Today I decided to take the merc on its first road trip, to Iowa, a 4.5 hour drive. Approx. 2hrs into the trip, we noticed a funny smell, pulled over, and opened the hood. I noticed the smell came from the battery which was hot and was hissing. I also noticed it was leaking some fluid. I took it to a local shop where I purchased a new battery. I thought maybe the old battery was going bad. Well, 2 hrs later, we made it to our destination, but after getting out of the car, I noticed the smell again. Opened the hood and the new battery was hot, but not leaking. I've owned the car for several months now, and the car always started in the cold Minnesota winter. The only electrical problem I have is my climate control fan sometimes works and sometimes doesn't. I also noticed in the past that when the climate control fan used to go to high, my lights would brighten. Am I in for a lot of expense? Could my alternator be overcharging? Please help, thanks

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2012 550CLS
1982 380SL
1985 300D
1998 ML-320
1997 E-320
1984 380SE