There are three vital factors for rating a Power Tools Battery Performance for use in DeWalt, Sealey, Ryobi, Draper and Black and Decker Power Tools.
This is a measure of the batteries energy. As Voltage increases so does power and run time. Voltage can be thought of as the size of a car engine.
This is a measure of the capacity of the battery which effects run time. This can be simply compared to the size of the petrol tank in a car.
A measure of a batteries life; how many times can the battery be recharged. This is never marked on the battery and can vary considerably depending on the usage of the battery during its life.
The other three factors of course for the user are the weight, size and cost.