In a Parallel connection, batteries of similar voltages and capacities are connected to increase the capacity of the bank of batteries.
When you connect two identical batteries in parallel, you double the output capacity while keeping the output voltage the same as either battery. This means that the batteries would last longer. Conversely, in the case of two identical batteries being connected in series, the output voltage is doubled but the output capacity stays the same as either battery.
Connecting batteries in parallel increases total current capacity by decreasing total resistance and also increases overall amp-hour (Ah) capacity.
NOTE: All batteries connected in parallel must have the same voltage rating. Batteries can be damaged by excessive cycling and overcharging.
The illustration shows how multiple batteries are connected in parallel to increase battery capacity (Ah) or, in other words, increase the battery backup time.
To join batteries in parallel, use a jumper wire to connect both the positive terminals, and another jumper wire to connect both the negative terminals of both batteries to each other. Negative to negative and positive to positive. You can connect your load to one of the batteries, and it will drain both equally.
Notice that both batteries are rated at 6 volts and 10 Ah (amp hours) each. When the batteries are connected in parallel, the voltage remains the same at 6 volts, however, amp hours are doubled to 20 Ah.