The S number is the number of cells in series of a Lipo battery. 1S = 3.7V, 2S = 7.4V and so on. You connect the voltage detector to the balance plug of a lipo battery. The balance pug has S+1 pins, so 2S has 3 pins, 3S has 4 pins and so on. 1S batteries don't need a balance plug. You can plug the voltage detector to the balance plug and leave it on while your connected tothe main power plugin to your model. This is important if your voltage detector has a low-voltagealarm, which you would could hear while driving the tank, and then know that the voltage per cell of your lipo is as low as you want to take it.

Russ