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Service battery charging system message

Diagnose and fix a service battery charging system message

Service battery charging system message, generator light on, alternator light on, not charging, what does Service battery charging system mean?

In the old days all cars has a generator and a voltage regulator. Then car makers switch to an alternator and voltage regulator. When solid state came along, car makers started building the voltage regulator into the alternator. Then they decided to let the ECM take over the job of regulating alternator output. Now we’ve entered a whole new realm of power management. First off, alternators are now called generators again—even thought they produce alternating current. And the generator is now controlled by several different components. I’ll describe GM’s new Electrical Power Management (EPM) system. But GM isn’t the only car maker to use a system like this. So if you’re working on a late model vehicle with an electrical problem, you darn well better buy a subscription to an up-to-date shop manual like alldatadiy.com or eautorerpair.net or you’ll be throwing parts at a problem based on out of date information.

Let’s start with the basics.

Car makers want a charging system to do three things: 1) maintain battery charge above minimum levels, 2) lower fuel consumption when electrical generation isn’t really needed, 3) Get the longest life out of the battery to reduce unnecessary replacement. Let’s look at that last issue first.

Late model cars have computers that must remain awake to a certain extent and that means they draw power constantly. To reduce this power consumption when the car is turned off, computers are programmed to go into sleep mode. However, even if all the computers enter sleep mode, they can still drain a battery to unacceptable low levels in as little as 30 days. Once battery voltage gets that low, the lead plates can sulfate. In the old days, you’d simply slap it on a battery charger. But sulfated plates need a special charging technique, and these late model EPM know just how to do that.

So an EPM’s first job is to monitor the condition of the battery and select a charging routine that matches it condition. It monitors the battery’s state-of-charge (SOC) during ignition off and ignition on cycles by measuring open circuit voltage when the battery hasn’t been used for several hours. Once it know initial SOC, it can compute battery needs based on actual current usage, generator output, battery temperature, and battery capacity. Knowing this information, EPM can calculate optimum voltage regulation operation to maintain battery voltage, meet the vehicle’s electrical needs, and extend battery life.

The body control module (BCM) and engine control module (ECM) communicate with one another over a data buss to determine how much field current to deliver to the generator based on electrical needs and current engine RPM. The BCM relies on input from a 3-wire battery 3current sensor. It converts current measurements into a pulsed 5-volt signal to the BCM. How long the pulses stay in either ON or OFF position is referred to as pulse-width-modulation, or duty cycle. Normal duty cycle for the current sensor is between 5-95%. The ECM ultimately controls the field current to the generator, but bases its decisions of data from the BCM.

The ECM issues a generator “turn on” signal and monitors generator performance through the generator duty cycle signal circuit. Once again, this is a on/off pulsed signal and the ECM communicates performance back to the BCM.

Here’s an example of the correlation between duty cycle and voltage output

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

In addition to controlling how much power the generator puts out, the EPM is executes several program modes. Here’s the explanation for each mode.

Start Up Mode

The BCM sets a targeted generator output at startup. The charging voltage is set to 14.3 volts for 30 seconds. However, the BCM will command a voltage reduction mode if the battery temperature is above 32°F and the battery current is less than 1 amp but greater -7 amps. In voltage reduction mode, the ECM targets generator output voltage 13 volts. The BCM commands an exit from this once the following criteria are met for Charge Mode.

Charge Mode

The BCM will enter Charge Mode when all these conditions are met:

• Windshield wipers operate for more than 3 seconds.
• The climate control unit requests increased idle speed due to A/C or Defrost activation in an acceptable temperature window.
• The high speed cooling fan, rear defogger and HVAC high speed blower is activated
• Battery temperature is less than 0°C (32°F).
• Battery State of Charge is less than 80 percent.
• The vehicle Speed is greater than 90 mph
• A current Sensor Fault Exists
• System voltage is below 12.56 volts
• Tow/Haul Mode is enabled

The ECM will command charging between 13.9-15.5 volts, depending on SOC and battery temperature.

Headlamp Mode
The BCM will enter Headlamp Mode whenever it detects the headlight are on. Voltage is regulated between 13.9-14.5 volts

Fuel Economy Mode
The BCM will command Fuel Economy Mode when it measures air temperature to be at least 32°F but less than 176°F, and the battery current is less than 15 amps but greater than -8 amps, and the SOC is greater than or equal to 80 percent. It will target generator output voltage 12.5-13.1 volts. If you test the charging system while it is in Fuel Economy Mode using traditional charging system methods, you will falsely determine that the generator is defective. Do NOT make this mistake. The only way to determine if the system is in Fuel Economy Mode (besides observing the criteria), is to view the current mode on a scan tool. If you don’t know, don’t start replacing parts!

Battery Sulfation Mode

The BCM commands sulfation mode if it sees charging voltage dip below 13.2 volts for 45-mins. The BCM will command charge mode for 2-3 minutes and then determine which mode to enter depending on voltage requirements

In addition to all these charging modes, the old ALT/BAT light has been changed to
“Service Battery Charging System.” The indicator will illuminate under these conditions:

• The ECM detects generator output is less than 11 volts or greater than 16

• The BCM determines system voltage is less than 11 volts or greater than 16 volts.

• The BCM indicates there is a system voltage range concern.

The vehicle will set a trouble code for a charging system fault. Rely on a scan of trouble codes before acting on any charging system tests.


© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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