The W211 electrical system is flagged as the biggest ownership drawback by some, with cars described as extremely finicky about voltage and prone to parasitic drain; a fresh AGM main battery and healthy aux battery matter because the cars draw power even when parked. Loose battery connections can masquerade as bigger faults.
Electrical and voltage gremlins, battery drain
Mercedes-Benz E55/E63 AMG (W211) · mechanical