Agree with others a battery in Mercedes / bm etc is no use unless it’s fully charged / bad battery some things not working as should is big possibility.Also when getting one check it’s high power suitable for your car , cranking etc. there’s many m bs starting ok with the batteries in them but the likes of s/s
Memory seats , keyless entry not working as should , radio shutting down few minutes after you turn off engine etc , been there a few times , never had a mb with a battery over 5 yr old that was any good