My 06 E320 CDI is/has been evidencing the same message. It's the main auto battery in the trunk under the false floor is the spare tire and battery, not the small, auxiliary sport battery under the hood (bonnet).


My CDI is now going on it's 3rd battery since new. Similar main battery-need schedule as my '99 E300's need frequency - about every 4-5 years.

The brains of the computer systems are always 'on' in these cars. A trickle charger may be something to consider, but I do not know that they would extend the life of a battery that is never depleted of it's charge, and used / run often.

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'06 E320 CDI
'17 Corvette Stingray Vert