A lesson in quick-and-dirty solar charging

Unfortunately this caused the set-up to either turn on or off depending on the amount of sunlight on the panels. A better solution is to adjust the charge current with respect to the solar panel voltage so that charge current varies with the level of sunlight on the solar panels. In other words, when the solar panel output voltage drops, so should the charge current.

One approach to synthesizing the behavior is to use an op amp (LT1006) and MOSFET to control the PROG pin on the LTC1435 (DC133A). This pin controls the level of charge current output in a ratio to the amount of current sunk from the pin. The op amp tracks the solar panel voltage, adjusts the Rds-on of the MOSFET and controls the current, depending on the voltage level of the panels.

When the panel voltage is at its maximum, the op amp turns the MOSFET on, letting maximum current sink out and deliver 4 A of charge current. When panel voltage is low, the charge current should also be low to maintain proper power output. A potentiometer can adjust MOSFET current biasing such that it cuts charge current in half when the solar panel voltage reaches its midway point, 10 V. At maximum, 20 V, charge current should be 4 A. The approach in this case was to set the potentiometer for maximum current when testing at a solar panel voltage of 20 V and to reduce the voltage to 10 V while adjusting the potentiometer slowly to a 2 A charge current without latching up.

Resources

Linear Technology Corp. www.linear.com

Thanks to Graham Freeman for lending the BP solar panels for this project. Simon Lim performed the initial prototype design for the solar power charger and helped throughout the entire design. Mark Thoren provided source code on the DC590B and ongoing software support. Fran Hoffart and Jay Lin helped with debugging. Thanks to Doug Coker, Mike Fahmie, and Dev Gopalkrishnan for conceiving of and inspiring me with the original idea of a mist system and for providing contact to Graham Freeman.