I would slow charge a new battery(with a Battery Tender, etc) before you install it and not replace it in the parking lot. It may have been setting partially discharged on the shelf for a while. Also as I found Parasitic drain shortens battery life.

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Tony H
W111 280SE 3.5 Coupe
Manual transmission

Past cars:
Porsche 914 2.0
'64 Jaguar XKE Roadster
'57 Oval Window VW
'71 Toyota Hilux Pickup Truck-Dad bought new
'73 Toyota Celica GT