eneloop D size battery adapters are designed for use with eneloop AA rechargeable batteries. eneloop C and D size spacers allow for eneloop AA batteries to be inserted and utilized in C and D size battery applications. The battery cells maintain their original eneloop AA rechargeable battery performance characteristics including 2000mAh typical (1900 minimum) up to 2100 recharges and will hold 70% of their charge for up to 10 years (when not in use). These adapters are a great alternative when C and D size batteries are not available. Battery life will vary based on the application.