I've got a 25-year-old Benz that over the last two years has developed an appetite for batteries.  Somewhere in my car is this little demon who couldn't be found by at least three mechanics and AAA but who within four days of inactivity will drain the battery.  The local Benz dealership said that it could take days of analysis to pinpoint the home of the demon (which could cost $125 x countless hours) or I could get a trickle charger (imagine, a Benz service specialist suggesting that!).  So I found the best one I could!  This is made to mount either directly onto the chassis of the car or onto this neat bracket that slips under the battery.  In my case because of the density of the area of the battery, neither would work.  But this charger is so small, that I was able to lodge it snugly into the cables and such so that it doesn't budge.  Too bad they don't provide a longer power cord --- I had to use a 6' extension which I cable-tied to 'stuff' out to the front bumper where I have a convenient port for attaching towing lines.  So now I plug in my car every time I come home.  If you do something similar, here's a hint:  Tie a red ribbon to the door handle or windshield or do something to remind you that the car is plugged in --- you don't want to drive off with the extension cord attached --- LOL!!!