Apart from the shape of the battery, only Volt value must be identical ; Amper is equal to the capability of energy storage. More the amper value is high, longest the battery will keep without to be reloaded. Watts is function of Volt * Amper ; And Watts-Hour = https://en.wikipedia.org/wiki/Kilowatt_hour. N900 batteries must be genuine to be recognized by the BMS circuit of the phone.
https://wiki.maemo.org/Batteries