The Shimano SM-BTR2 is an older generation model internal battery that weighs about 50g, has a rated capacity of 500 mAh and a rated voltage of 7.4 V.
Its normal operating temperature range is between -10C and 50C. You charge the battery by connecting your charger to the Junction box.
If storing the battery away for a long period, remove it when the battery level is 50% or higher (the indicator is illuminated green) in order to prolong its useful life. It is recommended that you charge the battery about once every half a year.
Store the battery or bicycles with the battery installed in cool indoor places (approx. 10 to 20 °C) where they do not become exposed to direct sunlight or rain. If the storage temperature is low or high, the performance of the battery is reduced, and its available time will be shorter. When you use the battery after a long storage period, be sure to fully charge it before use.
Note that this older battery is not synchro-shift compatible. You will need to use the newer BT-DN110 for that.