First of all, you need to know that there are two main ways to connect two or more batteries to a power inverter.You can do it in series as this type of connection adds the voltage of the two batteries, but it keeps the same amperage rating. For instance, two 6-volt batteries joined one by one now produce 12 volts, however, they still have a total capacity of 10 amps.
The second way is to connect batteries in parallel. With this method, you will increase your current rating, but the voltage will remain the same. In the case of parallel connection, we’re back to 6 volts, however, the amps increase to 20 AH. As for the second option, you have to find a heavier-duty cable to protect the cables from burning out since the amperage of the batteries is going to grow.