So what if you remove the second factory auxiliary battery (AGM) and associated relay and replace it with a B2B and lithium. That's what I've done, it all appears to be working great. The van is VW Grand California.It resets the charging regime and tells the van to relearn the state of charge of the battery etc
It’s not ultra critical as the van will adapt to it after it’s been driven X amount of times, it’s more important if you change the capacity of the battery as the van may get a bit confused over state of charge etc if it thinks the batt is a different capacity than it is