Hi,
After some help! I've got a MultiPlus 12/800/35-16 on my boat with start and leisure batteries. It has worked perfectly in the past but recently it is not charging the leisure battery or inverting when I remove its power source. I've charged the leisure battery with a separate portable Victron charger so I think the battery is ok. I've got a fuse between the MultiPlus and the leisure battery so I've changed that. I've got a SmartShunt on the negative of the leisure battery so I can accurately check its status.
Any further troubleshooting ideas?
Thanks!