Lithium battery technology is one of the fastest growing trends in the tool community, and certainly for good reason. Lithium Ion batteries have the best energy to weight ratio, meaning they pack the most power with the smallest amount of bulk.
They also experience no memory effect or, lazy battery effect. Li-ion batteries, conversely, have absolutely no memory and can continually accept a maximum charge. Additionally, Lithium Ion batteries have a very slow rate of charge loss while the battery is disengaged.