The Volt's 12V battery functions as a dual purpose, startup and cycling, battery. The startup load when powering on the Volt is not as large as when turning over an ICE engine. The Volt's 12V electronics use ~500W when powered on. The startup load on the Volt's 12V battery is ~ 40 amps. After a few seconds the Auxiliary Power Module (APM) takes over and powers the Volt's 12V electronics by drawing power from the high voltage traction battery for as long as the Volt is powered on. When the Volt is powered on I have observed the voltage supplied by the APM vary between 12.4V and 14.8V as measured at the accessory outlet.Just a question is the AGM battery a starter battery or the deep discharge leisure type as used in RVs for house functions.
As I understand it the Volt does not need the high current to spin a starter, but may need the flatter output voltage curve of this type.
The 12V battery also needs to be able to maintain the Volt when the vehicle is parked for up to 30 days while still being able to sporadically communicate vehicle status over the 4G LTE network. GM specified the Volt's 12V (BCI Group 47 AGM) battery so that the Volt can be parked for up to 30 days and still have enough reserve power to start up the Volt when the driver returns. Any replacement 12V battery needs to be an absorbent fiberglass mat (AGM) type lead-acid battery to be properly charged by the Volt's 12V APM circuitry.