A charge controller is very important in a solar power system. It prevents damage to the rechargeable battery which can occur due to overcharging or due to deep discharging. Electronic switches are used to disconnect the solar panel when the battery is fully charged to prevent overcharging. Another switch disconnects the LED lamp or other load if the battery voltage is below a threshold to prevent deep discharging. The micro energy systems are designed to work with lithium iron phosphate (LiFePO4) batteries because they have a much longer life expectancy than other rechargeable batteries. In order to allow an easy adaptation and a modular system we use microcontrollers as the core of the system. The family of processors which are used are the MSP430 value-line processors. The reason why they were chosen is the low cost below 1$ and the availability of the open source tool chain (MSPGCC) which
allows software development in the C programming language. Another advantage of this family is the very low cost development and programming system which costs below 5$.
- SPV1040 - 5 W solar battery charger http://www.st.com/internet/com/TECHNICAL_RESOURCES/TECHNICAL_LITERATURE/DATASHEET/CD00287506.pdf
- LT3652 - 2 A LiFePO4 solar battery charger http://cds.linear.com/docs/Datasheet/3652fc.pdf
The most important areas of application for such small photovoltaic systems are lighting (indoor LED lighting, street lights), communication (cell phones and wireless network devices), small computers, and entertainment devices like radios, MP3 music player etc. Other interesting areas of application include water pumping systems and electric fences.Here's a list of some typical devices and their power usage:
- LED lighting: 3.2 V, 0.5-3 W
- cell phone charging: 5 V, 1 A, 5 W
- satellite telephone: 15 V, 2.8 A
- WLAN router: 12 V, 1 A
- Mesh Potato (http://villagetelco.org/mesh-potato/) 9-12 V, 2-4 W
- Netbook computer: 9.5V, 2.5 A
Therefore the most important output option will be 5 V, 1 A. For the XS design (single cell LiFePO4) this can be achieved with a DC/DC converter. Designs which have two LiFePO4 cells in series can use a simple linear regulator to provide 5 V or also a DC/DC converter.
Work is currently focussed on the XS charger. It is intended for use with a single LiFePO4 cell and can handle a charging current of 1 A. The system voltage is 3.3 V.
Based on the first design for pico power systems, a bigger design which can handle higher currents and several cells in series with system voltages up to 13.2 V will be designed.