DIY USB 5V Solar Power Pack

DIY USB 5V Solar Power Bank

Solar energy is a renewable, free, widely available, and clean form of energy. For many years, it has been considered an important energy source because large amounts of energy can be obtained for free if utilized with modern technology. Many people are familiar with so-called photovoltaic cells or solar panels, which can be found in devices such as spacecraft, rooftops, and handheld calculators. These cells are made of semiconductor materials, similar to those in computer chips. When sunlight shines on the cell, it knocks electrons out of the atoms. As these electrons flow through the cell, they generate electricity.

In this project, we are building a portable power bank that harvests energy using solar panels. The energy harvested by the solar panels is stored in lithium polymer batteries. These batteries then provide a stable 5V voltage for use with USB devices. The power bank can also be charged via an external 5V power source. The best thing about this power bank during the day is that you don't need to remember to charge it. It charges itself using sunlight, so you won't encounter a situation where it runs out of power.

The circuit consists of two stages. The first stage is the battery charger state based on MCP73831, and the second stage is the boost converter based on LT1302-5 to convert the battery voltage to 5V.

The MCP73831 is a miniature, single-cell, fully integrated lithium-ion/lithium-polymer charging management controller. With an input voltage range of 3.75V to 6V, any solar cell rated within this range can be used as an input source. An additional 5V mini-USB input is also included, allowing you to charge a power bank in low-sunlight conditions. The controller will safely charge the battery to 4.2V. An LED connected to the controller's STAT input illuminates throughout the charging process.

The output stage is a boost converter that converts the battery voltage to 5V. It is based on the LT1302-5 fixed 5V DC/DC converter. The converted power is delivered to a Type-A female USB connector. The LT1302-5 can operate at input voltages as low as 2.2V, therefore your lithium polymer battery should have an internal low-voltage cutoff circuit.

The solar panels used in the project are rated 6V and 150mA, providing approximately 0.9Wh under ideal conditions, while the lithium polymer batteries are rated 3.7V and 4000mAh, providing approximately 15Wh. We can see that the charging time will far exceed 15 hours because the efficiency of storage and boost conversion will be less than 100%, and the energy we harvest from the sun depends on the time of day and the angle of the beam. We can easily say that a full charge using these solar panels would take several days. Since solar energy is free, any percentage is a gain.

Quantity Part Description Identifier

1 MCP73831-2ACI Lithium-ion/Lithium-polymer Charging Controller U1

1 LT1302 Fixed 5V Boost Controller U2

3 SS14 Schottky diodes D2 and D3

1 SMD LED Red 0805 D1

1 4.7uF 16V Tantalum Capacitor C1

2 100uF 10V tantalum capacitors C4 and C5

1 22uH SMD Inductor L1

1 0.1uF ceramic capacitor C2

1 0.01uF ceramic capacitor C3

1 330Ω 0805 resistor R2

1 1kΩ 0805 resistor R1

1 20kΩ 0805 resistor R3

1. 4000mAh Lithium Polymer Battery B2

1 6V 150mA Solar Panel B1

1 Female Mini USB Connector J1

1. Female Type A USB connector J2

2 2×1 Terminal blocks B1 B2