Design Process Analysis & Evaluation Part II Example Design: Solar Candle by Prof. Bitar

Current System Block Diagram Solar Panel Charge Controller Rechargeable Battery 1.2V NiCd 700 mAhrs Zetex LED Driver 70% Eff. LED 20mA 3.2V(min) Switching Control Mode Selection Photo Sensor Timer

Changing Focus to Charging Question: How much charge is removed from the battery during a typical evening? Answer: LED requires 20mA x 3.2V x 6hrs = 384 mW hrs (energy)LED requires 20mA x 3.2V x 6hrs = 384 mW hrs (energy) Converter is only 70% efficient, so energy taken from battery is 384 mW hrs / 0.7 ≈ 550 mW hrsConverter is only 70% efficient, so energy taken from battery is 384 mW hrs / 0.7 ≈ 550 mW hrs Dividing by Battery Voltage: 550 mW hrs / 1.2V ≈ 460 mA hrs (charge) removed from battery in 6 hrs, which will need to be restored.Dividing by Battery Voltage: 550 mW hrs / 1.2V ≈ 460 mA hrs (charge) removed from battery in 6 hrs, which will need to be restored.

On to the Solar Panel… After taking the Home Depot Landscape Light apart, I made the following measurements (in direct sun): I SC = 50mA V OC = 4.3V

Solar Panel V-I Characteristic

Solar Panel Considerations Make Conservative Assumptions 10 Hours of Daylight10 Hours of Daylight 50% Incident Light50% Incident Light How much charge is restored if the panel were connected directly to the battery? 50 mA x 10 hrs x 50% = 250 mA hrs50 mA x 10 hrs x 50% = 250 mA hrs We need DOUBLE this amount !

Available Solar Cell Power

Available Solar Cell Energy LED Needs: 20mA x 3.2V x 6hrs = 384 mW hrs (ie: Joules) Solar Cell Provides: 170mW x 12hrs x 50% = 1030mW hrs (ie: 3672 Joules) Seems feasible! The solar cell is capable of providing more than enough energy … ASSUMING WE OPERATE AT THE MAXIMUM POWER POINT !! But what if we don’t ???

Reduced Solar Cell Power at Lower Voltages

Reduced Solar Cell Energy LED Needs: 20mA x 3.2V x 6hrs = 384 mW hrs (ie: Joules) If we charge directly at the lower voltage of 1.2V, Cell provides: 60mW x 12hrs x 50% = 360 mW hrs (ie: 1296 Joules) NOT ENOUGH !! 

Charge Options? Find (or construct) a similar panel (same area) configured for higher current and lower voltage (ie: I SC = 100mA, V OC = 2.15V ) Use a Buck DC-DC Converter to efficiently lower the voltage for charging the battery. Use two solar cells in parallel to boost the current (although we would be severely under-utilizing the available energy).

Modified Solar Cell Close inspection of the Solar Cell reveals it is actually a panel made up of 8 independent cells wired in series. Therefore, if these were reconfigured to be two groups of four in parallel, the current could be doubled, and the voltage halved.

Modified Solar Panel Configuration

Modified Solar Cell (Panel) V-I Characteristic

Check Solar Cell Energy Again… LED Needs: 20mA x 3.2V x 6hrs = 384 mW hrs (ie: Joules) Modified Cell provides: 120mW x 12hrs x 50% = 720 mW hrs (ie: 2592 Joules) MORE THAN ENOUGH !! MORE THAN ENOUGH !!

Solar Cell Update to System Block Diagram Solar Cell I SC = 100mA V OC = 2.15V I AVE = 50mA Δt = 12hrs Charge Controller Rechargeable Battery 1.2V NiCd Zetex LED Driver LED 20mA 3.2V(min) Switching Control Mode Selection Photo Sensor Timer

NiCd Charge Control Methods (Panasonic)