Source: https://www.chevybolt.org/forum/82-charging-batteries/10290-12v-battery-charging-2.html
Timestamp: 2019-04-24 08:22:46+00:00

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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.
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.
If you keep the car plugged into the portable EVSE or an L2 240V EVSE it will keep the 12V battery charged.
^ it would be easy enough to verify with a voltmeter connected across the 12V battery.
Take reading.... plug Bolt in... take another reading.
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.
Funny. h.e.l.l gets censored. Sigh.
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?
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
It takes corrective actions by adjusting the regulated voltage.
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.
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 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.
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.
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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