What exactly does the 12V battery do?

[CarZin]

It is my understand the 12V battery is used to basically start the car, but at that point, the 12V accessory high voltage DC->12V DC power module is actually what powers the accessories as well as keeps the 12V battery charged. It essentially acts as an electric alternator to provide current to the 12V circuits, keep the 12V battery charged up, and provide the real energy for system components.

is this an technically accurate description?

[Skotty]

I think it would be more typical to say the 12V battery is what powers the accessories. I think how the accessories are powered is no different than a typical car, with the exception of what is doing the charging (be it either an alternator or a power inverter). So if you would say the 12V battery in a typical car powers the accessories, then the 12V battery in the Volt also powers the accessories. Likewise, if you say that the power inverter is what powers the accessories in the Volt, then you could say it is the alternator that powers the accessories in a typical car. But that would only be true while the car is on; when the car is off, you can only say the 12V battery will power the accessories.

One thing that is different and worth pointing out to anyone who might not be more knowledgeable, is that when you say "start the car", what you mean is basically powering up the control systems. However, it is the main traction battery and not the 12V battery that is used to start the engine if it is needed. You probably already know this, but it is something those less familiar with the Volt usually do not, so I like to point it out.

[scottf200]

Summary of the related APCM.

Accessory DC Power Control Module Description and Operation

Location
The 14 V Power Module, also called the Accessory DC Power Control Module, is located in the rear trunk compartment underneath the floor stowage trim panel. It is attached to the chassis with four fasteners and connected to a plastic cooling duct. The 14 V Power Module has a high-voltage pigtail that passes through the floor of the rear compartment and connects to a high voltage cable that comes from the hybrid battery pack. Also connected to the 14 V Power Module are the 12 V cables + positive and - negative, and a signal connector for serial data.

Operating Functions
The 14 V Power Module is an electronic device that takes the place of the generator on a traditional vehicle. On a hybrid or electric vehicle the 14 V power module converts high voltage (300 V) direct current (DC) to low voltage (12 V) DC for accessory electrical operation and to charge the 12 volt battery.

Normally, the 14 V power module only supplies 12 volts DC when the vehicle is under a normal drive cycle. However there are certain instances when the vehicle is being charged through a standard wall socket where the14 V power module operation is required to sustain the 12 volt battery.

The 14 V power module is capable of supplying up to 180 Amps of 12 volt DC.

[vdiv]

Thanks Scott.

Only in GM land a 14V Power Module would convert 300V from a 360V battery pack down to 12V...

180A at 12V is an impressive power, I suppose peak, not sustained.

[DCFusor]

I've tested the inverter I hooked to my 12v battery to 1kw, sustained (about 80a at 12v). But that's iffy usage.
I think that battery is also there to handle real nasty peaks when things like hydraulic pumps (brakes) etc have a big startup surge. If one of those happened while my inverter was also in use, I'd fear burning something up. Hopefully they have a smart current limit in the APU...

[CarZin]

Thanks guys!