XS charger

Designed for 7 solar cells, each 1/2 W; together 3.5 W, 3.5-4.2 V
Power supply for the uC: from battery with one diode (maximum voltage at the processor: 3.0 V, minimum voltage: 2 V)

LiFePO4 battery: http://www.a123systems.com/products-cells-26650-cylindrical-cell.htm Charge algorithm:
  • CCCV (constant current followed by constant voltage)
  • standard charge method: 3A to 3.6V CCCV, 45 min
  • fast charge method: 10A to 3.6V CCCV, 15 min
  • the solar cells will deliver 1 A maximum and charging time is about 2.5 hours

The discharge characteristics are very "flat" and the output voltage is most of the time around 3.3 V. At 2.6 V battery voltage the load will be switched off.
This is necessary to prevent deep discharging.

Touch sensor as on/off switch

Some of the MSP430 value line processors have special peripherals for capacitive proximity sensing applications. In order to save the money for a switch and because it is really cool we are going to use this feature as the on/off switch for the solar-charged LED light.

Experimental system: MSP-EXP430G2 Launchpad with M430G2252 processor

Datasheet
Family guide
Errata

There's a library provided by TI for touch sensing. It has many advanced features and options. But in order to keep the code size small and everything as simple as possible we wrote a small demonstration code which is available here: http://www.opensource-solar.org/projects/chargers/repository/show/touch-test?rev=master

The source code repository can also be cloned using git:

git clone http://git.opensource-solar.org/chargers/