The C Spacer Pack with 2 AA NiMH Batteries from Watson provides you with two identical plastic spacers and two 1.2V, 2300mAh batteries. Each spacer is the size of a C battery. One AA slides into the spacer, which can then be inserted into any device that accepts C batteries--effectively turning your AA into a C. The spacers will accept any AA battery.
The included AAs feature a higher charge capacity than ordinary nickel-cadmium batteries. They drain themselves completely of their capacity before recharging, ensuring that they don't develop memory effect--so that their full capacity continues to be usable after each charge. The AAs are rechargeable up to 1000 times, environmentally friendly, and free of both mercury and cadmium.