2005 Mercedes Benz E220 warning on dash

It is very common on this vehicle to have this message caused by a failed voltage regulator which is part of the alternator. However the message can also be caused by a failed alternator or battery that is not holding a charge.

I'm not clear, did you replace the volatage regulator on the alternator?

If its going away after 20 mins then its more than likely not a voltage regulator, You may have a slight amperage draw from a component not turning fully off and drawing down your battery. Does the vehicle ever have a long crank before it starts? This warning in this case is telling you your battery voltage is low and will not turn off until the alternator charges the battery past the warning level. If this is an amperage draw it will eventually kill your battery when the car sits a couple days or even over night. If you have noticed the vehicle cranking for a longer time when you start it up in the morning then this is probably the case.

You can use a trickle charger in the mean time however if this is an amperage draw you will want to remedy this soon. The testing for this will be time consuming and will require a lot of patience. You will need to first roll down all the windows and open the hood and trunk. Close the latches on both sides of the hood and at the trunk latch. Once these latches are closed go to your battery. Loosen the negative cable but do not remove just yet. With a multimeter set on amps clamp the negative lead on the post and the positive on the cable. Lock the vehicle with the remote key. Now remove the battery cable with the meter held firmly on both the cable and post. DO NOT BREAK THIS CIRCUIT once you have the cable and post appart. With your multimeter firmly attached it will read the amp draw from the vehicle. Let the vehicle set fro 20-30min while all the components go to sleep mode. You will see the amperage drop lower and lower. The reading when all components are asleep should be roughly 0.030amps. Anything much over this and you have a problem with a component not going to sleep. Now comes the fun. Note: do not open or close any doors or the trunk or hood as this will reactivate the vehicle and you will need to start over. Now with the vehicle asleep start unplugging fuses one by one. Check the meter after each is pulled to see if it drops into the correct range. If it does this is fuse is to the circuit of the component that is not going to sleep. This process can take some time so be patient. Also make sure you can access all of your fuse boxes to do this before you begin.