Dead Battery

2006 Sierra     RVC Charging System

The customer complaint is that the battery keeps going dead and that he has had to call out AAA multiple times to jump start his truck. He also states that he thinks there is something wrong with the charging system since his gauge drops to just a little over 12v at times while driving. Normally he states it is much higher than that.

I grabbed my tech 2 and went for a test drive. This is a capture of the complete data list for the GBCM (generator battery control module) as I was cruising down the road at about 40 mph.

Click here for instructions on how to use the snapshot plotter below.

Here is the schematic for the charging system:

What do we do to make this customer happy?
     

  Part 2

From part one:
The customer complaint is that the battery keeps going dead and that he has had to call out AAA multiple times to jump start his truck.

The battery was tested and it failed a conductance test. A conductance test is different from a load test in that it measures a battery's ability to produce current and does so while at the same time ignoring the state of charge (within certain limits). If a battery fails a conductance test, it is bad. If it passes, it might still be bad since the conductance test cannot stress the battery's internal connections.

So now for the other concern...

From part one:
He also states that he thinks there is something wrong with the charging system since his gauge drops to just a little over 12v at times while driving. Normally he states it is much higher than that.

This is where we need to educate the customer. The truck is equipped with an RVC (regulated voltage control) charging system. In this earlier knowledge base article, some of the purposes of RVC are:

  • Improved fuel economy
  • Extended battery life
  • Increased switch life
  • Enhance lamp longevity
  • Maintain a battery state of charge at or above 80%

Earlier GM charging systems relied on the generators internal voltage regulator to set target output voltage. The decision was based on temperature and load. Fuel economy was lost since the generator was always charging (to some degree). Battery life was degraded since the unnecessary charging causes some of the water in the electrolyte to be broken up into hydrogen and oxygen gases and then to leak to the atmosphere. Higher than necessary voltages also put more stress on switches and lamps.

So now when the battery's SOC (state of charge) is above 80%, target output voltage can be decreased and bring about the savings mentioned above.

It is important to remember that with RVC, although the target output voltage is now controlled externally, the regulator is still in charge of deciding the duty cycle of the applied rotor current. The generator L circuit is an input which tells the regulator what the target output voltage is supposed to be and the F circuit is an output that tells the ECM the duty cycle that the regulator is applying to the rotor fielding (to attain target output voltage) so that the ECM can anticipate engine load.

So let's take a look...

The battery's running state of charge is well over 80%, so there is no need to raise the target output voltage any higher. The voltage is right where it should be. Now keep in mind that the vehicles voltage gauge isn't exactly a precision instrument... In this vehicles case it tended to read a little lower than what the actual voltage is. And hence the customers concern.

All that was wrong with the vehicle was that the battery had a bad cell.

Convincing the owner that there was nothing wrong with the charging system was another matter.. But that's not my job, I left that to the service writer ;-)

Before deciding to use this case study I searched the iatn archives to see what has been covered by others. One of the things that struck me was that there was some disagreement in how to go about testing the generator output. The different operating modes were mentioned, as well as being very sure to follow the service manual procedures.

We really don't need to make it real complicated... Disconnect the regulator wiring. This will cause the regulator to default to a commanded output voltage of 13.8v. Now put your amp clamp around the large generator output wire, raise the engine speed, and use the carbon pile of your choice to drop the battery voltage low enough to squeeze out all the amperage you can from the generator and compare it with the specs in the service manual.

With a new battery installed, I grabbed the following waveforms.

The new battery was only at a 70% SOC, so the system decided to ramp up the voltage to bring it up to speed.

It is important to note that RVC systems do not measure the total generator amperage output but rather only the current that is entering or leaving (charge/discharge) the battery. The schematic below is overly simplified, but I think it makes the point.

Ok, this case study was a warm up for the next one... Please be sure to leave feedback so that I can know what kind of job I am doing. Feedback is anonymous so let it rip ;-)

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