Every once in a while Lidl sell a battery conditioner and a battery condition monitor. I use the conditioner on the SLK whenever she's in her garage (which is a lot of the time) connected to the emergency +ve under the bonnet and the suspension top mount nut, works fine, only downside is that in the event of a power cut you have to manually re-set it. I also have the condition monitor hooked up to the battery of the S204 (which lives on the driveway) and hung in a way that I can see it through the tailgate. It has 3 indicator lights, green, amber and red, and from experience as long as the amber light is on the car will start OK. However, on the odd occasion it goes red I run an extension cable out and connect up my SIP Chargestar Smart 18 Automatic battery charger, which among other tricks switches itself off when it has fully charged the battery.
The Lidl kit when they have it costs a lot less than any CTEK, some find the Lidl conditioner quits within 3 years but many have used them for years without problem - I've been using mine for over a year now - but still have my 1999 original (not CTEK, but at the time of purchase was the only one approved by a major car manufacturer - sadly BMW) as back-up!
