Finally I changed my batteries to a brand new B&B set.
I thanks everybody for all informartions I gathered.
So while changing the batt set I rewired them into 2P4S (Batteries are linked in parallele and the four pairs are connected in serial)
I had to remove one charging socket (no more needed as batts won't be able to be turned into 2x48V series)
->so I remove B+ and Bx connections on the BlackBox (which is white so I'll call it WB from now).
->the two more wires from the charging socket go to 0V and +48V of the original second serial off batts (B)
I rewired everything using original cables (especially the longs ones form relayB to batts).
Relays are connected together in parallel in the relaybox (below feet).
I used 6^2mm as often as possible bute I had to do somme parts with 2x4^2mm.
Everything can be seen here (in french but with many pictures
Nb: I finally kept Ax connection on the WBox.
On this picture I still have originals batts