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For a 2014 it is charged in any one of the following states:
1) vehicle is powered ON (just like any other car when on, it simulates an alternator and charges the battery from 12V power source with ~15V and then drops to ~13.x)
2) when actively charging the main high voltage battery, 12V battery may be charged as well
3) when plugged in and not actively charging the HV battery, 12V will be periodicially checked and charged as required.

Scenario 3 did not exist prior to 2014 and may be the reason for many users having to prematurely replace their batteries.
We'll know in a couple years when 2014s pass the 4 year mark (when many early ones had to be replaced).
 

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For a 2014 it is charged in any one of the following states:
1) vehicle is powered ON (just like any other car when on, it simulates an alternator and charges the battery from 12V power source with ~15V and then drops to ~13.x)
2) when actively charging the main high voltage battery, 12V battery may be charged as well
3) when plugged in and not actively charging the HV battery, 12V will be periodicially checked and charged as required.

Scenario 3 did not exist prior to 2014 and may be the reason for many users having to prematurely replace their batteries.
We'll know in a couple years when 2014s pass the 4 year mark (when many early ones had to be replaced).
I think that both 2 & 3 did not exist prior to 2013 models. So 2011 and 2012s only charged while turned on like an ICE vehicle.
 

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The 12 volt battery is charged by the 14 volt power module only, does not get power directly from the battery charger as a low voltage but as a high voltage converted in to low voltage. There are many modes of operation for the 14 volt power module to charge the 12 volt battery. Battery Sulfation Mode, Normal Mode, Fuel Economy Mode, Headlamp Mode, Voltage Reduction Mode, Plant Assembly Mode. The 14 volt power module maintains the low voltage system for the car and charges the battery. It takes high voltage from the drive motor battery and converts it to a useable low voltage for this system. The 14 volt power module acts like a alternator would on a ICE car.
 

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I think that both 2 & 3 did not exist prior to 2013 models. So 2011 and 2012s only charged while turned on like an ICE vehicle.
Very simple to confirm if your Volt is also charging the 12V AGM while plugged into L1/2 and charging:

Begin a charge cycle then pull the negative lead off the 12V AGM battery removing it from the circuit.
There should be @13VDC between B+ terminal of the 12V AGM and chassis ground.
Unplug the Volt or just wait for the charge cycle to complete and this voltage will be removed.
 

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There's also an OBD PID that's less "invasive" to be able to monitor.
 
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