I installed an inexpensive battery maintainer in both my '79 450SL and my 2001 ML320. Harbor Freight sells a unit that will sit right next to the battery and directly connect to it. It is a three stage charger with an anti-sulphation mode on the trickle charge, and has kept my batteries up really well. Without the maintainer my ML320 would go dead in less than two weeks, and the 450SL would be dead in about 3 months. I ran a short extension cord out the front of the grille and just plug in the car when I park it. HF regularly sells the units for around $20 on sale. They are switching units, seem to work very well.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C