Attention - Attention. Xfires cook the front cell of the battery each time you park hot. Merc SLK's have a compartment around the battery to prevent this. I have now simply wedged a wad of soft white fluffy stuff, called cortel I think, taken out of an old speaker box, in front of my new battery. If you can look inside your cells, you will see that the front one is going white from sulphation. The only cure for that is to drill into the connector bar and separately charge just that front cell. A new battery is easier. Parked drain is 13mA unlocked - theoretically 34 weeks to flatten the 74Ahr battery.
This advice should have been on a RECALL !!! Regards, Jim in New Zealand, where Chrysler wanted $US600 for the genuine battery !!!