For a Parallel connection, batteries of similar voltages and capacities can be connected to increase the capacity of the batteries. This means that the backup batteries will provide the same power needed and will provide power for a longer duration.
In the above illustration, both batteries are rated at 6 volts and 10 Ah (amp hours) each. With the batteries connected in parallel, voltage remains the same at 6 volts, and amp hours are doubled to 20 Ah.
Connecting two identical batteries in parallel will give you double the output capacity and keeps the output voltage the same as either battery. This means that the batteries will last longer. Conversely, if two identical batteries are connected in series, the output voltage is doubled but the output capacity will stay the same as either battery.
NOTE: All batteries connected in parallel must have the same voltage rating. Batteries can be damaged by excessive cycling and overcharging.