[OldEngineer]

I think the Main Battery and the Support Battery get out of sync.
Charging through the single connecting point seems to sort it out but if not, disconnecting the main battery will make the Battery Monitor recalculate.

I replaced my support battery earlier in the year for the message 'Service Needed' with one from a Volvo dealer on line. It takes seconds to fit. But I also bought an intelligent charger that got rid of the message before I found the discounted battery.