OE battery is $110-$120. Yours is eight years old it doesn't owe you anything. Remember modern batteries usualy work fine until they fail, sometimes there is no warning. If you don't plan on going anywhere your probably fine. You can call the number on your glove box door, and MB will bring out a new battery when it dies.

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2006 CL500
2009 C300 4matic
1969 280SE
2023 Ram 1500
2007 Tiara 3200