This applies for model: FIV series.
This inverter has 2 modes: UPS mode and Inverter mode. Also, it has two ways for charge: 10 Amps and 20 Amps. The connector is at the back or behind the inverter (See image).
For FIV-1500 / 750W:
This image shows how needs to be connected the batteries for this FIV-1500:
Paralell connection with 6V Batteries on Serial
(Gives 12V on each array)
Paralell connection with 12V Batteries
For FIV-3000 / 1500W:
This image shows how needs to be connected the batteries for this FIV-3000:
Paralell connection with 12V Batteries on Serial
(Gives 24V on each array)
Paralell connection with 12V Batteries (One array)
Keep in mind that in a parallel connection the voltage will remain, however each battery currents are summed. In serial mode, the voltages are summed and the current is equal to the value held by each battery.
Also, the initial charge current will be at least of 10A to use the inverter.
What's the difference between the UPS Mode and Inverter Mode?
On "UPS Mode", the inverter will give a normal input (AC mode). On the meantime that the power from the AC outlet goes off, the Inverter will change his state to "Battery Mode", giving the requested backup time.
On "Inverter Mode", the inverter could be connected to the batteries without having energy.