Dual Alternators and Aux Battery Questions

[jeffd]

My 2020 3500 LTZ has dual alternators and aux battery (and plow package). Help me understand (or point me) a few questions.

• What do each of the alternators do/charge? Do they work independently or supply sum charge to both batteries?
• Is one of the batteries always used for starting? Is the other aux only?
• Are they isolated like a normal aux battery set up?

Just looking for an overview of what they do, why and how.

 

Thanks,

Jeff

 

[Tomato]

All of the HD's have 2 batteries that work in parallel - both power the same things.  I am not certain, but I would guess the same for the alternators - both wired to the same system and charge both batteries.

[redwngr]

The alternator output is controlled by the computer.  It is NOT a simple voltage regulator like what controlled alternators for many years. 

 

Output is varied base on many of the items that the computer is able to monitor. 

 

There is a multipage write-up / explanation in the GMupfitter documents.

[jeffd]

I will study the upfitter docs, thank you!

[cdnSpinalTap]

6 hours ago, jeffd said:

I will study the upfitter docs, thank you!

If you happen to find some specific links are relevant documents, would you mind posting them?  I will also take a look and post any relevant links that I find.

[redwngr]

From a post of mine on another forum:  

 

Small section of the info available in the GM upfitter manuals:

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 issent 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 more 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)
• Current sensor fault exists.
• 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.

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.