starter battery

Hi guys, Had some issues with a low battery in the boot of my R230 so am charging it with a CTEK charger at the moment. Just wondered if I had to buy a new one what the price would be so took a photo of it and the battery model is A0009823108 (Genuine Merc part). However, when I google this...

I've read that a battery charger can be fitted permanently to the car and just needs plugging-in put the vehicle on charge. I'd like to use a portable 5 amp charger on my s211 wagon, which is continually losing charge due to lack of use, but the charger instructions insist that the battery must...