For those who store their cars there are a couple of possible options.

For long term or a very low battery, I disconnect battery negative or positive and charge battery. After long termstorage, hook up and car starts right up. Just have to set clock and on W210, the radio code (easy to do)

However, with the current smart trickle charges, such as C-tek (even MB sells one as an accessory), you leave battery connected and charger maintains it as required. I now leave mine connected all winter while away.

On cars like W210, there is quite a current draw even when parked, so something has to be done. On some cars, battery is hard to get at, so the small maintainers are handy.

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Graham
85 300D,72 350SL, 98 E320, Outback 2.5