Hi Michael. i do have a LiIon Hard Carbon external battery pack but i would not use Li Polymers in my pocket because of the fire hazards. i use 10 1/2 "AA" (250mAH) plus one regular "AA" NiCad in my KX1. Send me an email and i will send you the modification details. The KX1 is usually hand held so the batteries stay warm. Paul w0rw at aol.com