BMV-600S connecting to Cerbo

I have connected it to a Cerbo via the serial port.
How accurate it is I am not sure.

Serial Cable form Victron then to a Serial-USB adapter into Cerbo


Ah, that’s a ~13-year-old unit, which is probably why there was an initial issue. The 600S is not listed as possible to connect - see here: 14. Compatible Victron products
A modern battery monitor would have the current standard VE.Direct port, but once you go beyond a decade, some variances come into play.