I have the original Vx1 and the battery life was not great but it did kept its charge even after 6 months of storage.
Upgrades mentioned here: Thoughts on Flipsky VX1
Batteries like these.
https://www.amazon.com/MakerFocus-Rechargable-Protection-Insulated-Development/dp/B07CXNQ3ZR/ref=sr_1_15?dchild=1&keywords=MakerFocus&qid=1603463779&sr=8-15