First of all, a fully charged battery will give you about 14.5 Volts with no load. I agree with the others, it sounds like your battery is not good. To me it seems that it won't hold a charge.

If it is the original battery it may be at the end of its life. I'd also check the connections on the battery terminals.

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2004 ML350 Black with charcoal leather, OEM Hitch, Rhino step, Bose, Navigation, OEM front guard, Comfort package, Entertainment package, Sunroof, heated seats.