The brief
The B200 had been throwing the odd electrical warning and a few systems acting up, and the cause traced to the auxiliary battery. A lot of owners don't realise a modern Mercedes carries two batteries: the main starter battery, and a smaller auxiliary battery that keeps certain electronics steady, supports the start-stop, and bridges the voltage dip when the engine restarts. The auxiliary battery wears out like any battery, and once it's weak the systems it feeds get unreliable, you see warnings and odd behaviour, and start-stop stops working properly. A worn auxiliary battery doesn't recover, so it needs replacing with the correct type, and registered to the car if the system calls for it.
The diagnosis
A battery and charging check went over both batteries. The main battery was fine, but the auxiliary battery was down, failing the load test and unable to hold voltage, which is what was feeding the warnings and the niggles. The charging side was fine, it was just the auxiliary battery at the end of its life. So it was an auxiliary battery replacement, the correct type and rating, fitted and the faults cleared.
The work
The old auxiliary battery was removed and a new genuine Mercedes-spec auxiliary battery of the correct type fitted, the terminals cleaned and the connections checked. The battery was registered to the car where the system required it, and the stored low-voltage faults cleared with the voltage now steady. A quick run confirmed steady voltage, no warnings, start-stop working, and the electrics behaving normally.
The outcome
Steady voltage, no electrical warnings, start-stop working, and the systems back to normal on a fresh auxiliary battery. The B200 went home sorted. A weak auxiliary battery quietly upsets the electronics it supports until it's changed, so renewing it and clearing the faults put the car right before it became a breakdown.
