Probably because some part of the starting system is weak so a bandaid fix of adding a second battery was done. Not a good sign, could be low compression. But their is no way to tell without an inspection.

Carefull with W115's they rust very badly. Wasn't their a thread about this car last week?

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200