The Shimano SM-BTR1 is a rechargeable 7.4 volt lithium-ion battery for the Dura-Ace Di2 and Ultegra Di2 component systems.
Shimano recommends re-charging about every 1,000 miles. There is no charge memory so the battery can be charged again and again at any intervals. The battery can go through 300 charge cycles without diminishing and will slightly and slowly diminish afterward.
The SM-BTR1 weighs 70 grams and charges is 1.5 hours. Its charger, wiring kit, and frame mount are sold separately.