many other newer cars, controls the Lead Acid battery charge to save energy! No longer the old "Voltage Regulator" all like most things in the C8 computer controlled.

To simplify, in the daytime, no rain wipers off, the battery will ONLY charge to 80%.

I monitored my 2020 C8 for 4 years. Retired, although it's my DD only have to drive a few days each week BUT most drives are ~60 mile round trip to town.
If car sat for ~4 days it would start reading about 14 volts on the dash voltmeter (actually reading alternator voltage) as it brought the battery back to 80%. Could take most of the ~30 miles trip to town.

But for sure on the way home, once the battery reached 80% the voltage control computer would cause a STEP change to about 13 volts. That was enough to run the car's electrical needs, AC Fans, sound system etc BUT not enough to further charge the battery! It was at its Max 80% charge.

SIDEBAR
BTW my E-Ray does the same with it's Li-Ion 12 volt battery. In that case the alternator charges the battery to 80% for a Li-Ion, 13.3 volts.
When i start it may may be 13.4 or 13.5 volts BUT in that case within 5, max 10 miles it drops to 13.3 volts (80% charge.)

 

As you know, your owner's manual give absolutely no clue about how to interpret those voltages. The voltage can be anywhere between 12.5 and 15.5, depending on the load, battery soc and accessories and lights that may be active. You can verify that its within that range (the owners manual does not even tell you that but the service manual does), but I think trying to interpret what is going in in the range is futile - going down a rabbit hole. I do not ever display the battery voltage tile except when questions about it come up on the forum.