I currently have a Werker in my 250. It came with the car in 2008, and I've managed to abuse it by draining it dead at least once, as well as letting the car sit with the battery still connected for many weeks at a time. So far, so good.

The battery in my 240D is the one that came with my 300D in 2005. I can't think of the brand name right now, but I think it's black with a red top. Definately NOT an Optima. I swapped it into the 240 when the 240's battery fried from a failing (overcharge) voltage regulator. Kragen pro-rated the replacement battery, which went into the 300, since it gets the best of everything and the 240 gets the dregs. It still works just fine, and I can check the brand tomorrow if you're interested.

The 300 currently has about a year old battery of whatever manufacture Kragen sells as their standard. They retail for about 106 bucks in group 49 size. No complaints as of yet.

I would disconnect the battery before letting the car sit for extended periods, or if you do it a lot, go ahead and put in that battery disconnect switch.

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Whoever said there's nothing more expensive than a cheap Mercedes never had a cheap Jaguar.

83 300D Turbo with manual conversion, early W126 vented front rotors and H4 headlights 401,xxx miles
08 Suzuki GSX-R600 M4 Slip-on 26,xxx miles
88 Jaguar XJS V12 94,xxx miles. Work in progress.
99 Mazda Miata 183,xxx miles.