Presumably your talking about your VE Calais?
These cars have electronic smarts in them that control the alternator and how the battery is charged and how the battery power is consumed. If the smarts detects the battery voltage is getting low, it may decide to start shedding battery load. Generally the first thing it starts to shed in the entertainment system and then HVAC, all to allow the vehicle to continue to drive. So if your stereo and aircon/heater are switching off at odd times, this may be the cause.
Your best cause of action is to
- Get the vehicle scanned for Diagnostic Trouble Codes which will aid in the diagnostics process.
Just remember that DTC's are an entry point into the diagnostics process and not a definitive statement in what needs to be replaced 
- Get your battery load tested and replace it if it fails.
- Get the charging system tested (because a fault could cause your battery to not be charged and that will kill your new batteries in short time).
If the fault is not due to battery and or charging issues, you'd need to look at the wiring schematics and determine what is common between the sound system and HVAC in terms of power and ground as you could have a loose connector/bolt somewhere...
Good luck.
