The Shimano SM-BTR1 is an older generation model external battery. Most bikes these days will be equipped with internal batteries such as the BT-DN110.
It weighs about 50g, has a rated capacity of 500 mAh / 3.7 WH and a rated voltage of 7.4 V. You charge the battery by taking it out of the bike and putting it in the SM-BCR1 external battery charger.
When storing the battery away for a long period, remove it when the indicator is at the half-way level (for the FLIGHT DECK) or when the green indicator is flashing (for the SM-EW79A) in order to prolong its useful life. It is recommended that you check the condition of the battery about once every half a year.
Note that this older battery is not synchro-shift compatible. You will need to use the newer BT-DN110 for that.