2013 Sierra running at 12.5 volts

[zedbear]

Several months ago, I bought a new 2013 Sierra w/5.3L. 3 days later, the battery took a dump. They said it was probably defective the moment it was made so the dealer replaced and all was well. After awhile, I noticed the voltage gauge, which usually runs near the middle would sometimes drop down to here:

 

 

I'm not sure if it did this before the battery failure since I only had it for 3 days at that point. Anyways, the needle drops down to the position shown in the pic at random. Doesn't seem to matter if I'm idling or crusing along, low or high electrical load. The other day, I finally got around to plugging in my scan gauge and then drove around town to take care of some business. Voltage showed between 13.5 and 14 volts. At some point during the trip, I noticed the needle had dropped down like in the above pic and voltage showed 12.5 while driving along. I reached my destination and had to idle for a bit with the A/C on and it stayed at 12.5 for about 20 mins of idling and low speed driving and also for about another 10 mins at 55mph. Is this normal for these things? My 2000 never did it but that was 13 years old..............

 

I noticed a thread several lines down from here that asked about low voltage and the A/C cutting out. Mine (auto climate control) also does this sometimes but for the most part, its not to that extent. Seems like when I'm coasting to a stop, it will sometimes blow warm humid air for a few seconds as if the compressor stopped for a moment. A/C performance seems sorta weak in general. In one instance a few months ago, the A/C compressor refused to engage for at least an hour. Is it possible the above problems are somehow related? Its going to the dealer soon but I just wanted to get as much info as I could first. Thanks.

[zedbear]

Had I actually conceived that something like this would have been mentioned in the owners manual, I would have checked. And no, I don't read it front to back but I read most of the areas related to operating the vehicle's systems like the auto A/C and the transmission modes, etc. Makes for good bathroom reading time, lol. I don't need to know about how to use a seat belt or about "highway hypnosis" and such. I will have to check the manual on this tomorrow. My first battery really was defective, though.

[teddyv2]

I have noticed the same thing on my 2014 Suburban on the second day of ownership during daytime driving. When the charging needle dropped to a flat 12v, I figured I was destined for my first dealership visit to check the charging system. I know the manual talks about it, but I would never have thought the BCM control had such a significant role.

 

I bought this Suburban used, so there was no sticker to refer to for actual options. So I went to the dealer parts counter and had them run the VIN to get all the RPO codes on my truck. That's when I realized my new used Suburban had LMG … RPO code for FlexFuel and cylinder deactivation. It wasn't mentioned on the options sticker while on the "preowned" lot, but had I known, I probably wouldn't have traded my 2000 Suburban. So I went to the owner's manual to read up on it, but still haven't found anything at all as to how LMG works. I do know that while looking at the DIC display at slow speeds, that the 8 drops to 4 and shows 32 miles per gallon … something I know I'll never see in reality LOL

 

I like this '14 Suburban and it still provides truck like utility and comfort driving long distances, but I have my reservations on the fact that so much of the truck is governed by damned computers.

[MikeNH]

Pinning this to the top as it is a very common question. Thanks to everyone who contributed!

[daxr]

Good information. I would have jumped to the conclusion that the alternator was faulty, having spent many years (back in the 90's mostly) diagnosing electrical systems. Its a good reminder that modern cars can regulate system voltage down as a mpg-saver depending on conditions.

 

The old rule to check the battery first, and to not diagnose electrical systems with a battery of unknown or weak charge still applies. At my dealership it hasn't come up in my work, but the new "smart-chargers" we have ask the year and model before going into diagnostic mode, and they do charge and test the battery before anything else.

[zedbear]

Interesting, thanks for the info. I had wondered if it were some sort of more closely regulated charging system. Glad I checked here first.

[gmtech4]

normal,,here is some light reading ,,,

 

 

Charging System Description and Operation

Electrical Power Management
Overview

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:

• It monitors the battery voltage and estimates the battery condition.
• 
  
• It takes corrective actions by boosting idle speeds, and adjusting the
  regulated voltage (if equipped).
• 
  
• It performs diagnostics and driver notification.

The battery condition is estimated during ignition-off and during
ignition-on. During ignition-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.

The state-of-charge can be used as a diagnostic tool to tell the customer or
the dealer the condition of the battery. Throughout ignition-on, the algorithm
continuously estimates state-of-charge based on adjusted net amp hours, battery
capacity, initial state-of-charge, and temperature.

While running, the battery degree of discharge is primarily determined by a
battery current sensor (if equipped), which is integrated to obtain net amp
hours.

If equipped with a battery current sensor, the electrical power management
function is also 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.

The Charging System Description and Operation is divided into 3 sections. The
first section describes the charging system components and their integration
into the electrical power management. The second section describes charging
system operation. The third section describes the instrument panel cluster
operation of the charge indicator, driver information center messages, and
voltmeter operation.

Charging System Components

Generator

The generator is a serviceable component. If there is a diagnosed failure of
the generator it must be replaced as an assembly. The engine drive belt drives
the generator. When the rotor is spun it induces an alternating current (AC)
into the stator windings. The AC voltage is then sent through a series of diodes
for rectification. The rectified voltage has been converted into a direct
current (DC) for use by the vehicles electrical system to maintain electrical
loads and the battery charge. The voltage regulator integral to the generator
controls the output of the generator. It is not serviceable. The voltage
regulator controls the amount of current provided to the rotor. If the generator
has field control circuit failure, the generator defaults to an output voltage
of 13.8 V.

 

Body Control Module (BCM)

The body control module (BCM) is a GMLAN device. It communicates with the
engine control module (ECM) and the instrument panel cluster for electrical
power management (electrical power management) operation. The BCM determines the
output of the generator and sends the information to the ECM for control of the
generator turn on signal circuit. It monitors the generator field duty cycle
signal circuit information sent from the ECM for control of the generator. It
monitors a battery current sensor (if equipped), the battery positive voltage
circuit, and estimated battery temperature to determine battery state of charge.
The ECM performs idle boost.

 

Battery Current Sensor (if equipped)

The battery current sensor is a serviceable component that is connected to
the negative battery cable at the battery. The battery current sensor is a
3-wire hall effect current sensor. The battery current sensor monitors the
battery current. It directly inputs to the BCM. It creates a 5-volt pulse width
modulation (PWM) signal of 128 Hz with a duty cycle of 0–100 percent. Normal
duty cycle is between 5–95 percent. Between 0–5 percent and 95–100 percent are
for diagnostic purposes.

 

Engine Control Module (ECM)

When the engine is running, the generator turn-on signal is sent to the
generator from the ECM, turning on the regulator. The generator's voltage
regulator controls current to the rotor, thereby controlling the output voltage.
The rotor current is proportional to the electrical pulse width supplied by the
regulator. When the engine is started, the regulator senses generator rotation
by detecting AC voltage at the stator through an internal wire. Once the engine
is running, the regulator varies the field current by controlling the pulse
width. This regulates the generator output voltage for proper battery charging
and electrical system operation. The generator field duty terminal is connected
internally to the voltage regulator and externally to the ECM. When the voltage
regulator detects a charging system problem, it grounds this circuit to signal
the ECM that a problem exists. The ECM monitors the generator field duty cycle
signal circuit, and receives control decisions based on information from the
BCM.

 

Instrument Panel Cluster

The instrument panel cluster provides the customer notification in case a
concern with the charging system. There are 2 means of notification, a charge
indicator and a driver information center message of SERVICE BATTERY CHARGING
SYSTEM if equipped.

 

Charging System Operation

The purpose of the charging system is to maintain the battery charge and
vehicle loads. There are 6 modes of operation and they include:

• Battery Sulfation Mode
• 
  
• Charge Mode
• 
  
• Fuel Economy Mode
• 
  
• Headlamp Mode
• 
  
• Start Up Mode
• 
  
• Voltage Reduction Mode

The engine control module (ECM) controls the generator through the generator
turn ON signal circuit. The ECM monitors the generator performance though the
generator field duty cycle signal circuit. The signal is a pulse width
modulation (PWM) signal of 128 Hz with a duty cycle of 0–100 percent. Normal
duty cycle is between 5–95 percent. Between 0–5 percent and 95–100 percent are
for diagnostic purposes. The following table shows the commanded duty cycle and
output voltage of the generator:

Commanded Duty Cycle

Generator Output Voltage

10%

11 V

20%

11.56 V

30%

12.12 V

40%

12.68 V

50%

13.25 V

60%

13.81 V

70%

14.37 V

80%

14.94 V

90%

15.5 V

The generator provides a feedback signal of the generator voltage output
through the generator field duty cycle signal circuit to the ECM. This
information is sent to the body control module (BCM). The signal is PWM signal
of 128 Hz with a duty cycle of 0–100 percent. Normal duty cycle is between
5–99 percent. Between 0–5 percent and 100 percent are for diagnostic purposes.

Battery Sulfation Mode

The BCM will enter this mode when the interpreted generator output voltage is
less than 13.2 V for 45 minutes. When this condition exists the BCM will enter
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 ever one of the following conditions are
met.

• The wipers are ON for than 3 seconds.
• 
  
• GMLAN (Climate Control Voltage Boost Mode Request) is true, as sensed by the
  HVAC control head. High speed cooling fan, rear defogger and HVAC high speed
  blower operation can cause the BCM to enter the Charge Mode.
• 
  
• The estimated battery temperature is less than 0°C (32°F).
• 
  
• Battery State of Charge is less than 80 percent.
• 
  
• Vehicle speed is greater than 145 km/h (90 mph)
• 
  
• Battery current sensor fault exists (if equipped).
• 
  
• System voltage was determined to be below 12.56 V

When any one of these conditions is met, the system will set targeted
generator output voltage to a charging voltage between 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 estimated battery temperature
is at least 0°C (32°F) but less than or equal to 80°C (176°F), the calculated
battery current is less than 15 amperes and greater than −8 amperes, and the
battery state-of-charge is greater than or equal to 80 percent. Its targeted
generator output voltage is the open circuit voltage of the battery and can be
between 12.5–13.1 V. The BCM will exit this mode and enter Charge Mode when any
of the conditions described above are present.

Headlamp Mode

The BCM will enter Headlamp Mode when ever the headlamps are ON (high or low
beams). Voltage will be regulated between 13.9–14.5 V.

Start Up Mode

When the engine is started the BCM sets a targeted generator output voltage
of 14.5 V for 30 seconds.

Voltage Reduction Mode

The BCM will enter Voltage Reduction Mode when the calculated ambient air
temperature is above 0°C (32°F). The calculated battery current is less than
1 ampere and greater than −7 amperes, and the generator field duty cycle is less
than 99 percent. Its targeted generator output voltage is 12.9 V. The BCM will
exit this mode once the criteria are met for Charge Mode.

Auxiliary Battery
Charging (TP2)

The auxiliary battery provision (TP2) can be used to supply electrical power
to additional equipment that the customer may choose to add, such as a slide-in
camper or trailer, without discharging the vehicles primary battery. The
auxiliary battery relay closes when the engine is running, in order to allow the
generator to charge the auxiliary battery. The relay opens when the engine is
off, so that the accessories will not discharge the vehicles primary battery,
which is used for engine starting. If the vehicle is equipped with an auxiliary
battery, the relay will be located on the driver's side of the vehicle, next to
the underhood electrical center. Generally, a fuse should not be used in the
STUD 1 Fuse 68 position of the underhood fuse block, if the vehicle is equipped
with an auxiliary battery. A plastic plug may be installed in this position
instead of a fuse. If a fuse is installed in this position, the accessories will
discharge the primary battery in addition to the auxiliary battery.

Instrument Panel Cluster
Operation

Charge Indicator Operation

The instrument panel cluster illuminates the charge indicator and displays a
warning message in the driver information center if equipped, when the one or
more of the following occurs:

 

• The engine control module (ECM) detects that the generator output is less
  than 11 V or greater than 16 V. The instrument panel cluster receives a GMLAN
  message from the ECM requesting illumination.
• 
  
• The instrument panel cluster determines that the system voltage is less than
  11 V or greater than 16 V for more than 30 seconds. The instrument panel cluster
  receives a GMLAN message from the body control module (BCM) indicating there is
  a system voltage range concern.
• 
  
• The instrument panel cluster performs the displays test at the start of each
  ignition cycle. The indicator illuminates for approximately 3 seconds.

Display Message: BATTERY NOT CHARGING SERVICE CHARGING SYSTEM or SERVICE
BATTERY CHARGING SYSTEM

The BCM and the ECM will send a serial data message to the driver information
center for the BATTERY NOT CHARGING SERVICE CHARGING SYSTEM or SERVICE BATTERY
CHARGING SYSTEM message to be displayed. It is commanded ON when a charging
system DTC is a current DTC. The message is turned OFF when the conditions for
clearing the DTC have been met. During cold weather warm-up and extreme
electrical demand , the generator capacity can be briefly exceeded causing this
message to be displayed for up to two minutes

[Jsdirt]

Oh these new computer wonders are more than capable of roasting a battery, I can assure you of that.

 

Since I was 16 years old & first started driving, I have yet to replace a fried battery (except for one in a motorcycle). Wasn't until recently that I owned a vehicle long enough to reach the end of a battery's useful life. Got 12 years out of the Die-Hard battery I bought in Phoenix, AZ for my '72 El Camino, 8 years out of my '94 CBR600F2 battery (took a constant 20v blast from a fried voltage regulator on a 100 mile trip and left me stranded - thank God I found a battery in a store, and was able to get enough juice from just adding the acid to get me home), and I just recently sold my 8 year old OEM Delco battery out of the Silverado for $40. Still going strong thanks to my 3-stage charger.

 

The right charger makes a big difference, especially if the vehicle sits any length of time, and if it has a computer constantly drawing mA ...

[Gone Again]

Doesn't anyone read the owners manual anymore?

[Jsdirt]

My 07 did the same thing at the same time - 3 days into ownership. Dealer took care of it under the bumper-to-bumper warranty and haven't had a problem since.

 

I got the SERVICE CHARGING SYSTEM message after charging the battery, to get the truck to even start. Voltage must've been higher than the system expected. Apparently this system is adjustable by the tech, because after the visit I noticed the average needle position on the gauge to be a bit higher than it had been previously, but it still occasionally rides low on the gauge like the pic there for the reasons explained in prior posts above. Why our trucks didn't leave the factory the way mine is set now, I don't have an answer for.

 

One thing the owners manual states this truck is equipped with that has never worked, is a "low battery warning" system -- so if your listening to your radio at full blast, the system is supposed to kill the radio before your left stranded. Each and every time my radio has killed the battery (while working in the barn), there isn't enough voltage/amperage left to do anything but make the solenoid "click". Just be aware of that if anyone parks at the beach with doors open and stereo cranked.

 

I'm still on the OEM battery at year 6. Every so often, when I think of it, I throw my 3-stage charger on it to freshen it up. So far, so good, but I normally get anywhere from 8-10 years out of batteries of all shapes and sizes here.

[txab]

Nope.

[bobejones]

I feel anyone interested in why GM and other OEMs do this, should seek some reading material on lead acid and AGM battery tech. Battery life and performance are largely affected by how the battery is used and charged. Under charging and over charging a battery is a big problem. One will cause sulfation of the plates, the other will boil the electrolyte and expose the plates.

 

Charge voltage and amperage is a balance based on the battery state of charge. A battery will have full charge at 12.7V and considered depleated at only 12.3V! These values will fluctuate with use but the surface voltage and loaded voltage can all tell you something about the system.

 

You can charge a battery at 15-18V if the DOD (depth of discharge) is low but you pull that when the battery is near full charge and you will boil it. There is a lot involved and I for one and VERY happy to see OEMs include this technology!!! It not only saves batteries, it saves money, hassle, and resources such as lead to get the most you can from a battery.

 

Not very long ago it was common place to see charge systems roast batteries.

[LaserBlueZ71]

I remember when I bought my 2010, I thought the exact same as you. I phoned the service shop the second day I had the vehicle and told them the battery was pooched. They put me on the phone to a mechanic who confirmed these trucks have a smart charging system, it is normal operation.

 

Honestly, I wonder how much difference it makes on fuel economy, if anything at all.