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How is the 12v battery maintained under normal operating conditions ? Is the main battery always connected or only when the car is 'ON' ?
 

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How is the 12v battery maintained under normal operating conditions ? Is the main battery always connected or only when the car is 'ON' ?
Good question and I don't know the answer, but I would assume it only charges when the car is on. It would be sweet if it topped up and maintained the 12v battery whenever the car is plugged in to charge the high voltage battery. Maybe it does? Someone with access to the service manual could find out.
 

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I was wondering the same thing. What is the 12v battery used for, anyway? Things like "legacy" equipment shared with other GM vehicles like fans, AC compressors, lighting, etc.?
 

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I was wondering the same thing. What is the 12v battery used for, anyway? Things like "legacy" equipment shared with other GM vehicles like fans, AC compressors, lighting, etc.?
Everything but the drive motor is 12V DC (radio, instruments, power windows, lights, etc.).
 

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The ancillary system component like the infotainment system, lighting, fans, defroster grids, cameras, basically everything but the traction motor, still run off 12VDC.

IIRC from a Bolt walk-through video- the 250+ DC battery voltage is converted to 3-phase AC for the traction motor, and that 3-phase AC is then stepped down and converted back to DC for the 12VDC circuit charging.
 

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How is the 12v battery maintained under normal operating conditions ? Is the main battery always connected or only when the car is 'ON' ?
The normal operation of vehicles with high voltage battery packs is for the battery pack to be disconnected from the rest of the car by a power relay when the car is turned off. This is to eliminate any possibility of people electrocuting themselves when working under the hood, for example.

This is obviously not true when the car is charging, and it may or may not be true if the car is connected to a charger but not actually charging. But if the car is turned off and not connected to the charger then it's very likely that the high voltage battery is disconnected and not maintaining the 12V battery. Basically just like an ICE vehicle when you turn it off.
 

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You can put the car in "Service Mode" by holding down the start button for 5 seconds without foot on the brake which activates all systems, but not the propulsion system. The manual warns not to leave the car in this mode for an extended time, as it runs solely off the 12V battery. If you run the 12V battery completely down, the manual states that you may not be able to start the car, probably due to the fact that the main computer needs power to run all the startup/safety checks before it connects the high voltage systems.

You can probably make the assumption that this means that the 12V battery charged separately only when the car is actually 'ON'.
 

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Discussion Starter #9
Thanks for all the answers. It would appear that turning the car ON every other week for 20-30 minutes would keep the 12volt battery charged ?
 

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Thanks for all the answers. It would appear that turning the car ON every other week for 20-30 minutes would keep the 12volt battery charged ?
If you keep the car plugged into the portable EVSE or an L2 240V EVSE it will keep the 12V battery charged. No need to turn on the car every couple of weeks or so.

Even if you don't plug the car in all the time, or don't turn it on every so often, the likelihood of the 12V battery going flat after sitting for a month is pretty slim unless you do something like leave a door ajar, or a light on in the car and even then it appears that the car is smart enough to turn the lights off to prevent the 12V battery from getting drained.
 

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Thanks for all the answers. It would appear that turning the car ON every other week for 20-30 minutes would keep the 12volt battery charged ?
If it won't be driven for 4+ weeks, GM recommends hooking up a trickle charger (along with having the high voltage battery at 2-3 bars).
See pg 259 of the owners manual.
 

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Discussion Starter #12
As regards leaving it plugged in - my understanding of charging Li batteries (I also own a Tesla) is they like to stay within a range of 20 percent to 80 percent charged for longest life. If you charge to a higher percentage then you should use the car to 'drain off' some of the charge. If all that is correct then might leaving it attached to my charger (Juice Box) leave it fully charged for a long(ish) period ? I do not know what the parasitic drain is on the 12volt battery but assuming it is like ICE cars then it would likely go flat in about a month or so if it were not charged - hence the idea to turn on the car every other week for a little while.
 

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As regards leaving it plugged in - my understanding of charging Li batteries (I also own a Tesla) is they like to stay within a range of 20 percent to 80 percent charged for longest life. If you charge to a higher percentage then you should use the car to 'drain off' some of the charge. If all that is correct then might leaving it attached to my charger (Juice Box) leave it fully charged for a long(ish) period ? I do not know what the parasitic drain is on the 12volt battery but assuming it is like ICE cars then it would likely go flat in about a month or so if it were not charged - hence the idea to turn on the car every other week for a little while.
The manual states that if you are not going to be using the car for up to 4 weeks to leave it plugged into the EVSE so that the car can automatically run the battery TMS as necessary. Certainly not charging to 100% is a good idea. If you are going to park the car for more than 4 weeks, then you are supposed to drain the main battery down to 3 or 4 bars (15 to 20 percent), and hook up a trickle charger to the 12-Volt battery.
 

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If you keep the car plugged into the portable EVSE or an L2 240V EVSE it will keep the 12V battery charged.
What is your source for this statement? (I'm not saying you are wrong, I just want to know where you got the info.)

I happen to know that the LEAF definitely did NOT keep the 12V accessory battery fully charged if the car was plugged into an EVSE but not charging - in fact it drained the accessory battery more quickly as there was more activity on the CANbus, which used up more electrons from the 12V battery. (The 'keep the 12V battery charged' logic in the LEAF was seriously brain-dead.)
 

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^ it would be easy enough to verify with a voltmeter connected across the 12V battery.
Take reading.... plug Bolt in... take another reading.
 

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More like : "open and prop up hood so that opening car door(s) doesn't wake up car or its computer ... plug in vehicle ... Fully charge up main battery. unplug vehicle (leave hood up). Wait 3 hours. Take reading on 12V battery ... plug Bolt in... make sure car isn't charging ... take another reading (and then additional readings every few minutes to make sure car isn't doing something weird, for about 30 minutes of 'no change') ... leave hood up ... come back every 6-10 hours and check again over the next day or two"

The above is pretty close what you had to do with the LEAF to figure out what was going on if all you had was a simple voltmeter. Opening a door woke up the car and put a load on the 12V battery, for example. One guy left a voltmeter with data recorder connected over the course of several weeks (when not driving it) to get a graph of the bizarre behavior going on with the battery.

After babysitting a friend's LEAF for a couple of weeks (and finding the 12V battery dead after the car was sitting unused for 4 days) I spent quite a few days charting and trying to analyze the charge/usage/drain pattern of the 12V in my Spark EV. The algorithm in the Spark seems a **** of a lot better than the one in the LEAF. Driving it puts about 13.6V into the battery, for example. When fast charging, 13.4V is fed into the accessory battery. (I still bought a $50 battery trickle maintainer for my Spark, which I hook up overnight 2 or so times a month.)
 

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Funny. h.e.l.l gets censored. Sigh.
 

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The 12 volt chassis battery is charged while the vehicle is in run mode as well as while the HVB is charging.
 

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The 12 volt chassis battery is charged while the vehicle is in run mode as well as while the HVB is charging.
But NOT charging when the HVB is fully charged and the vehicle is plugged into an EVSE?

Do you happen to know the charging profile(s)? (Say, 13.6V whenever the vehicle is in run mode, 13.2 when the vehicle is charging?)

Thx.
 

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Charging System Description and Operation
12 V Battery The following information is for the 12 V battery only.
For information about charging the high voltage drive motor batteries, refer to Drive Motor Battery System Description.
Charging System Operation The purpose of the charging system is to maintain the battery charge and vehicle loads. The main difference between a conventional generator charging system and this system is that the generator has been replaced by the 14V Power Module. The 14V Power Module provides the power to charge the battery from the high voltage system. There are 7 modes of operation and they include:

  • Battery Sulfation Mode
  • Normal Mode
  • Fuel Economy Mode
  • Headlamp Mode
  • Voltage Reduction Mode
  • Battery Maintenance Mode
  • Plant Assembly Mode

Charging System Components 14 V Power Module
The 14V Power Module provides the power to charge the battery from the high voltage system. The ECM provides a pulse width charge request signal (L Terminal) to the 14V Power Module. The 14V Power Module returns a PWM feedback signal (F Terminal) to the ECM. The 14V Power Module reports status and fault modes as a function of duty cycle. The 14V Power Module charges the battery based on the ECM signal.
Drive Motor/Generators
The drive motor/generators are serviceable components located within the transmission housing. When the rotors are spun, an alternating current (AC) is induced into the stator windings. This AC voltage is then sent to the drive motor generator power inverter module (PIM) where it is converted to high voltage direct current (DC) power. The output of the PIM is converted into low voltage electrical power by the accessory DC power converter module 14V Power Module for use by the vehicle's electrical system to maintain electrical loads and battery charge.
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 operation. The BCM determines the desired voltage set point and sends the information to the engine control module (ECM) which sends this information to the 14V Power Module. The BCM monitors a battery current sensor, the battery positive voltage circuit, and estimated battery temperature to determine battery state of charge.
Battery Current Sensor
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 V 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)
The ECM receives control decisions based on messages from the BCM as well as the HPCM2
Instrument Panel Cluster
The instrument panel cluster provides a means of customer notification in case of a failure and a voltmeter. There are 2 means of notification, a charge indicator and a driver information center message of SERVICE BATTERY CHARGING SYSTEM.
Hybrid/EV Powertrain Control Module 2 (HPCM2)
The Hybrid/EV Powertrain control module (HPCM2) is a GMLAN device. It communicates with the engine control module (ECM) to control the voltage set point sent to the 14V Power Module during the battery maintenance mode.
Battery Sulfation Mode Battery sulfation mode is used to help maintain the battery life. The charging system will enter a battery sulfation mode which tries to increase the vehicle charging when the charging system voltage is less than 13.2 V for about 30 minutes. Once in this mode, the BCM will set a targeted output voltage between 13.9–15.5 V for about 5 minutes. Following this 5 minutes, the BCM will then determine which mode to enter depending on the system voltage requirements.
Normal Mode The BCM will enter Normal Mode whenever 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).
  • Vehicle Speed is greater than 145 km/h (90 mph)
  • Current Sensor Fault Exists
  • System Voltage was determined to be below 12.56 V
  • Tow/Haul Mode is enabled
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 ambient air temperature is at least 0°C (32°F) but less than or equal to 80°C (176°F), the calculated battery current is greater than −8 A but less than 5 A, and the battery state of charge is greater than or equal to 85 percent. Its targeted 14V Power Module set-point voltage is the open circuit voltage of the battery and can be between 12.6–13.2 V. The BCM will exit this mode and enter Normal Mode when any of the conditions described above are present.
Headlamp Mode The BCM will enter Headlamp Mode whenever the high or low beam headlamps are ON. Voltage will be regulated between 13.9–14.5 V.
Voltage Reduction Mode The BCM will enter Voltage Reduction Mode when the calculated battery temperature is above 0°C (32°F) and the calculated battery current is greater than −7 A but less than 1 A. Its targeted 14V Power Module set-point voltage is 12.9–13.2 V. The BCM will exit this mode once the criteria are met for Normal Mode.
Battery Maintenance Mode That battery maintenance mode is designed to ensure the 12V battery has a good state of charge. It accomplishes this by checking the voltage of the 12V battery and providing a charge if needed.
When the vehicle cord is plugged in
  • The Hybrid/EV Powertrain control module (HPCM2) will check the 12V battery every 6 hours if the ignition is off. If the voltage is below a temperature dependent threshold ranging from 12.1 (cold) to 12.4 (warm)V, the Hybrid/EV Powertrain control module (HPCM2) will send the voltage set point to the engine control module (ECM). The engine control module (ECM) will send this to the 14V Power Module. Battery maintenance mode will charge the battery for 2-3 hours. If the Ignition is ON, the APM will cycle on as needed to maintain the 12V SOC.
When the vehicle cord is not plugged in
  • The Hybrid/EV Powertrain control module (HPCM2) will check the 12V battery every 4 days (2.5 to 3 days) and if the voltage is below a threshold of 12.0 may activate battery maintenance. If the high voltage battery state of charge is greater than 40% and the propulsion system is not active, Hybrid/EV Powertrain control module (HPCM2) will send the voltage set point to the engine control module (ECM). The engine control module (ECM) will send this to the 14V Power Module. Battery maintenance mode will charge the battery for 45-90 minutes..
Plant Assembly Mode The BCM will increase charging voltage for the first 500 miles of operation in an effort to ensure that the 12 V battery is fully charged when the vehicle is delivered to the customer.
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. This electrical power management system primarily utilizes existing on-board computer capability to maximize the effectiveness of the charging system, 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 adjusting the regulated voltage.
  • It performs diagnostics and driver notification.
The battery condition is estimated during Vehicle OFF and during Vehicle in Service Mode. During Vehicle 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. During Vehicle ON mode, 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.
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.
Instrument Panel Cluster Operation Charge Indicator Operation
The instrument panel cluster illuminates the charge indicator and displays a charging system warning message in the driver information center when the one or more of the following occurs:

  • The engine control module (ECM) detects system voltage less than 11 V or greater than 16 V. The instrument panel cluster receives a GMLAN message from the ECM requesting illumination.
  • The BCM determines that the system voltage is less than 11 V or greater than 16 V.
  • The instrument panel cluster receives a GMLAN message from the BCM indicating there is a system voltage range concern.
  • The instrument panel cluster performs the displays test at the start of each Vehicle ON cycle. The indicator illuminates for approximately 3 seconds.
  • Vehicle ON, with the engine OFF.
Battery Voltage Gauge Operation
The instrument panel cluster displays the system voltage as received from the BCM over the GMLAN serial data circuit. If there is no communication with the BCM then the gauge will indicate minimum.
This vehicle is equipped with a regulated voltage control system. This will cause the voltmeter to fluctuate between 12–14 V, as opposed to non-regulated systems which usually maintain a more consistent reading of 14 V. This fluctuation with the regulated voltage control system is normal system operation and NO repairs should be attempted.
SERVICE BATTERY CHARGING SYSTEM
The BCM and the ECM will send a GMLAN message to the driver information center for the SERVICE BATTERY CHARGING SYSTEM message to be displayed. It is displayed whenever the charge indicator is commanded ON due to a failure.
 
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