"Series" connectors are used to double the voltage of (2) batteries tied together as 1 battery. An example would be (2) 2100mAh 3S 11.1V batteries being tied together in series via this connector - with this adapter, the voltage would be doubled to 22.2V, but the mAh of the batteries would remain the same, and these (2) 2100mah 3S 11.1V batteries would now equal (1) 2100mah 6S 22.2V battery.
Note: Be sure to only use (2) batteries of the same mAh, voltage and C rating. Using mis-matched batteries can damage the batteries, and/or the electronics in use. Also, be sure both batteries are fully charged and balanced before use; if one battery has a lesser charge, it may be drained below a safe capacity, thus damaging the cells.
(2) EC3 female connectors for (2) batteries, and (1) EC3 male connector - in the form of a "Y".
Using 14AWG wire, thermal resistance: 200°C