That's why I keep a Lidl battery condition tester on the battery of the S204 (which lives on the front driveway), visible through the tailgate without having to open the car. If/when it goes red I know from past experience that I need to get the extension lead out with the Sip Chargestar Smart 18 Automatic charger, connect that to the battery and close the tailgate over the extension lead cable so car is locked and alarmed, then leave it at least overnight or longer if possible. I also know that if the indicator light on the monitor is amber the car will start. Easier with the SLK - she's a garage queen, so parked there she's always on a conditioner!!