For the last 3 months I have the "Visit Workshop" with a picture of battery message displayed continously. The car drives fine with no issue. I drive this car about 2-3 times a week and there have been occasions where the car has sat in the garage idle for 7+ days. When I do start the car, it starts immediately and runs fine. However the above message is displayed continously.

The battery is about 4 years old. The car 2006 E350 is about 5 years old and i have driven approx 27000 miles.

I am wondering if I should just replace the battery or could it be an issue with the auxillary battery.
What is the best way to troubleshoot this.
Thanks