Just to be perfectly clear, once again:
If you have a BmPro J35 and a battery installed, AND the battery is flat:
THE BMPRO J35 DETECTS THIS AS A FAULT CONDITION AND SHUTS DOWN THE ENTIRE 12V SYSTEM:
If I can get this one simple message across, I will feel I am not stuck in a real life version of groundhog day.
And again, just one more time:
If you are at a site with 240V and you have a flat battery, you will have no 12V to any part of the van. You can however get around this: Disconnect the battery. The BmPro detects there is no battery, hence no flat battery, and will then happily supply 12V to the van.
The BmPro will not charge or trickle charge a flat battery. The battery needs to be disconnected and charged with another source to get the battery voltage up as explained in detail by
@Boots in Action. Once the battery voltage has been restored, it can be reconnected to the J35 which will then happily supply 12V to the rest of the van AND maintain the battery,
Both
@Boots in Action and I have spent a lot of time on the BmPro J35 reading and understanding how it works even though neither of us even has one.
take care
Mike