Batteries are made up of one or more individual cells. Each cell has a voltage. NiCd and NiMH cells are rated at 1.2 Volts per cell, so 7 of them connected together in series make 8.4 Volts. Each time you add a cell, the Volts jumps by 1.2.

LiPoly cells are rated at 3.7 Volts per cell. Connect 2 of them together in series and you get 7.4 Volts. Connect 3 cells together in series, and you get 11.1 Volts. Each time you add a cell, the Volts jumps by 3.7.

A 7.4 Volt LiPoly will work fine in place of an 8.4 Volt NiMH or NiCd. It is lighter than the NiCd or NiMH battery, so you don't need as much power. The power difference is very small, anyway.