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Discussion Starter #1 (Edited)
While I was at the SF Unplugged test drive event I ask one of the GM reps what was under the door in the hatch area that was labeled Battery. He said it was the 12vdc Aux battery but did not know any details on how it was charged. I asked several times for a session with the Engineering Tech but he was never available or within sight. I assume there is a separate inverter that charges it from the Li battery.

Does anyone know what loads the 12vdc battery supplies and where the charging power comes from?

Norm
 

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Does anyone know what loads the 12vdc battery supplies and where the charging power comes from?

Norm
I've gotten curious about that lately as well. If the aux battery doesn't drain current during CD mode, does that imply aux battery loads don't affect AER? For example, if the radio runs on aux, you can crank the tunes without reducing EV range? Could that explain the difference between eco and comfort mode on the HVAC? Do DRL and indicator LEDs run on aux?

The "little" battery may be a little sleeper...
 

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Funny, I just wrote up the explanation below in response to a similar question on ChevroletVoltage.com ... (dont' bother going there, there really is FAR more activity here, and anything new posted there first usually shows up here within hours anyway)

Some of this comes from a WopOnTour post here a while back.

First, some background. Hybrid vehicles like the Volt and Prius have two separate battery systems: the big, high voltage "traction" battery and the small, low voltage "accessory" battery. The traction battery obviously provides the power to move the car, but it also provides the power to start the gas engine.

During operation, the 12 Volt battery's voltage is maintained by the "accessory power module" (APM) whenever the Volt is "ON", and maintained by the main battery charger assembly (On-Board Charging Module) when the Volt is plugged in and charging is ACTIVE (steady green LED). The APM is a DC to DC converter that takes high voltage (380V?) from the Volt's traction battery and converts it to ~13.0-15.5 Volts in order to maintain the low voltage accessory loads (including the Volt's computers and modules). It also charges the 12V system's battery, also know as an "absorbant glass mat" (AGM) battery. The 12V battery's voltage is maintained when the car is running or charging (by the APM or charger, respectively), but not when parked and unplugged.

Both the APM and the 12V auxiliary battery are located in the rear of the vehicle under the removable rear hatch compartment floor.

Someone had asked a question about jumping the Volt. The scenario that you'd need a jump for is when you've somehow managed to drain the 12V battery (e.g. leaving lights on). In that case, you just need to connect to the 12V battery and give it a little juice, to literally boot up the brains so that the car will start up.

Even if it wasnt (plugged in), you wouldn't be able to drain the 12V battery by leaving a door open, map/dome light on, or even headlights on as after approximately 10 minutes the Body Control Module (BCM) will shut down the various fused feeds to these devices via the inadvertent power relay (aka battery run-down protection).

The Volt is also equipped with a "Battery Saver" mode used for periods of storage. The battery saver mode reduces the parasitic load of some of the electronic modules during long shipment or during vehicle storage conditions. This improves the drain time on the battery (up to 70 days without the battery going dead). (more on this here)

You can't jump the car if it has a dead traction battery (the HV battery), but it's nigh impossible to run that battery dead anyway. The accessories don't drain the traction battery when the car is parked, and the car won't let you drive it until the traction battery is completely dead -- it stops at 25% SOC (we think) and certainly well before 0% SOC.

The Volt manual has quite a bit of detail on A) jump starting the Volt and B) using the Volt to jump start another car. See pages 10-72 through 10-76. Note that the 12V battery is in the rear of the car, and you can attach the jumper cables there, but for convenience there are also jump connection points in the front of the car, as shown in this post. However you should only use those front points when jumping the Volt; if jumping another car use the rear points. This is probably because of a lighter gauge wire used to feed the front points.
 

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. I believe it only does this (charges the 12V battery) when the car is running, but I'm not sure.
Good post.
FYI The On-Board Charging Module (not the APM) maintains the 12V battery but only during "active" charging. This is because numerous charging electronics that operate off the 12V system, including the HV contactor relays that connect the charger output to the 360V battery during charging sessions. Without OBCM output, the 12V sytem would be drained and like you said results in a "No Go" scenario.
HTH
WopOnTour



EDIT: The above 12V charging behavior is currently correct for 2011 and 2012 models only. For 2013 and newer Volts a new "12V Battery Maintenance Mode" has been added to better deal with keeping the 12V battery SOC up during OFF - NON-CHARGIING periods fro up to 30 days.
For more information see my post #73 later in this thread- WOT
http://gm-volt.com/forum/showthread.php?5409-Auxillary-Battery&p=1292514#post1292514
 

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Thanks WOT! I've updated my post above with that info.
 

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This sounds very similar to the Prius. The 12-volt battery is used for the typical vehicle electrical loads such as lights, stereo, etc. as well as the computers. With OnStar there is going to be some continuous low-level drain for the cellular modem to allow remote unlocking, remote start, etc. but this should be quite low drain all things considered.

The bad news is that this battery is smaller than a conventional car battery in terms of capacity which means that it can be run down by leaving a dome light on much faster. The good news is that without a starter motor to crank, you can jump-start it relatively easily from a lantern battery or the like as you just need enough juice to boot the computers and kick in the contactor to the traction battery. Then the DC/DC converter fires up and supplies the 12-volt loads.
 

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With OnStar there is going to be some continuous low-level drain for the cellular modem to allow remote unlocking, remote start, etc. but this should be quite low drain all things considered.

The bad news is that this battery is smaller than a conventional car battery in terms of capacity which means that it can be run down by leaving a dome light on much faster.
Figuring that a modern cell phone (which is basically what OnStar is) can last a week on a 2.5 Wh battery (and much better, if OnStar has a longer slot cycle - which it should), I don't think the phone drain's a big deal.

What *is* the capacity of the Volt auxilliary battery? A local (at work) mailing list notes the Prius as having a 36 AH (430wH) battery. The concern there was somebody left their dome light on today, and it was worried that the battery would be dead by the end of the day...
 

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I believe it is 70ah but will try to verify. In any case it has a parasitic drain managment system that would prevent someone from killing the battery with the dome light. :D
WOT
 

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I believe it is 70ah but will try to verify
Hmmm.... If I read it right, wiki says standard lead-acid tolerates an 80% discharge, while AGM prefers 50%. So that would make the Prius aux have ~350 wH capacity and the Volt ~420 wH. So about the same, with the added advantage that the Volt battery is basically in the passenger compartment and not directly exposed to the elements.
 

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Hmmm.... If I read it right, wiki says standard lead-acid tolerates an 80% discharge, while AGM prefers 50%. So that would make the Prius aux have ~350 wH capacity and the Volt ~420 wH. So about the same, with the added advantage that the Volt battery is basically in the passenger compartment and not directly exposed to the elements.
Actually the Prius aux battery is in the passenger compartment as well. Plus keep in mind the aux battery really isnt subjct to any levels of deep discharge as essentially it's SOC is constantly being maintained to the upper levels by the APM when "ON" and by the main battery charger (OBCM) whenever plugged in and charging.Periods where the vehicle's 12V rail is relying soley on the 12V AGM battery are normally relatively short transitional periods during "power ON".
WOT
 

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Plus keep in mind the aux battery really isnt subjct to any levels of deep discharge as essentially it's SOC is constantly being maintained to the upper levels by the APM whenever plugged in or "ON".
Really? Even in CD mode if it's fully charged, they transfer energy from the motive battery to the aux battery? I would think they'd let it drain to "aux customer empty" before they recharged it in CD mode. CS is fair game (and obviously when plugged in).

Interesting that the Prius (which I've never much looked in to) has the aux in the cabin. Makes me wonder why Nissan put the Leaf's (rather large) aux under the hood.
 

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Really? Even in CD mode if it's fully charged, they transfer energy from the motive battery to the aux battery? I would think they'd let it drain to "aux customer empty" before they recharged it in CD mode. CS is fair game (and obviously when plugged in).
The charging scheme is much like any other 12-volt lead-acid automotive battery. Not a lot of attention given to SoC. The charging system (DC-DC converter in Volt and Prius, alternator in conventional cars) operates in a float voltage mode. It has a voltage regulator that keeps the 12-volt bus at a fixed voltage up to some current limit. Typically 2.3 volts per cell depending on battery chemistry, or 13.8 volts on a 12-volt system. As electrical loads such as headlights increase, the alternator or converter increases its current output to maintain the float voltage. At this voltage, the battery will consume current to charge itself if discharged and just "float" across the line when fully charged, neither charging nor discharging. Should electrical loads exceed the capacity of the charging system (think subwoofer in several cars in my neighborhood!), the battery will discharge and then recharge when the load is removed.
 

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Auxiliary Battery Information Display - Where is it?

I had a chance to talk with a GM representative about the Volt's auxiliary battery at an open house in Gaithersburg, MD yesterday and was surprised to hear that without this 12 volt lead-acid battery the Volt won't run. After I got home I found this thread and the discussion verifies that statement along with a lot of other useful information about how the auxiliary battery fits into the Volt's system (one of the many benefits of the gm-volt forums). Since this is a fairly critical part of the Volt's system is there any way to monitor the health of the auxiliary battery from the Volt's information system? Will this information show up on the diagnostic information available through OnStar? If the auxiliary battery is about to fail it would be nice to get some warning prior to the Volt refusing to start.
 

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About a week ago, WOT posted the list of all of the possible status and warning messages that can appear on the Driver Information Center display, and some of them were:

BATTERY SAVER ACTIVE--This message displays when the vehicle has detected that the 12-volt battery voltage has dropped and vehicle features are being disabled. The 12-volt battery saver system starts reducing certain features trying to save the charge of the 12-volt battery. Turn off unnecessary features to allow the battery to recharge.

LOW BATTERY--This message displays when the 12-volt battery voltage is low. See Battery for more information.

SERVICE BATTERY CHARGING SYSTEM--This message displays when there is a fault in the 12-volt battery charging system. Take the vehicle to your dealer for service.
There you go :)
 

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Interesting that the Prius (which I've never much looked in to) has the aux in the cabin. Makes me wonder why Nissan put the Leaf's (rather large) aux under the hood.
Funny you should mention the Prius 12V battery, I'm in the process of replacing mine before I sell my '04 Prius to buy the Volt! The Prius 12V battery is sort of in the passenger cabin; it is located under the floor in the back cargo area. It is ventilated with air from the cabin (as is the big traction battery) to heat it a little in the winter and cool it a little in the summer.

Mine is 6 years old, and I've read they usually last about 4 years. Mine reads "9.6V" (in Accessory mode) on the diagnostic screen, but should read 12.4V to 12.8V. The dash sometimes beeps at me for no apparent reason after I turn off the car, then if I leave it overnight in that state I'll need a jump in the morning. I believe the new 12V will clear that all up.

Just don't feel right selling it to the next guy before I fix that. Maybe I can haggle the price of the car a little as I'm selling it by pointing to the receipt for the battery they would have had to replace anyway...
 

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What options does the Volt have for attaching 12V accessories? I'm wondering about simple things like tire inflators that plug into a lighter outlet as well as more permanently attached things like a 2M amateur radio transceiver.
 

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I think you'll find the Volt has adequate 12V accessory plugs (front,rear and inside of center console) and as far as more permanant acc wiring there is a couple of fuse block accessory kits avaialble for connecting to both B+ and switched IGN+ feeds.
WOT

PS> I've verifed the 12V AGM battery is 60ah (not 70 as previously stated)
 

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Discussion Starter #20
Chris,

This description of jump starting the Volt needs to be updated per the Instruction Manual. For your FAQ items it would be good to also reference the Owners Manual section when there is a description available there.

For this description it would be good to mention the +,_ posts under the Hood for jumpering to start the Volt and the need to jumper directly to the battery in the rear for starting another car with the Volt battery.

Keep up the good work.
 
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