The "S" in 2S batteries stands for "series." In the context of batteries, a series connection means that the positive terminal of one battery is connected to the negative terminal of the other battery, creating a single voltage that is the sum of the individual battery voltages.

For example, a 2S battery pack consists of two individual batteries connected in series. If each battery has a nominal voltage of 3.7 volts, the total voltage of the 2S battery pack would be 7.4 volts.

Battery packs are often labeled with a number and an "S" to indicate the number of batteries that are connected in series. For example, a 3S battery pack would consist of three batteries connected in series, while a 4S battery pack would consist of four batteries connected in series. The number of batteries in the pack determines the total voltage of the pack.

In the context of RC vehicles, the voltage of the battery pack is an important factor to consider when selecting a battery. Higher voltage packs can provide more power to the motor, but they may also require more advanced electronic speed controllers (ESCs) to properly regulate the flow of current.