Visit Workshop Message with Battery Image

BabsInNC

My 2003 E-320 Mercedes has only 55,000 actual miles on it. It is my "Sunday" car. Recent red message came on saying Visit Workshop, with image of battery. This happened a year ago when the serpentine belt broke. The shop I use checked the car out yesterday and could find nothing wrong with the 2 batteries under the trunk. They changed the oil, installed new oil filter & refilled all fluids. The light is still on. What OTHER BATTERY is there in a Mercedes besides the 2 under the trunk space? What could it be? I am a 76 year old single lady so my knowledge about cars is fairly limited. Thank you.