Charge Controller

The solar plane incorporates a charge controller that’s connected between the solar array and the plane’s small Lithium Polymer (LiPo) rechargeable battery.

The purpose of having a battery in the plane is safety. It’s imperative that every radio controlled model have a safe and reliable source of electrical power for the radio control receiver and the servos that drive the rudder & elevator.

Without this battery, a banked turn away from the sun’s rays might shadow the solar cells, interrupting the solar array’s output. In this situation, the unpowered receiver would result in complete loss of radio control. The plane could easily enter a non-recoverable flight attitude and crash.

Carrying a small battery alleviates this possibility. In normal flight it is not used, and the charge controller automatically maintains it at, or near full charge. As an added benefit, the battery can be drawn upon to provide a bit of extra motor power during the hand-launch of this large, heavy plane. Once flying, the throttle is reduced to a solar-only power setting.

Theory of Operation

All of the plane’s electrical loads are connected to the LiPo battery. The solar array also connects to the battery through low-resistance electronic switches in the charge controller. This configuration provides the electronics with a safe, reliable source of power during flight.

The charge controller continuously monitors the voltage of the 2-cell 3.3A-H LiPo battery. If the battery is at 8.4 volts (or higher) it is at full charge, and must not be charged further. During such moments the solar panel is electronically disconnected from the plane’s battery and loads. Briefly, all of the power for the motor, receiver, servos, and telemetry system is draw from the battery. This causes the battery's voltage to ramp downward, and after a moment the charge controller detects the reduced voltage, signaling that the battery needs a bit of charging. The solar panel's output (up to 10 Amps) is electronically connected to the battery and loads. The solar cells power the entire plane and surplus solar current recharges the battery, causing its voltage to ramp upward toward the full charge voltage of 8.4V. When this voltage is achieved, the controller disconnects the array from the battery and the cycle repeats endlessly and automatically.

Long term, the battery remains at (or extremely close to) full charge, and the flight is 100% solar powered. An analogy to this operation is the flywheel on the back of a piston engine.  

To control the speed and stability of the switching, the controller incorporates 0.4V of hysteresis plus lowpass filtering in the control loop.

Additional voltage detectors drive high intensity LEDs mounted on the outside of the fuselage that are visible from the ground during flight. These LEDs indicate the battery's state of charge in increments of 25%, 50%, 75%, and 100%. An additional LED displays the instantaneous state of the controller's electronic switch.

I included provision to include a second, identical 7.4V, 3.3A-H battery for flying in conditions of low sun-angle (e.g. winter mornings) or on cloudy days. Two batteries provide a flight time of over 30 minutes with no solar contribution.

Charge Controller Schematic (PDF)

Charge Controller Photos

Back to the Solar Plane Main Page