Solar charger for three AAA batteries
This project does not directly involve a microcontroller. However, it is designed with a goal to power a PIC-based device. I used electronics from a garden light unit. This device has a solar cell mounted on the top if the light, an LED, a board with a circuit that turns LED on in the dark, and a battery. The battery block is a set of two AA Nickel Metal Hydride Batteries, the reverse engineered schematics of the control board is shown below. The photo cell has a high resistance in the dark (more that 1Mohm), while its light resistance is only about 150Ohm. This way the transistor, which serves as a key, is closed during the day time and the LED is not powered, allowing the solar cells to charge the battery.
The solar cells are very robust and good sealed. The unit comes with two solar panels connected in series. It delivers about 7.5V at full sun, and its short-cut current is about 22mA. This voltage was too high for my purposes, so I connected the panels in parallel and got 43mA at 3.8V. The back side of the unit has plastic clips that hold the AA batteries. I removed two out of four clips and glued up a holder for 3 AAA NiMH batteries instead.
After quite a bit of research I decided to give a try to a circuit described in Application Note 484 "Harnessing Solar Power with Smart Power-Conversion Techniques" available from the MAXIM web site. The note has a pretty clear description of the underlying approach and a schematics, which uses MAX856 DC/DC converter and micro-power dual comparator IC MAX982. I like the idea the circuit is based on and the way of controlling the DC/DC converter. It works exactly as described and delivers up to 100mA at 4V when powered from an external 3V power supply. Since the solar cell cannot provide 75mA needed for a standard charge of my batteries, the trickle charge is applied instead. The starting charge current was about 23mA in my experiment right after attaching deeply discharged batteries. Although, MAX856 is designed for output voltages of 3.3V or 5V, the output voltage of the entire block is 4V (at full sun) due to periodic shut-downs of the converter by the comparators. This, however, cannot be considered as a reliable voltage regulation and does not eliminate a need in a voltage stabilizer, if a constant voltage is desirable. I use LTC3204 regulated charge pump attached directly to the rechargeable batteries to get stable 3.3V at required 20mA. The last IC is not a part of this unit and is mounted on the application board powered from that charger.
The PCB is intended to be mounted on place of the original one, so it mimics its geometry. The board is fixed by three plastic clips located on the posts at the back side of the solar battery case and hangs right above the batteries. The top part of the board has a placeholder for assembling a circuit similar to the original one, if I need a sensor for the dark time of the day. It accepts a transistor in the SOT23 package, The green inductor I used is from the P0751/2 series manufactured by Pulse and available from Digi-Key. The Schottky rectifier diode is BAT42W in the SOD-123 package, and the resistors are in the 0603 package.
Downloads:
