A lesson in quick-and-dirty solar charging

The BP solar panels, BP380U, in place at Burning Man.

The Burning Man event is well-known for its tribute to alternative lifestyles and an ecological creed. But among the most notable facets of Burning Man is its Black Rock City desert locale, a hundred miles north of Reno, Nevada. It is characterized by daytime temperatures routinely exceeding 100°F with extremely low humidity and dry weather which rapidly and continually wicks moisture from the human body. At nearly 4,000 ft above sea level, the atmosphere provides much less filtering than at lower elevations.

For many, staying cool in the desert heat poses a challenge. But it is also an opportunity to demonstrate what can be done with a photovoltaic power source in an area far removed from conventional ac mains. This was the motivation for a solar-powered charging system for a marine deep-cycle battery to power lights, radios, and a water mist system consisting of a water pump feeding a mist hose with nozzles.

The resulting assembly is an interesting illustration of what can be done with circuitry consisting of not much more than a few demo boards.

A block diagram of the charger system design. A current-sense system goes in series at the battery using a shunt sense-resistor to measure the input charge current and battery discharge current.
Select figure to enlarge.

The charger takes input power from two BP solar panels (BP380U), each outputting a peak power of 80 W at 17.6 V and 4.55 A. A principle task in devising a charging circuit is to accommodate the output voltage from the solar panel which varies widely, between 0 and 20 V depending on the sun position. The circuit uses a regulator that will accept this wide input range and maintain low current intake (Max output current from the panels is about 4 A each.) while regulating a fixed voltage on the output. Principle components include a Linear Technology µModule dc/dc switching regulator, the switching mode buck- boost LTM4607 power supply µModule regulator mounted on a demo board (DC1198A-B).

The LTM4607 is a LGA-packaged (15×5×2.8-mm) chip that houses all support control components of a buck-boost dc/dc switching regulator. The switch-control circuitry and power FETs are built into the regulator, making it easy to use. The result is a clean layout with just a egulator, inductor, and a few capacitors and resistors. The 4.5 to 36-V input voltage range regulated to a fixed 20-V output well suits the solar panel 0 to 20-V output. The device can load up to 5 A in boost mode and in 10 A buck mode. The circuit is 91% efficient at the solar panel peak power and actively uses the benefits of a buck-boost wide-range input.

The full system in operation. An LCD reads out V_s (solar panel voltage), V_c (charge circuit voltage), V_b (battery voltage), C (charge current) and is controlled via the DC590B PIC microcontroller. The ‘C’ in the display indicates charging. When in discharge, the display will change to a ‘D’.

For the purposes of this design, output is regulated to 20 V. This 20 V powers an LTC1435/LT1620 synchronous step-down switching regulator controller and current-sense amplifier which regulates average output current independent of input and output voltage variations. The result is a high-efficiency low-dropout battery charger with over 95% efficiency.

An LTC1435/LT1620 demo board (DC133A) controls charge current to a steady 4 A at a regulated 14 V. The demo board for this application is quite close to the application circuit on the first page of the LT1620 data sheet with the exception of the FB resistor (110 kΩ). It is changed to a variable potentiometer which hand-adjusts the output voltage to set the battery float voltage to 14 V.