I can not see an aux battery in the spare wheel well so I'm guessing it does not have one?ECU doesn't need to be re-set. Buy a MB battery and ask for a discount. The margins are massive. MB batteries are great quality and last a good amount of time, they will fit first time and be 100% correct for your car. You may have an Aux battery.
What should I pay for a new battery from MB, any idea, part number is A 001 982 81 08