Back-UPS Pro 900 (BR900G-FR) doesn't charge the batteries

[Anonymous user]

Hello,
 
I recently purchased a second hand APC Back-UPS Pro 900 inverter. Diagnosis: batteries (original) empty.
 
After replacement with 2 'standard' batteries, new but not APC, previously loaded externally, the inverter has started well and seems to work normally ..... save for this: the battery voltage decreases gradually, over several days, without any recharge, despite the declaration of change in the Powerchute monitoring software, or even after a hard reset procedure found on another forum.
 
I'd appreciate any information on the return to function of the battery charge, and if possible the way to get the diagram of this device so to be able to fix it.

Thanks in advance & Rgds,
Robin