Voltage gauge dropping, then charging, then dropping and staying there? Possible battery?

Pale Rider · March 1, 2022

Hi fellas, been awhile, busy with commuting for work.

A few days ago, the voltage gauge dropped to what looks like 12+ volts 1/2 way home during a 70 mile commute. usually the gauge always sits at 14, or just a hair over 14V.

Today on the way home it did it again. Now in the morning when I leave its a steady 14+ for 70 miles no issues. On the way home today it dropped and stayed there. The gauge will ocassionally come up to 14V but not stay there very long and back down to 12. When I got home I checked the system while the truck was running and saw 12.5 - 12.7 when the gauge was low, and 13.7-14.5 when the gauge was high. With the truck off, the voltage drops steadily from 12.71 down to 12.2

whats happening here, it the alternator having problems or a bad battery?

history, 2019 Used Silverado purchased a few months ago with 3060 miles. truck is 3+ years old original battery. Truck now has 8500 miles and this just started... everything else looks good.

is this from intelligent voltage control or similar?

Thank you.

Edited March 1, 2022 by Pale Rider

Gerry Mitten · March 1, 2022

following

JimCost2014 · March 1, 2022

1 hour ago, Pale Rider said:

Hi fellas, been awhile, busy with commuting for work.

A few days ago, the voltage gauge dropped to what looks like 12+ volts 1/2 way home during a 70 mile commute. usually the gauge always sits at 14, or just a hair over 14V.

Today on the way home it did it again. Now in the morning when I leave its a steady 14+ for 70 miles no issues. On the way home today it dropped and stayed there. The gauge will ocassionally come up to 14V but not stay there very long and back down to 12. When I got home I checked the system while the truck was running and saw 12.5 - 12.7 when the gauge was low, and 13.7-14.5 when the gauge was high. With the truck off, the voltage drops steadily from 12.71 down to 12.2

whats happening here, it the alternator having problems or a bad battery?

history, 2019 Used Silverado purchased a few months ago with 3060 miles. truck is 3+ years old original battery. Truck now has 8500 miles and this just started... everything else looks good.

is this from intelligent voltage control or similar?

Thank you.

The ECM, BCM, battery, and alternator work as a system. The power management system has 6 modes of operation

Battery Sulfation Mode-determines the correct charge protocol to correct plate sulfation condition. The BCM enters this mode if the alternator output voltage is less than 13.2 V for 45 minutes. The BCM will enters Charge Mode for 2-3 minutes. The BCM will then determine which mode to enter depending on voltage requirements.

Charge Mode–The BCM will enter Charge Mode when it detects one of the following conditions:

The wipers are ON for than 3 seconds.

Climate Control Voltage Boost Mode Request) is true, as sensed by the HVAC control head. Ie, you’ve turned on the AC

High speed cooling fan, rear defogger and HVAC high speed blower operation are on.

The battery temperature is less than 0°C (32°F).

The BCM determines that the battery state of charge is less than 80 percent.

The vehicle speed is greater than 90 mph. (No need to save gas at that point)

The battery current sensor is showing a fault

The system voltage is below 12.56 V

When any one of these conditions are met, the system will set targeted alternator output voltage to 13.9-15.5 V, depending on the battery state of charge and estimated battery temperature.

Fuel Economy Mode–The BCM will enter Fuel Economy Mode when the battery temperature is at least 32°F but less than or equal to 176°F, the calculated battery current is less than 15 amps but greater than -8 amps, and the battery state-of-charge is greater than or equal to 80 percent. At that point the BCM targets alternator output to 12.5-13.1 V. to save gas.

Headlamp Mode–The BCM boosts alternator output to 13.9-14.5 V whenever the headlights are turned on.

Start Up Mode–The BCM commands a voltage of 14.5 volts for 30-secs after startup.

Voltage Reduction Mode–The BCM enters Voltage Reduction Mode when the ambient air temperature is above 32°F, the battery current is less than 1 amp and greater than -7 amps, and the generator field duty cycle is less than 99 percent. The BCM targets output to 12.9 V. The BCM exits this mode once the criteria are met for Charge Mode.

Jay P · March 1, 2022

Normal operation. The ECM & BCM controls the alternator output and will lower the system voltage when it thinks it can (not much electrical load). This is just a way to save fuel for fuel economy. This is not new to GM, they have been doing this for years. It's in the owners manual somewhere.

Jay

Edited March 1, 2022 by Jay P

fastk9dad · March 1, 2022

2 minutes ago, Jay P said:

Normal operation. The ECM or BCM controls the alternator output and will lower the system voltage when it thinks it can (not much electrical load). This is just a way to save fuel for fuel economy. This is not new to GM, they have been doing this for years. It's in the owners manual somewhere.

Jay

They also have a TSB for the customer concerns. SMA actually had a video on it yesterday as well.

Pale Rider · March 2, 2022

Thank you everyone, since posting this I have researched, read, and watched on info pertaining to this. You guys said it the best. I understand now what's going on.

Awesome