https://m.aliexpress.com/item/1005005847389753.html?browser_id=b993e6b920ca474594606ef6940716d3&aff_platform=msite&m_page_id=srrhouwgwgcavjfj1898dc427d116eca525

https://m.aliexpress.com/item/1005005847389753.html?browser_id=b993e6b920ca474594606ef6940716d3&aff_platform=msite&m_page_id=srrhouwgwgcavjfj1898dc427d116eca525621a4dc&gclid=&pdp_npi=3%40dis%21GBP%2167.20%2167.2%21%21%21600.00%21%21%402100b18f16903005900425077d0733%2112000034562776259%21sea%21GB%210&algo_pvid=2c58bd0c-3abc-4119-a8ee-32ab4b87e1a8 https://m.aliexpress.com/item/1005005847389753.html?browser_... Just search sodium ion on AliExpress and you will get all sorts from 18650s andmother sizes as well as pouch batteries in various quantities. The voltage curve on them is really extreme compared to Li-on, should make estimating SOC very reliable.