When I bought my 97 E420, I noticed that the past owner put in an aftermarket EXIDE group49 battery. It didn't have a vent tube, but on the side it said "APR 06". So I'm guessing this is a fairly new battery.

Then last month I left the doors open (made sure all lights were out), and traveled for 6 days and the battery was dead when I came back. Car wouldn't even crank and all lights were dead.

It was a struggle to even jumpstart the car, but I managed to start her again and all seemed fine. But recently I'm noticing that it takes alarmingly long time (sometimes 3-4 seconds) of cranking to start the engine. I'm driving the car everyday but I'm afraid if I park the car for 4-5 days it will die again.

Does an even relatively new battery get destroyed if you let it completely drain out? I wonder if I should just bite the bullet and replace the battery now. Hopefully I don't have another bigger problem like a battery drain somewhere... argh

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Current cars:
2001 MB E55 AMG
2003 Honda S2000
2000 Honda CRV

Gone but not forgotten:
2004 BMW 325i 5sp sport
1999 BMW 540i sport
1997 MB E420
1995 BMW M3 coupe