I am setting up a simple solar charger in my 2020 sprinter van. It has a starter battery and an aux battery charged by a “smart alternator” and seperated with an isolation relay. This is all from the factory and works fine. I planned to simply add a few solar panels and a Smart Solar 100/30 MPPT charger. This would connect to the aux battery. I found the Sprinter documentation says that external charging of the aux battery must only be done when the engine is OFF. So, the recomendation from my MPPT vendor was to add a Victron Smart Battery connect between the MPPT and the battery. When the Smart battery connect detects engine running it will disconnect the MPPT form the battery, leaving the PV array connected to the MPPT, but nothing connected to the MPPT output. He said the MPPT would go to “float” mode. Seems reasonable, but someone pointed out that all/most solar charge instructions say to connect to the battery FIRST so it auto detects the voltage, than connect to the PV array. So, it seems like it would NOT be good to put disconnect a batttery from a solar charger while the PV array is active. Will this be an issue?
I don’t see why there should be an issue with charging the aux battery from a correctly configured MPPT even when the engine is running.
The restriction is probably to prevent any charger overvoltage from damaging the smart alternator.
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Thanks, I suspect it would be OK, but a new alternator could be expensive 
Just to be safe, I’m going to add a relay/disconnect on the PV side of the MPPT charger and trigger it with a 12v trigger that is supplied in the Sprinter. Easy enough to do.