Series(S) is a way to assemble lithium batteries to increase voltage. If you take 2 cells that have a nominal voltage of 3.7v and build them together in series, the voltage would increase to 7.4v on the battery. 3 cells would be 11.1v etc. If you sort our site by 4s packs, they would all have a voltage of 14.8v with different mAh options. If you have any other questions about series builds, contact our battery tech team here.