integrated battery would be very helpful

This part is working properly. The sturdy rate of return you can get a small fountain at slightly cloudy sky already.
However, it is not possible to obtain a consistently high fountain. Varies the light intensity, which indeed very often the case, then it shoots sometimes too high and sometimes too low. As a built-in battery / capacitor would be helpful to add a bit constant current. But that is probably required for the price too much. In this respect, 4 stars.