Solar-Powered Power Bank
This article introduces a power bank that can be charged via a solar panel.
Supplies
1x Solar Panel
1x TP4056 battery charging module
1x USB-A Breakout Board
1x Lithium Polymere Battery 3.7V
1x Connector JST-PH 1.25mm (matching battery)
4x Screws 2.2 x 13mm
3D printed enclosure
The Circuit
The solar panel consists of two sets of 10 solar cells. Each set of 10 cells is connected in series, resulting in a voltage of approx. 5V. A solar cell works practically like a constant current source, whose current depends on the incident solar energy. Depending on the internal resistance of the load, the voltage at the module changes accordingly: U = I * Ri. However, the voltage never exceeds approx. 5.6V.
A charge controller with the TP4056 chip is used to charge the lithium battery. This charge controller has an input voltage range of 4 to 7V, so 5V is typically suitable for the solar panel. Charging takes place even in low light, but the charging current is then low. Depending on the state of charge, the voltage at the battery is up to 4.2V when the battery is fully charged. The charge controller has two LEDs, one red and one blue. When charging is taking place, i.e. when charging current is flowing, the red LED lights up. When the battery is full, the blue LED lights up. If there is no battery but voltage is present from the solar cell, the blue LED also lights up and the red LED flashes.
To ensure that 5V is present at the USB socket at the output of the circuit, a step-up converter is required to increase the battery voltage from 3.4 to 4.2V to 5V.
One more note about performance. Under a cloudless sky, a charging current of approx. 150 mA can be achieved with optimal alignment. Under an overcast sky, 30 to 80 mA. Caution! Behind insulated glass, the charging current drops to one fifth.
Assembly
First, connect the individual modules using connecting wire according to the circuit diagram and solder them in place. Always ensure that the polarity is correct. Now connect the battery and set the output voltage to 5V using the potentiometer on the step-up converter.
The two housing parts are made on a 3D printer. Keep in mind that the power bank will be exposed to the sun for long periods of time and will therefore become quite warm. PLA is not very suitable as this material softens at temperatures above approx. 50 to 60°C and can deform. PETG is more suitable. The picture shows how the individual modules are housed in the case. The status LEDs can be seen from the outside through the slot in the charge controller. The solar module is glued to the frame with double-sided adhesive tape.