Actually, the OEM Mercedes battery is decently priced at the dealership...and you can have the confidence knowing that it's the correct battery, with the correct dimensions and terminal orientation.

The other alternative is Interstate Battery...their alternatives match the OEM offerings (including the weird trunk mounted units required for the SL and CLK)...and they are cheaper than the dealer.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car