Lithium Ion cells are available, and custom packs are getting less expensive. I've been using them in my 'homebrew' bicycle headlight for about 9 months now.

Check out batteryspace.com for Li-Ion cells, packs, chargers, and protection circuits.

IMHO I would be very leery of Li-Ion cells in power tool applications, but am willing to believe that they could work in a well designed system. NiCd and NiMH cells are really far easier to use and more robust, but Li-Ion is a better chemistry (lighter, greater energy density, better for the environment, etc.), and if the protection circuitry is carefully designed, and the battery is large enough to supply the load, then they should be quite good.

Battery power tools are often 'specification gamed' with battery voltage. But battery voltage does not tell the whole story; you also need to consider the total _energy_ stored in the battery (watt hours) and the _power_ available at any given time.

In general, of NiCd, NiMH, and LiIon, NiCd is the heaviest and has the least energy storage per unit mass, but has the best power output per unit mass. NiMH has better energy density, but worse power density, and LiIon has the best energy density and the worst power density.

Power tools are generally used in a very bursty fashion, high power for short periods with delays between use. This means that a LiIon power tool will need to be carefully designed to have enough _power_ available for the bursts, but if this is taken care of, all of the other factors (weight, capacity, etc) will be _great_. [Linked Image]

-Jon