How the 12 volt battery works on the Volt

[silverton38]

I got this directly from an engineer at Chevrolet.

How is the 12v battery is charged?

The Volt does have a 12V battery, just like any other vehicle on the road. It is used the same way in the Volt as in other vehicles as well - that is to power electronics, computers, and accessories within the car. For example, things like radios, power windows, etc., are all powered off the 12V battery. It is only the propulsion battery & drive motors that are powered by the high voltage battery. The 12V battery is actually not used to start the engine/generator – the vehicle uses the high voltage battery to do this.


When do you know if the 12V battery is dead?

Just like any vehicle, you would know if the 12V battery is dead because nothing would work. Things like power locks, windows, radios, etc., would not function. Although we do not use the 12V battery to start the gasoline generator, it cannot be started either, because all of the computer & control systems in the car are powered from the 12V battery, so if they cannot be powered, then the engine cannot be started.

Does it charge while the vehicle is moving?

Not in the sense that vehicle motion puts energy into the 12V battery, but yes, the 12V battery is charged whenever the car is "on". Again, it helps to compare to a gasoline vehicle here. In a gasoline vehicle, the battery is charged when the engine is running (by the alternator). In the Volt, whenever the vehicle is "running" (i.e. powered on), the 12V battery is being charged, although not by an alternator, but rather by a converter that takes power from the high voltage battery.

If I leave my volt plugged in and leave my lights on will it drain the 12v ?

This depends on whether the car is "on" or not. Again, think of a gasoline car. If you leave it parked, with the engine off, and the lights on, then your 12V battery will be drained. If you leave the engine running, then you will not drain the 12V battery (as the engine will keep it charged). Same is true in a Volt. If the car is "powered off" and you leave the lights on, this will drain the battery. If the car is "powered on" then the battery will be kept charged.

Does the 12 volt slave off of the home charger if needed?

No, it does not. The charger that charges the 16kWh traction battery converts 120V AC or 240V AC into high-voltage DC to charge the Lithium Ion propulsion battery. There is a separate module in the car that charges the 12V battery. This "Accessory Power Module" converts high-voltage DC (from the 16kWh battery) into 12V DC (for the 12V battery). This module is only "turned on" when the car is "running" or powered on. (again, think, just like an alternator in a gasoline vehicle - it is only "on" and producing power when the car is running).

[Nrsk4u]

That's interesting, I wonder how much further I could go on the main battery if I turn the radio and daytime running lights off....

[mikeg3]

One role the 12V battery plays is that no Volt functions, even the power door lock, will operate in a powered off Volt with a fully charged main battery and a discharged 12V battery. Once the Volt is turned on, the main battery supplies power to all Volt systems with no help needed from the 12V battery. The computers, etc., are powered by the 12V bus which is supplied both by the 12V battery and by the Accessory Power Module powered by the main battery.

I found my 12V battery dead one morning while the Volt was still 240V plugged in. The moment 12V was applied to the jumper terminals under the hood, the Volt started and ran normally. The main battery was fully charged, which one would expect since it was 240V plugged in all night. The dealer found no problems except a drained 12V battery.

If the Accessory Power Module is on whenever the Volt is on, then leaving the Volt on and plugged in should never fully discharge the 12V battery.

WOT's theory was that we had left our Volt on all night and drained the 12V battery.

Is silverton38's Chevy engineer wrong or is WOT wrong?

[bonaire]

>> In the Volt, whenever the vehicle is "running" (i.e. powered on), the 12V battery is being charged, although not by an alternator, but rather by a converter that takes power from the high voltage battery.
<<

It seems that with the number of 12V batteries that have been reported going dead over the last year or so, some type of sensible feature should be enabled such that the Volt could turn on this secondary DC power converter to charge the 12V even when plugged into the wall. This is one power management area that may have not been fully engineered to guard against dead 12V's. Yes, you can jump start it with a simple jump-start kit or 2nd battery. But with a full traction battery, having the 12V go dead and leaving you stranded has always sounded a bit limiting whenever someone posts about it.

If you're into the RC hobby, A small 12V LiFePO4 RC-grade pack could be kept around at home or in the car in case it goes dead. You'd think a 4Ah 13V 4-cell pack could be a simple jump-pack. I don't suggest a Li-Poly pack due to its issues with heat and volatility. Of course, there are the consumer-grade jump packs available too which are charged by plugging into the wall. I only suggest the packs above if you know what you're doing with them, otherwise call the auto-service or do a regular car-to-car jump.

[WopOnTour]

I got this directly from an engineer at Chevrolet.

.

Your "Chevy Engineer" has it wrong on a few counts actually (Even if he/she actually is an engineer with GM it's doubtful they have much to do with the Volt IMO)

Q: Does the 12 volt slave off of the home charger if needed?
WOT's A: Actually yes it does (well sort of, but remember the charger is in the car, not in your home). So despite what your Chevy Engineer says, the 12V does actually get an auxillary charge when OFF and plugged in but only while charging is actively taking place.(i.e. steady "ON" green LED on dash)

However charging current DOES NOT come from the DC-DC converter as it normally would when the car is "ON". Instead it comes from a totally separate 12V charger that is built directly into the on-board charging module. So as long as your car is actually charging the Lithium Ion battery (again, steady green LED) then so too is your 12V battery. This is necessary because there needs to be a certain number of 12V modules and devices electrically participating during charging operations and it doesnt really make sense to draw current FROM the Li-Ion source when our primary goal is to charge it, and since the DC-DC converter is only ON when the car is ON, there is a neccesity for this auxillary 12V charger.

Q: If I leave my volt plugged in and leave my lights on will it drain the 12v ?
WOT's A: It's pretty obvious that the 12V battery won't just immediately go dead if the car is on isn’t it? (more on that scenario in a bit) But I'm pretty certain the question was being posed assuming the car was turned OFF and someone had “left their lights on”. (assuming I guess they had been manually turned ON while driving as opposed to using the AUTO feature)
The Volt, like most GM cars actually, is equipped with a battery run-down feature. If you had inadvertently left your lights on (including a map light or other interior light) the system is designed to automatically turn OFF these lamps after approximately 20 minutes. Since a single module (the BCM) is “in charge” of ALL of your interior or exterior lighting this feature is easily accomplished by the ability to detect the switch inputs and software driven timers (Note: in some documentation it will say 10 minutes which is technically true but doesn’t include the potential for 10 minutes in Retained Accessory Power mode or re-entry into another temporary power mode after plugging in )

Now there are a few caveats to this anti-rundown operation as it techncially CAN be manually overridden by turning any switch left ON, to OFF then back ON again. This is a manual override that would indicate to the system you need your light/s on for some emergency purpose and in this case, the lamps will stay ON until the 12V battery is depleted. I suspect a few people have actually been “caught” by accidentally going into this over-ride mode.

As far as your 12V battery not ever being run down while the Volt is "On" that of course depends on how long it will take for the Li-Ion battery to reach a critically low state of charge after it runs out of fuel. (basically do different than of running out of gas idling on a conventional car) It won't be able start the engine in this event, so instead the HV system "load sheds" (to protect the Li-Ion battery from going too deep) and when it hits the hard floor ~15% SOC all DC-DC conversion also ceases (the last load to shed) and since the power mode is still "ON" everything in the car is being powered by the 12V battery eventually it will be dead as a door nail.

HTH
WopOnTour

[Jim Fallston Md.]

This is from the owners manual page 6-6.

Battery Power Protection

The battery saver feature is designed to protect the vehicle's 12‐volt battery. If the exterior lamps or any interior
lamp is left on and the vehicle is turned off, the battery rundown protection system automatically turns the lamps off after about
10 minutes.

So in theory if you leave any of the lights on it will not drain the 12 volt battery. This has been a feature on all GM vehicles since they started adding auto headlamps if I remember correctly.

[mikeg3]

As far as your 12V battery not ever being run down while the Volt is "On" that of course depends on how long it will take for the Li-Ion battery to reach a critically low state of charge. (basically the equivalent of running out of gas idling on a conventional car) It won't start the engine in this event. Instead the system "load sheds" (to protect the Li-Ion battery from going too deep) and DC-DC conversion ceases andsince the power mode is still "ON" everything is being powered by the 12V battery eventually it will be dead as a door nail.

HTH
WopOnTour

This doesn't explain how a Volt's 12V battery can be completely discharged in 12 hours when the main battery is fully charged and connected to the 240V charger (these two facts I guarantee) while the car is on (your theory, not mine).

BTW, our headlight switch is always on AUTO.

[cnicholson]

As far as your 12V battery not ever being run down while the Volt is "On" that of course depends on how long it will take for the Li-Ion battery to reach a critically low state of charge. (basically the equivalent of running out of gas idling on a conventional car) It won't start the engine in this event. Instead the system "load sheds" (to protect the Li-Ion battery from going too deep) and DC-DC conversion ceases andsince the power mode is still "ON" everything is being powered by the 12V battery eventually it will be dead as a door nail.

Is that true about the ICE not firing up when "On" and the main battery has been drained from charging the 12V? I thought in the thread about using the Volt as an emergency power source/generator it was reported that the ICE will kick on periodically to give the main battery a bit of a charge and then shut off as this new charge gets depleted by further charging the 12V. In the example, the 12V power power a load from a connected inverter.

Back to the main discussion. Although an edge case for sure, since the car can "know" that you are on the path to having a disabled vehicle (by watching the voltage on the 12V), wouldn't it be preferable in all cases (even when car if "off" and NOT plugged in) to periodically fire up the 12V charging function (once every 48 hours?) to avoid this bad scenario? Perhaps have the additional rule to not go below 33% SOC (or whatever) in so doing...

[scottf200]

<snip>Yes, you can jump start it with a simple jump-start kit or 2nd battery. But with a full traction battery, having the 12V go dead and leaving you stranded has always sounded a bit limiting whenever someone posts about it.

If you're into the RC hobby, A small 12V LiFePO4 RC-grade pack could be kept around at home or in the car in case it goes dead. You'd think a 4Ah 13V 4-cell pack could be a simple jump-pack. I don't suggest a Li-Poly pack due to its issues with heat and volatility. Of course, there are the consumer-grade jump packs available too which are charged by plugging into the wall. I only suggest the packs above if you know what you're doing with them, otherwise call the auto-service or do a regular car-to-car jump.

See post here on a $27 option that does just that (jump car from lighter/accessory power plug):
http://gm-volt.com/forum/showthread....715#post138715

[scottf200]

From FAQ in my sig:

All about the 12V accessory battery, including how it stays charged, and jump starting the Volt
http://gm-volt.com/forum/showthread....4692#post44692