Do not fully discharge a lithium battery - you could ruin it! Lithium batteries are designed to be charged mid-cycle.

See here for more info...

and here...

I still have the original battery for my K10D and it can take hundreds of shots on a single charge but I make sure it never fully discharges.

Sorry to hijack this thread but I too am looking for a K20D as an upgrade to my beloved K10D.

John - if you're still looking to sell your K20D, could you post some details (condition, shutter count, price, etc).

Many thanks,
Matt