Step 1

Storing the Energy

In this activity you will need

• Rechargeable battery (1.2 volt Ni-MH or wireless phone battery)
• Battery holder
• Solar Array (or 2 if available)
• Wind Generator (DC motor and propeller)
• Stepper Motor
• Rectifying Diode (Skottky diode is best)
• Alligator clip leads

Step 2

Before You Start

There are a number of issues associated with recharging batteries.

• The source must have a higher voltage than the batteries being charged or current will not flow to the battery. Think of two ponds connected by a stream. Water will not flow between them unless one is higher.
• With a solar cell, the voltage will drop off as light levels fall. If the battery has a higher voltage, current will flow in the opposite direction, discharging the battery.
• Leaving the charging circuit on for too long may result in overcharging the battery. This could shorten its operational life and for some types, might cause damage or explosions.
• Another issue is the amount of charging current. Some cells cannot tolerate high currents and may be damaged. Commercial battery chargers are specifically designed for the properties of the cells they charge. Never use a charger which is not designed for the cell!

In this part of the activity, you will only charge the battery for a short time and you will need to use the number of cells that matches your solar array.

Pictured below are two types of rechargeable cells.

• The first is a battery of Nickel Cadmium cells. These are surplus wireless phone batteries and each cell has a voltage of 1.2 volts. The battery can be taken apart and the individual cells used. Either use the leads on the batteries or tape wire leads to the terminals. Never solder wires to the cells as there is a risk of explosion!

Figure. Nickel Cadmium cells

• The second is a Nickel Metal Hydride cell in AA format. You can purchase standard holders and combine the cells in series to make higher voltage batteries.

Figure. Nickel Metal Hydride cell

Because of their special charging properties it is NOT recommended that Lithium Ion cells be used in this activity.

Step 3

Connect the Circuit

If you are using a single solar battery, use one NiCd or NiMH cell. Make sure you pay attention to the polarity (+ -). The positive terminal of the solar battery must be connected to the + cell terminal.

Figure. Solar battery connected to a single NiCd cell

• Shine an incandescent light on the solar battery and leave the circuit on for about 10 minutes. You can connect a voltmeter to make sure the voltage is high enough to charge the cell. Make sure you have the probes in the correct jacks and the selector switch on the 2 (or 20) volt DC range.

Figure. Solar Battery charging NiCd cell at 1.3 volts

Step 4

Test the Charge

Let the battery charge for at least 10 minutes.  Then

• Disconnect the cell (or battery) from the circuit.
• Measure the cell's voltage with the meter.
• Using alligator clip leads connect the cell to a small motor to demonstrate the charge.
  Note. A single cell will not light a LED (with resistor attached). If you used two solar batteries and two cells you can use the LED.

The next link is a short video clip showing a cell being tested.

Video. Cell Test

Make an appropriate entry in your module portfolio

Step 5

Going Further

You can connect the rechargeable cell to the wind generator you developed in a previous activity. Make sure you determine the polarity of the generator leads (positive and negative). You can do this by spinning the propeller and attaching the multi-meter configured as a voltmeter. The lead on the red probe is positive when the negative symbol on the meter display is NOT displayed.

Make sure the meter is connected so you can monitor the voltage. For one cell keep the voltage between 1.5 and 2.0 volts. You can do this by adjusting the fan speed or distance from the propeller.

When you stop the fan the cell will probably feed current back through the generator and spin the propeller!