I will answer to myself, just for the community ;-)
As I use my archos ALWAYS at home, I removed the dead batteries and did the following :
-removed the DC plug and modified it : cut the 3rd pin that tells the system if you have a plug inside or not, bend the + pin to connect it to a wire that is directly connected to one of the 2 + connector of the battery reciever, and let the ground connected as is.
-then, you can use your archos with the power supply, without battery.
-as my power supply died, I use now an old PC powersupply, that have a powerfull 5 volts output.
My Archos works fine with that, but now I need a truck to move it ;-)
(bloody big power supply)