The battery in question is located inside the passenger compartment, specifically in the end area of the centre tunnel.
To access it, first open the glove compartment, then by prying into the slot (1) located inside near the compartment release button, remove the upper panel (2).

In the same way, disengage the small armrest (3) located above the mouse. Then unscrew the three screws just released with a Torx T25 spanner (4), the two located on the armrest (5), remove the top cover of the tunnel and disconnect the connectors.

Continue with the disassembly of the battery (6), it is almost at floor level, inside the central tunnel it is light grey.

Please note that all operations must be carried out with the main battery disconnected.
Below are all the photos indicated by the numbers.
Thank you all and good work

• 5 - Two under armrests
• 6 - Battery
• 4 - Three Torx T25
• 3 - Armrest
• 2 - Upper panel
• 1 - Blocking slot