Over at Hackaday.io [AndyHull] popped open some cheap LiPo battery power ... a TP4221 controlling the charge cycle for up to four 18650 LiPo cells connected in parallel. The controller also ...

They tend to have a very low operating voltage, and due to their operating principle of storing charge on parallel plates ... with modern lithium polymer (LiPo) batteries so all in all they ...