How To Make A Solar Battery Charger

Materials:
     You will need to understand the difference between wiring in a SERIES or in PARRALLEL. A SERIES is when each Cell (Solar or Battery) is wired Positive to Negative to Positive to Negative and so forth. A SERIES will increase Voltage while maintaining Amperage. PARRALLEL is when each Cell is wired Positive to Positive to Positive and then Negative to Negative to Negative. PARRALLEL increases Amperage while maintaining Voltage.



     You will need to configure the Solar Cells to produce 1.2-1.5 VDC or Volts in Direct Current. To increase Voltage, while maintaining Amperage, wire the Cells in a SERIES. Three Cells producing 0.5 Volts each wired in a SERIES produces 1.5 Volts (in good sunlight of course). The alternative to wiring in a SERIES is to wire in PARRALLEL which will increase Amperage while maintaining the Voltage of a single Cell. Three Cells producing 0.6MA and 0.5 Volts each wired in PARRALLEL will produce 1.8mA at 0.5 Volts. mA is Millionths of an Amp.

     A Diode is a one-way switch for elecrical current. Current flows from Cathode to Anode but not Anode to Cathode. If the band marking Cathode is not visible, you must either contact the manufacturer to find out which end is which or use a Multimeter. A Multimeter can be purchased for less than $15 at your local Wal-Mart or Automotive Parts store and is useful for many projects including this one where you can test your solar charger's output in different lighting conditions. Using a Multimeter, set the dial to Ohms(Ω), this mode is used to test electrical circuits for continuity and the resistance (Ohms(Ω)) within the circuit. When using a Multimeter to determine which direction the diode is allowing current to flow, remember that with DC, electrons move from negative to positive. One thing to remember is that a good circuit with no resistance will be at 0 Ohms(Ω). This means that when the Multimeter displays 0 you have a good circuit, you can touch the two probes closely together on your skin to see what the Multimeter would display when a high resistance circuit is detected. Since we know that direct current flows from negative to positive we can determine Cathode and Anode in the following way: when connecting the probes to the two leads on the diode, if the circuit is completed, indicated by Ohms(Ω) going to 0 (or near 0 resistance) we know that the negative probe is touching the Cathode and the positive is touching the Anode. Once you have determined the Cathode, mark the Cathod side with a marker or small bend in the lead.



     Most if not all battery holders come prewired in a SERIES. You want the batteries to be in PARRALLEL with one another. They will currently be positive to negative to positive to negative, etc.. You want them to be positve to positive to positive and then negative to negative to negative as shown above on a SERIES circuit.

     Radio Shack sells a Silicon Solar Cell which is 2x4cm and delivers 0.3amp at 0.55VDC. We want the Voltage to be 1.2-1.5V so we will take 3 of these and wire them in a SERIES to give 0.3 amps at 1.65 volts. If we wanted more Amperage, we would wire another "set" of 3 Cells in a SERIES and wire the 2 sets together in PARRALLEL to produce 0.6 amps at 1.65 volts.

     To wire the Solar Charger together, you will put the diode on the negative wire between the Solar Cells and the Battery Cells. The Negative wire on the Solar Cells should be soldered to the Cathode and then the Anode should be soldered to the Negative wire of the Battery (holder). So the one-way current is from Solar Cells to Battery Cells which is Cathode to Anode. The positive wire on the Solar Cells will be connected directly to the positive wire on the Battery Cells.

     Packaging the Solar Charger is up to you. The 2x4cm cell used in the above example is very breakable and must be protected in some fashion. Radio Shack sells pre-packaged cells which have a plastic case with a translucent top. They come in a variety of Voltages and Amperage. You can modify the charger with plugs so you can change out battery holders to different sizes as needed.

Notes:

The primary concerns in designing your Solar Charger is how to connect the Solar Cells to produce enough Voltage to charge your Battery Cells. Since most all Solar Cell specifications do not equal the specifications of a Battery Cell you will need to do the math to determine the design for Parrallel and Series connections for both the Solar Cells and the Battery Cells. With small Solar Cells this will often require connecting your Solar Cells in a Series to produce enough Voltage to be equal to or slightly more than your Battery Cells to force current into the Battery Cells. If you want faster charging capabilities you will want to wire another set of Solar Cells in a Series and then take the two Solar Cell "arrays" and wire them in Parrallel. This configuration will give you the same amount of Voltage as your first set of Series wired Solar Cells but will increase the output Amperage of the array thus increasing the rate of charge.

The primary concept to remember during the design is that Voltage, not Amperage, will determine which direction current will flow. If the battery, or battery array (pack) has more Voltage than the output of the Solar Cells the diode will prevent the Battery Cell from moving current to the Solar Cells where energy would be lost. In this state no electrical current is moving with the exception that if you put highly discharged Battery Cells into a charger with lightly discharged Battery Cells while the battery tray is wired in Parrallel, the higher Voltage Cells will discharge current into the lower charged cells. This is the same concept as moving current from the Solar Cells to the Battery cells.

When the solar cells are producing more Voltage than the battery cells then current will flow thru the diode and charge the battery cells. Amperage will not determine the direction of electrical flow, it is a measure of capacity where Voltage is a measure of electrical force. Whichever has the most force (Voltage), the Solar Cells or the Battery Cells will determine the direction of electrical flow but by using a diode we essentially cut off electrical flow when the Solar Cell is shaded or no longer producing enough Voltage to charge the Battery Cells.

You cannot wire the battery pack into a Series as it would increase the Voltage of the battery pack and require not only a completely different arrangement of the Solar Cells but would also prevent charging in all but the brightest of conditions.