Aliexpress
Here are coupons that you can use.
Latest coupon and price update (UTC):
Banggood
Banggood warehouse is located in China. There is one great coupon!
Link: Banggood product page
Coupon (11.87 USD): BG10OFFAFF1
(CouponsFromChina.com tip: this code has to be applied in your 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): BGQBJ, linepayflt, BCNLXK, BGBFCR41, BgcLuBofErtA10, BGFLS15, BGAFF10OFF, BGTORCH, 4BLF, BGHONEY2110, BG10OFFAFF, BGwhielabel
(probably won’t work, but you can try them out)
Price history and price alert
Check out price history chart (for all/most countries) for 1x Basen 21700 30A 4000mah Battery.
Price records source(s): Banggood
Click on image to enlarge. 
You can set price alert (for Banggood).