Mercedes Backup Battery
This bulletin applies to 2020 Mercedes-Benz model series 177, 118 and 247. A backup battery message may be displayed in the instrument cluster “Service Required.”
Fault codes B21DC01 (buffer battery of the electronic ignition lock has a malfunction) or B11C113 (the additional battery has a malfunction...there is an open circuit) or b11c116 (the additional battery has a malfunction).
The limit value for electrical voltage has not been attained) may be found. Note that the warning message may be due to deep discharge of the backup battery after a non-operational time period of more than 10 days (12V starter battery disconnected or discharged due to lack of battery care).
Replacing the backup battery may fix this complaint. Replacement backup batteries are available as P/N N 000000 004039.