Solar cells are ideal projects for the classroom. They produce a low current and a low-voltage, so they are relatively safe when used properly. With solar cells, you can easily construct a circuit that will charge flashlight batteries. Fundamental to the design of such a circuit is the ability to calculate how many solar cells are required to charge the battery.
The answer to this question depends on how long you want to wait and the rated battery voltage. The more solar cells you use, the faster the battery will charge. And the higher the battery voltage rating, the more solar cells you will need.
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Things you need
- Solar cell specification sheet
- Flashlight batteries
Obtain the solar cell's output voltage rating and output short-circuit current rating. Look on the solar data sheet from the manufacturer, or contact the manufacturer directly. Solar cells usually have an output of 0.5 volts. Solar cells usually don't have the same short-circuit current ratings, or tolerance levels. They can vary from as low as 1 milliampere to as high as 900 milliamperes.
Determine the solar cell's charging current. Measure the charging current of your solar cell under sunlight conditions or use the short-circuit current rating as an estimate of the charging current. Connect an ammeter in series with your solar cell and the battery and measure the current. This is the charging current that will charge the battery under the current sunlight conditions.
Obtain your battery's milliampere rating. Look on the side of the battery or a specification sheet. The AA battery for this example has a milliampere-hour rating of 1,200 milliamperes-hours and a voltage rating of 1.225 volts.
Calculate the minimum number of solar cells needed to be placed in series to obtain a voltage greater than the battery voltage. Divide the battery voltage by the solar cell output voltage rating. Then round up to the next highest integer. For a 1.225-volt battery with a 0.5-volt solar cell, the division results in 2.45, and rounding up results in 3. So you will need three solar cells at a minimum.
Calculate the milliampere-hours required to completely charge the battery. Divide the milliampere rating of the battery by the charging current you obtained in Step 1 or Step 2. For a battery with a 2,200 milliampere-hour rating and a solar cell with an output current of 100 milliamperes, the battery will take 22 hours to charge because 2,200 divided by 100 is 22.
Tips and warnings
- Decrease the battery charging time by using more solar cells in a series-parallel arrangement. If 66 solar cells were arranged in a series-parallel arrangement; 22 parallel legs with three solar cells in series in each leg, the total charging current would be increased by 22, or 2,200 milliamperes. This would reduce the charge time to one hour.
- It may take less time to charge the battery if it is already partially charged. The battery may never charge if the battery has been depleted.
- Many solar battery charging circuits use a diode in the circuit to prevent overcharging. For this example, this would require that you add two solar cells in series to ensure that the diode is turned on. So if you include a diode for the example, you would need five solar cells to charge the battery.
- Batteries can explode or catch fire if overcharged or if faulty. Always connect solar cells to batteries using properly designed safety circuits.
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