Aliexpress
Click here… to search for Xiaomi Drone 5100mAh RC Battery on Aliexpress
Here are coupons that you can use.
Latest price update (UTC time):
Banggood
Banggood warehouse is located in China.
Link: Banggood product page
Coupon (71.70 USD): Mirc20
Coupon (80.67 USD): AFfbG10
(CouponsFromChina.com tip: apply code in your basket (cart) after clicking “Proceed to checkout”. Click to copy code and open shop)
Exclusive Allowance Coupons: https://bit.ly/3xWgD9k
Old coupons (from the oldest to the newest): BGMiB18
(sometimes old coupons work, so you can check them out (this site checks them on every price update))
Price history and price alert
Here is price history (for all/most countries) for Xiaomi Drone 5100mAh RC Battery.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).