after connecting all the battery correctly, dodging all the short circuit i finally managed to fire up the system and guess what : It work !

the UPS accepted the new battery withouth complaining and after some poking around it even allowed to reprogram their capacity. The UPS can now deliver up to 40kW of power to anthing that is attached to it (the modest desk lamp you see on top of it for now) it would take up to 2 and half hours to empty the battery at full power but for the average consuption I expect more like 10 days of autonomy before we run out of power.

Next step, Solar panel, wind mill and even maybe water wheel to charge the system