Of course the battery dying in a month is not common but depends on the condition & age of the battery. I've seen cars left at our shop for 6 months still start w/o jump! And others 4-6 weeks and there dead.
Most erratic gauges are from defective sending units!
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)