Solar energy is renewable, free, widely available and clean form of energy. It is considered as a serious source of energy for many years because of the vast amounts of energy that is made freely available, if harnessed by modern technology. Many people are familiar with so-called photovoltaic cells, or solar panels, found on things like spacecraft, rooftops, and handheld calculators. The cells are made of semiconductor materials like those found in computer chips. When sunlight hits the cells, it knocks electrons loose from their atoms. As the electrons flow through the cell, they generate electricity.

In this project, we are building a power bank which harvests energy by using a solar panel. The energy gained by the solar panel is stored in a LiPo battery. Then the battery is used to supply a stable 5V which is used by USB gadgets. The power bank can also be charged by an external 5V source. The best thing for this power bank during day that you don’t need to remember to charge it. It charges itself by using the sunlight and you don’t come up with an empty bank.

Circuit Design

usb-5v-solar-power-bank-schematic-620

The circuit consists of two stages. The first stage is the battery charger state based on MCP73831 and the second stage is the step up converter based on LT1302-5 which converts the battery voltage to 5V.

MCP73831 is miniature single-cell, fully integrated Li-Ion,Li-Po charge management controller. Since the input voltage range is 3.75V to 6V, any solar cell rated between these values can be used as the input source. An aditional 5V mini USB input is also included in the design which allows you to charge the power bank when sunlight is insufficient. The controller will charge the battery up to 4.2V safely. The led connected to the STAT input off the controller lights up during the entire charge process.

The output stage is a step up converter which converts the battery voltage to 5V. It is based on LT1302-5 fixed 5V DC/DC converter. The converted power is delivered to an A type female USB converter. The input voltage of LT1302-5 can be as low as 2.2V so your Li-Po battery should have internal low voltage cutoff circuitry.

The solar panel used in the project is rated at 6V and 150mA which provides about 0.9W/h in ideal conditions and the Li-Po battery is rated at 3.7V and 4000mah which can deliver approximately 15W/h. We can see that charging will last much more than 15 hours because the efficiency of storage and step-up conversion will be less than 100% and the energy we can harvest from the sun depends on the time of the day and angle of the light beams. We can easily say that it will take days to fully charge the battery by using this solar panel. Since the solar energy is free, any percentage will be of profit.

PCB Design

usb-5v-solar-power-bank-pcb

Quantity Part Description Designator
1 MCP73831-2ACI Li-Ion Li-Po Charge Controller U1
1 LT1302 Fixed 5V Step-Up Controller U2
3 SS14 Schottky Diode D2 D3
1 SMD LED Red 0805 D1
1 4.7uF 16V Tantallum Capacitor C1
2 100uF 10V Tantallum Capacitor C4 C5
1 22uH SMD Inductor L1
1 0.1uF Ceramic Capacitor C2
1 0.01uF Ceramic Capacitor C3
1 330R 0805 Resistor R2
1 1k 0805 Resistor R1
1 20k 0805 Resistor R3
1 Li-Po Battery 4000mAh B2
1 6V 150mA Solar Panel B1
1 Female Mini-USB Connector J1
1 Female Type-A USB Connector J2
2 2×1 Terminal Block B1 B2

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