Dual or Auxiliary Battery using Blue Sea Systems ML-ACR

RyF · June 7, 2019

I know I'm not the only guy who has gone with this setup, but I'm posting this to refresh and refine the topic. My setup is slightly modified, and I also have some questions at the end for others who already went this route as well. I deliberated on this project for quite awhile, because I've had negative experiences with electrical systems in the past. You can do everything right, and in spite of best efforts the vehicle's PCM just doesn't cooperate sometimes.

I initially wanted to make this OEM true using strictly factory auxiliary battery option components. The factory option for the auxiliary battery is "TP2." I ultimately decided, though, that there was a better way to get what I wanted while still keeping it very close to factory and much more cost-effective. The only real choice that needed to be made was how I would connect the two (isolator, solenoid, direct connection, etc). I researched and went back and forth for about a year before I finally chose the marine-grade Blue Sea Systems ML-ACR. The acronym stands for Manual Lever Automatic Charging Relay.

The ML-ACR is proven and very popular in the overlanding community with nothing but great reviews. It offers three settings with a magnetic lever under the hood.

1. Automatic - the batteries are isolated until the alternator provides amperage necessary to begin charging the starter battery. Once this happens, the magnetic button on the lever is activated automatically, connecting the batteries.

2. Manual Off - the batteries stay isolated from one another regardless of amperage provided to the system. This is achieved by turning the yellow lever to its off position. The batteries will never connect while on this setting.

3. Manual On - this connects the batteries even with the vehicle off. The benefit of this option is never needing jumped again. If your starter battery loses voltage required to start the vehicle, simply push the magnetic button in on the yellow lever. This connects the batteries, immediately channeling voltage (~12.5v) from the aux to the starter battery, allowing you to start the vehicle.

There's also a remote switch included that can be located in the cab, but it's use is optional. The remote switch is the typical rectangular auxiliary switch that fits aftermarket panels. I did not use it, because I like keeping my cab factory clean and didn't want to cut anything inside.

Tools needed:

Something to cut copper cable,

Any style terminal crimpers,

Something to shrink heat wrap,

Razor or knife to strip copper cable sheath,

Socket set.

Parts needed:

My model came stock with the aux battery tray, so I only needed the bolt and plastic clamp to retain the battery in that tray.

GM #11519527 (bolt) - $6.23

GM #14005061 (plastic retainer) - $5.49

GM #84043745 (fuse block) This is the same fuse block that clips on the top of your main battery. I wanted to have a few high-amp fuses for my winch connections, and I like that it looks factory. This is not necessary to complete the project. - $58.55

GM #84180633 (fuse block cover) This is the plastic cover that snaps onto the fuse block. - $12.76

Factory-specific battery model #48-AGM. I went with the Diamond model from Battery Source. - $179.99

Blue Sea Systems ML-ACR - $185.45

Ford OEM negative terminal clamp...I just needed something simple. - $12.75

Materials specific to wiring (all from Amazon.com):

10' combo pack of both 1/0 AWG copper red and black welding cable. This is high quality and rated for 300 amps at 25', which is much more than I need. I ended up with about 2' extra of each color when I was done. - $64.80

15-pack of 1/0 AWG marine-grade terminals. I only used six of them for this, but I needed the rest for another project, and it's cheaper to buy in bulk. - $19.61

4' red and 4' black adhesive lined marine-grade 3/4" heat shrink. I only used about 12" of red and 6" of black for this project, but again it's cheaper in bulk. - $13.00

2-pack of Fastronix 3/8" stud red terminal covers for the ML-ACR. - $7.99

20' black fire retardant 1" inside diameter split wire loom. I used this where I ran both pos and neg together across the back of the engine bay. It looks factory. I probably only used about 7' for this project. - $9.97

10' black fire retardant 5/8" inside diameter split wire loom. I used this to cover the individual strands of cable, which was only about 3' total. - $12.95

20-30 black zip-ties. I use zip-ties for everything, so they were already on hand.

My total for materials came to $589.54, but I'm sure I could have cut down on the cost if I hadn't purchased so much overage for other projects. I also used the OEM fuse block and cover, which aren't necessary. Given that, this project can definitely be completed in under $500. That's about 1/3 what any dealership will quote you, assuming they're even willing to retrofit your truck with the TP2 factory option. Most are for whatever reason hesitant to take on the project.

The actual job of connecting everything is pretty simple. I spent less than five hours in all, but it can be done in three hours. I spent a lot of time deciding how I wanted to route wiring and where I wanted to mount the ML-ACR. There aren't many places in the engine bay that make sense. I ended up mounting it on the fuse panel lid adjacent to the aux battery location. I used four mounting screws to do so, and I'm very happy with that location and method. The fastest process to complete the project follows.

1. Clamp the negative terminal on the stud, put the battery in the factory aux location, bolt it down loosely, and clip the optional fuse block on.

2. There's a fuse panel directly adjacent to the battery location. Remove the fuse panel lid and mount the ML-ACR in the center of the fuse panel lid with the terminals facing the body and the yellow switch facing the engine.

3. Route copper cables closely to what you intend to be their final positions. One single length of black cable should be routed from the starter battery neg terminal to the aux battery neg terminal. Two lengths of red cable will be used. One from the starter battery pos terminal to the ML-ACR, and one from the ML-ACR to the aux battery pos terminal. Mark the cables for cutting.

4. Remove the cables from the engine bay, cut, strip, and crimp the cables.

5. Reroute copper cables into their final locations. Use caution when connecting cables to avoid crossing and shorting. Tighten all connections.

6. The ML-ACR has a ground wire that must be grounded to the neg battery terminal on the aux battery.

7. Dress cables with the loom and zip-ties.

8. You can test the system with a multimeter and your dash voltmeter.

- Batteries should be at ~12.5v when the vehicle is off. When isolated, each battery will likely read a slightly different voltage output. When manually connected with the yellow button in (vehicle still off), both batteries should read identical voltages.

- Lock the yellow switch to isolate the batteries and start the vehicle. The voltmeter in the dash should read ~14v. The starter battery should be ~14v when tested with the multimeter. The aux battery should still read ~12.5v.

- Rotate the yellow switch to its automatic position and wait. Revving to 2,500 RPM will expedite this step. It could take as long as 5 min, so be patient. Observe the magnetic push button on the yellow switch, which will automatically retract once the alternator provides necessary amperage.

- Once the push button is automatically retracted, both batteries will read identical voltage at ~14v.

- Shut the vehicle off and immediately move back to the engine bay to observe the push button. It will automatically pop out in approximately 30 sec.

Edited June 16, 2019 by RyF

RyF · June 7, 2019

My question for others who have already completed this setup; have you noticed any abnormal fluctuation with the voltmeter? I had one singular event where the voltage never rose above ~12.5v during a drive that lasted about 30 min. I was concerned that the alternator had failed, so I got home and rechecked the entire system while off and while running. I was unable to recreate the failure. All numbers read correctly while off and after I started back up as well. That failure hasn't happened since, and had I not noticed the voltmeter, there were no other indicators of the failure. Could this have been a momentary issue with the PCM incorrectly modulating power? That's the only explanation I could come up with.

redwngr · June 7, 2019

It's important to remember that these charging systems are not a simple voltage regulator like what used to be typical.

Charging is controlled by computer (BCM iirc).

Charging rate (and voltage observed on gauge) on a long list of items.

Did you come across the documents about this during your research?

RyF · June 7, 2019

8 minutes ago, redwngr said:

It's important to remember that these charging systems are not a simple voltage regulator like what used to be typical.

Charging is controlled by computer (BCM iirc).

Charging rate (and voltage observed on gauge) on a long list of items.

Did you come across the documents about this during your research?

No, I didn't find anything specific to our computers' modulation of voltage. Of all things I could have messed up, this is the most ambiguous and really out of my control. With my limited knowledge, I have no way of knowing why the PCM might act a certain way. Luckily, I only noticed the low voltage that one time. What are my options in the future if this does become a recurring issue? Could a custom tune remedy the problem?

RyF · June 7, 2019

Conveniently, but unfortunately, I just experienced the issue again. The truck had a noticeable strain in 2nd, which was when I looked down and saw the voltmeter at 12.5v. I drove the truck at varying speeds with varying RPMs to troubleshoot. Driving style did not remedy the problem during about 10 min of different approaches. I then turned the radio up, the AC on high, and the heated seats on high. The voltmeter increased to 14v almost immediately.

The first time this issue occurred, I had the ML-ACR in the locked off position. That would mean that the only deviation the vehicle could have detected from the factory configuration was the ground cable from the starter battery to the aux battery. I will detach that ground cable from the neg terminal of the started battery and lock the ML-ACR off. If I drive for the next several days with no issues, then I can surely pinpoint my problem to the PCM. I'm open to everyone's opinions and thoughts here, and I'll definitely appreciate the help.

redwngr · June 7, 2019

1 hour ago, RyF said:

No, I didn't find anything specific to our computers' modulation of voltage. Of all things I could have messed up, this is the most ambiguous and really out of my control. With my limited knowledge, I have no way of knowing why the PCM might act a certain way. Luckily, I only noticed the low voltage that one time. What are my options in the future if this does become a recurring issue? Could a custom tune remedy the problem?

I'm no expert, but I suspect you're going to find that you don't have a problem and that it was a response to the changed load on the charging system. (and how it checks/ estimates the Amp-hour and voltage status)

The system will pop a message if it isn't charging and it thinks is should be.

If it becomes a problem, it's gonna tell you.

Anyway, the whole point of all this is that the charging output is not simply a voltage response to the voltage level that is being read by the gauge on the dash.

gm upfitter documents are a wealth of information.

The docs are on the site by year.

https://www.gmupfitter.com/

Electrical Best Practices' (link is 2018)

- https://www.gmupfitter.com/files/media/photo/842/2018_Elec_BPrac_093017.pdf

Charging system description starts about section 4-1; (page 157).

Description begins about section 4-12, page 168.

Again, this link is 2018, but description should be the same/similar.

https://www.gmupfitter.com/files/media/photo/836/18_Sierra_Body_Builder_2017AUG31.pdf

Here'a just a portion:

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.
• 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, which is integrated to obtain net amp hours.

2018 - Sierra, Silverado Electrical Body Builders Manual
12 V Starting and Charging 4-13
In addition, the electrical power management function is 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, the battery positive voltage circuit, and estimated battery temperature to determine
battery state of charge. The BCM performs idle boost.

Battery Current Sensor
The battery current sensor is a serviceable component that is connected to either the negative or positive 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 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.
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.

If wanting more information, the next section (Electrical Power Management Description and Operation) is the description is a multi-page chart/table showing the possible reasons that the charging system will send a message to the ECM to increase the idle rpm to one of the higher levels.

Idle level steps and load shedding techniques have separate charts for gas and diesel.

RyF · June 7, 2019

Thank you for that wealth of information. It's going to take me some time to get through it all. One thing I notice right away in the reading is that the BCM will enter Headlamp Mode of 13.9-14.5v anytime the headlamps are on. Turning the headlamps on had no result while I was troubleshooting during my last drive.

....In the meantime, I'm going to leave the system connected in hopes of the error occurring again. I'll also keep the multimeter in my truck. When it happens, I'll pull over and check voltage at both batteries with the headlamps on. If it is merely an error in the signal being sent from the BCM to the in-dash voltmeter, then there should still be a ~14v reading at both batteries. If the reading at the batteries is also only ~12.5v, however, then the computer is for some reason preventing the alternator from properly charging the batteries, correct?

Edited June 7, 2019 by RyF

crankman · June 7, 2019

What your seeing is normal operation, if it makes you feel better turn headlights and tow haul button and it will slowly climb up to 14 again. Mine sinks down all the time and I learned how to make it climb up by doing that. Dont overthink it to much its doing what the computer tells it to do. Its a very slow upward climb so give it plenty of time to move to 14v

Edited June 7, 2019 by crankman

RyF · June 7, 2019

I really hope you guys are right. I pay close attention to the gauges, and I have never seen this truck at anything but 14v in the 10,000 mi I have on it. It makes way more sense that the BCM would be more complex with variable output than just 12.5v or 14v with tech what it is nowadays. I just haven't had that experience. Thanks again for contributing.