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 How much energy is removed from the battery during a typical evening? LED requires 20mA x 3.2V x 6hrs = 384 mW hrs (power x time = energy)LED requires 20mA x 3.2V x 6hrs = 384 mW hrs (power x time = 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 How much charge? Dividing by the battery voltage gives the charge removed: 550 mW hrs / 1.2V ≈ 460 mA hrsDividing by the battery voltage gives the charge removed: 550 mW hrs / 1.2V ≈ 460 mA hrs

The Prior Art Dissected

On to the Solar Panel Requirements 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 How much charge is restored if the panel is connected directly to the battery? What assumptions should we make? How about 10 Hours of Daylight10 Hours of Daylight 50% Incident Light50% Incident Light This gives 50mA x 10hrs x 50% = 250mA hrs Is this enough? We need 460 mA hrs. No. 

Charge Options? Use two solar panels in parallel to boost the current (although we seem to be throwing away the excess voltage?) Modify the existing panel for higher current (at the expense of voltage).

Modified Solar Panel Configuration

Modified Characteristic I SC = 100 mA, V OC = 2.15 V (V OC still greater than V BAT )

A Possible Solution Now we have: 100mA x 10 hrs x 50% = 500 mA hrs. Is this enough? We need 460mA hrs. Yes! Is this enough? We need 460mA hrs. Yes!

Solar Panel Update to System Block Diagram Solar Panel I SC = 100mA V OC = 2.15V I AVE = 50mA Δt = 10hrs Q = 500mAHrs Charge Controller Rechargeable Battery 1.2V NiCd Zetex LED Driver LED 20mA 3.2V(min) Switching Control Mode Selection Photo Sensor Timer

And now the Charge Controller… Solar Panel I SC = 100mA V OC = 2.15V I AVE = 50mA Δt = 10hrs Q = 500mAHrs 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)

Which Charge Method to Choose? Semi-Constant Current Charge Most Typical Charge System Most Typical Charge System Simple and Economical Simple and Economical Typical Charge Time = 15 Hrs Typical Charge Time = 15 Hrs Typical Charge Current = 0.1 It Typical Charge Current = 0.1 It (0.1*700 mA Hrs = 70mA) Time Controlled Charge More reliable than Semi-Constant Current More reliable than Semi-Constant Current Slightly more complicated. Requires timer. Slightly more complicated. Requires timer. Typical Charge Time = 6-8 Hrs Typical Charge Time = 6-8 Hrs Typical Charge Current = 0.2 It (140mA) Typical Charge Current = 0.2 It (140mA)

Semi-Constant Current Charge Seems Viable With our low average current of 50mA, and charge time of 10 hrs, the Semi- Constant Current Charge method seems viable. Also, if we are concerned about over charge, we can extend the on-time beyond 6 hrs. This method is more economical and may not require a timer for this application.

Charge Controller Update Solar Panel I SC = 100mA V OC = 2.15V I AVE = 50mA Δt = 10hrs Q = 500mAHrs Charge Controller Semi-Const. Current Method Rechargeable Battery 1.2V NiCd Zetex LED Driver LED 20mA 3.2V(min) Switching Control Mode Selection Photo Sensor Timer