Take two batteries and connect them in parallel. Batteries may be wired in parallel or in series.
How To Connect Two Batteries In Parallel Caravan Chronicles
Connecting two 12 volt batteries in parallel. Take another set of batteries and wire them together in parallel as well. Finally take a jumper wire and bridge the positive terminal of one parallel connection to the negative terminal of the second parallel battery pair. Two 12 volt batteries in parallel give you 12 volts. Two 12 volt batteries in series give you 24 volts. Attach the positive terminals together and the negative terminals together. Connecting them in series will seriously damage the inverter.
Use one battery cable. But what happens if you wire batteries of different voltages and amp hour capacities together in parallel. Two 12 volt batteries wired in parallel continue to produce 12 volts but the capacity doubles. Connecting batteries of different voltages in parallel. This is the big no go area. Make a parallel connection for the two 12 volt batteries.
Following this example where there are two 12v 200ah batteries connected in parallel we will therefore have a voltage of 12v volts and a total capacity of 400ah ampere hour. Four 12 volt 2000 mah wired in parallel can provide 12 volt 8000 mah 2000 mah x 4. Connect the two positive terminals on both batteries together. The parallel connection of two identical batteries allows to get twice the capacity of the individual batteries keeping the same rated voltage. Two 12 volt batteries wired in series produce 24 volts but the capacity remains the same as one for a 12 volt battery. Charging 12 volt batteries wired in parallel requires a 12 volt battery charger.
If the input rating on the inverter no matter what the output wattage says 12 volts you must only connect two batteries in parallel.