What does it mean to run batteries in series or in parallel? Why would pilots do that? The short answer to the second question is: because they don’t want to buy a bigger battery. The first question requires some explanation.
Running two batteries in parallel will double the capacity (mah). A larger capacity translates to more flight time. For example, if you have two 11.1v 2200mah 50C batteries and you connect them in parallel, you will effectively have a 11.1v 4400mah 50C battery.
To connect two batteries in parallel, you will need a parallel battery harness. A parallel battery harness has two male connectors and one female connector.
The male connectors are plugged into the female connector on each battery, and the male connector is plugged into the ESC.
Running two batteries in a series will double the voltage, while the capacity remains the same. Going back to our first example, if you have two 11.1v 2200mah 50C batteries, and you connect them in a series, you will effectively have a 22.2v 2200mah 50C battery. Higher voltage means more power. You would use this setup if you have a large motor that needs greater power to perform.
You will need a series battery harness to connect two batteries in series. A series battery harness has three connectors: two male connectors and one female. One male connector has two positive leads into it. The other has one positive and one negative, as does the female connector.