If you require more than one battery, you have two main installation options, depending on if you'd like to increase the overall voltage or increase the runtime:
1. Increase the overall voltage (i.e. add the voltages of each battery together) by daisy-chaining batteries in a Series Configuration.
Four 10-volt batteries wired in series with one another will create a total of 40 volts. If one battery fails, the others will stop providing power to the load.
2. Increase capacity and extend runtime (i.e. amp/hours available) by connecting batteries in a Parallel Configuration.
If one 10-volt battery would normally power a circuit for two hours, two 10-volt batteries wired in parallel might power the circuit for four hours, but still supplying only 10 volts to the circuit. If one battery fails, the rest will continue to supply power to load.