where is the location of the battery under the hood? Thank you

I owned an 85 Audi 5000 which had the battery under the rear seat. There were a few screws at the front of the seat to disconnect it. Maybe your Mercedes is similar.

If you mean a mercedes A class 140 then your battery is fitted in the drivers footwell along with your fusebox. There should be two clips which open with 1/2 turn and you can lift the "floor" out, there you will find the battery and fusebox. Hope this helps.