Hello: first post

I have a 2003 SL500 with 2000 miles: hardly used, garage kept

the battery warning indicator stays on now and it says "electric consumers off line": previously I saw the warning but it would disappear after driving

I have taken it to the dealer twice: he says no problem

checking it myself, it seems that the rear battery is bad: it will not take or hold a charge: the start battery is fine

what troubles me is why the dealer cannot diagnose this: is it as simple as replacing the rear battery?

TIA