There were some earlier discussions on the reflector about using 10 "4/3 Fat A" batteries inside the K2 case. These are very high capacity (4.5 A-Hr) Ni-Cd or Ni-MH rechargeables, but not that common. As I recall, these should fit in the space allocated for the standard battery. The cost of a setup (~$60-70) dissuaded me from further exploration. 73 Jay K1UC