So i was gone for 3 months and my cars batterys dead
the lights were off and im 100% sure nothing was left on
could leaving the door unlocked do this??

If you haven't started your car in 3 months, Your battery is gonna die. Leaving the door unlocked has nothing to do with the battery being drained.

is it only our cars???? . . because my dads s550 works fine

I think its the S550 that is not the normal car...I dunno how it did.

My other cars, depending on the age of the battery, will only last from 3 weeks to 1 month...If the battery is really new, maybe a bit longer, but for sure it won't be 3 months.

Cars when left off still consume battery power, for the radio memory, alarm system and other minor stuff, and of course if you don't start the car, its only a matter of time when the battery will be consumed. If you plan to leave your car for that long, and it is in a secure location, you should get some kind of a battery tender or trickle charger...

Or just unhook the battery

best thing to do if you are not driving your car for awhile is to get a battery tender.

Try one of these Ctek battery chargers, they work great:

http://www.ctek.com/EN-US/home/Default.aspx

+1

Battery tender work very well. You just plug them into the wall then to your cigarette plug. Had one for the porsche, it actually came with the car.

Will a battery tender that plugs into a cig lighter port actually work if the cig port is a switched outlet? By that I mean, the cig lighter is not hot unless the key is in the on position.

It will not work in this case because no current will be able to get through to the battery. It will also not work if the lighter fuse is blown. Best bet is a direct connection of a smart charger to the battery terminals.

every year we leave it for 3 months and the battery doesnt run out

I herd from my dealer that the new merc has something that switch off everything when batter charge get low so enough is left to turn on the engine.. The only possible problem is that light and other computers would not be usable the first you turn it on, while it is charging....

Google this for the best price and pick one up:

http://batterytender.com/automotive/...-at-0-75a.html

I wonder if that is similar to this: http://www.harborfreight.com/automat...ger-42292.html