Such a product does already exist (with a hand crank).
The buffer battery is needed because phones generally don't like having variable voltage/current to charge from. They normally choose what speed they wish to charge at (depending on charger type), and if they can't draw their desired current and the voltage drops then they stop charging entirely.
Also, humans generally expect a crank to have constant torque. Whereas if you wanted constant charge power, then the faster you crank the easier it gets, and conversely the slower you go the stiffer it gets, which is very unintuitive.
By using a buffer battery, you can solve both problems. And users can choose to crank faster to charge the buffer quicker, which is what humans expect.
The buffer battery is needed because phones generally don't like having variable voltage/current to charge from. They normally choose what speed they wish to charge at (depending on charger type), and if they can't draw their desired current and the voltage drops then they stop charging entirely.
Also, humans generally expect a crank to have constant torque. Whereas if you wanted constant charge power, then the faster you crank the easier it gets, and conversely the slower you go the stiffer it gets, which is very unintuitive.
By using a buffer battery, you can solve both problems. And users can choose to crank faster to charge the buffer quicker, which is what humans expect.