Battery isn't charging apparently. While driving radio and dash will turn off gradually until car shuts off due to dead battery. Brand new battery has just been installed due to this issue and it happened again about 4 days later. Do I need a new alternator or is it the BMS. I don't mind buying and installing a new alternator just don't want to start throwing money at it if that is not actually the issue. Any help is greatly appreciated.

 

Throwing parts at a Jaguar soon gets expensive. I think you are right to suspect alternator or BCM as the most likely possible causes but there's 4 components directly in the loop in addition to overall control by the ECM:

1. Battery (90aH for your model)
2. Alternator
4. BMS (Battery Monitoring System module)
4. GWM (Gateway Module)

The BMS control module is able to generate DTC's (diagnostic trouble codes) to help diagnose battery or alternator power supply issues. These DTC's can be read using SDD (the Jaguar approved diagnostic system). SDD can also be used to implement a battery and alternator self test routine.

I'd always run a diagnostics session to look for relevant DTC's before changing parts.

Graham

 

Was it even checked if there is 13V+ across battery terminals when the car is running.

 

Yes, you need another alternator.