Schools OnLine Photovoltaics Student investigations

Solar Electricity: Instructions for connecting the PV module and using it for battery charging

The PV kit contains:

  • 1 Intersolar Phoenix B104W – PV module ( 300 mm x 450 mm approximately ) with an operating current at peak of 393 milliamperes. The peak short circuit current around 400 mA. Efficiency of module 4.2%. Or another PV panel of similar output.
  • 1 diode
  • 2 battery holders for AA batteries 1 rechargeable AA battery ( UNICOROSS RX6+ ) uncharged. The battery is available through Argos: catalogue number 980/1987, price £2.75 for 2 batteries or COMET etc. . This battery was chosen because it has a “superfast” charge rate of 350 mA for 2 hours, in addition to the normal charge rate of 65 mA for 14-16 hours and the rapid charge rate of 150 mA for 5 hours. One of these batteries can be left even in the full sun (with a warning not to leave it charging for too many hours).
  • 4 screw terminal connectors
  • red and black wiring
  • 1 low resistance ‘solar’ motor
  • 2 – 125 mA PV modules
  • 1 leaflet entitled: “Thin Film Silicon -the affordable energy choice for photo-voltaic power”

The school to add to the kit from its own stocks:

  • 1 torch bulb (1.5 V 0.3 A) and 1 socket for torch bulb to be used for monitoring charging current.
  • 1 multimeter or ammeter for monitoring the charge current Additional rechargeable Unicoross RX6+ batteries

Caution: the glass sheet in the PV module is fragile; if it breaks the PV module is ruined. Connecting the PV module will be fairly obvious to anyone familiar with electrical circuits.

These instructions are provided for those who aren’t.

  1. Turn the PV module over and lay it carefully on a flat surface. In one corner you should see a “+” sign. Remove the green corner piece nearest to the “+” sign, levering it off with a blunt screwdriver or similar. This will expose a brass screw. Loosen this screw, bare the ends of a length of red wire, and insert one end into the hole in the black plastic underneath the screw. Re-tighten the screw and replace the green corner piece. The red wire is now the positive outlet from the PV module.
  2. Identify the other corner having a hole in the black plastic. Bare the ends of a length of black wire and secure one end in this hole following the same steps as in No 1 above (remove the green corner piece etc). The black wire is now the negative outlet from the PV module.
  3. Now connect the diode to the negative outlet from the PV module. Take a single screw terminal connector and loosen the screws. Insert the black wire from the PV module into one side of the connector and the silver end of the diode into the other; re-tighten the screws. The silver ring on the diode should be towards the PV module. (The diode allows current to flow only in one direction, from the positive terminal of the PV module to the negative and not vice-versa; this means that the PV module can charge the battery in the day, but the battery won’t discharge itself back through the PV module at night).
  4. In fine weather you can charge two batteries at once. Otherwise it would be better to charge just one. Bare the ends of the wires on either one or two of the battery holders. To use two, twist the red wires together, and then separately twist the black wires together; (like this the battery holders are said to be connected in parallel). Take a single screw terminal connector and loosen the screws. Insert the red wire from the PV module into one side of the connector and the red wire (or wires) from the battery holder (or holders) into the other side of the connector: tighten the two screws. (The wires from the battery holders are rather fine; you may need to bare about 10 mm of the end and then double it back on itself, in order to make the wire thick enough to be held securely by the screw).
  5. Now take a third screw connector and connect the free end of the diode to the black wire or wires coming from the battery holder or holders. This completes the connections for battery charging.
  6. Insert a battery in the holder, (with the flat base against the spring). Insert the second battery in the second holder if required, (and if there is plenty of sun); the second battery must be inserted the same way round as the first one.
  7. Point the PV module in the general direction of the sun, and support it so that it can’t fall and break. You can place it on a window ledge if it can be secured there. However, it’s not necessary for the module to be close to a window; it can be in the centre of the room as long as sunlight is falling on it. Window glass (even when dirty) won’t much reduce the current; shade will. If possible move the module from time to time to keep it out of the shade. A single battery in direct sunlight in summer will charge in about 2 hours; in winter it may take 4 to 5 hours. If there are two batteries charging will take twice as long. In poor weather charging may take several days, or even longer.

Optional: Connecting an ammeter or torch bulb in the curcuit. If you have a suitable ammeter, capable of reading up to about 400 milliamperes (mA) you may wish to connect it in the circuit, to allow monitoring of the charging current. A further screw terminal is provided to allow this. Open the circuit at any convenient point; for example, disconnect the red wire coming from the cell from the battery holder(s), and connect the ammeter between the red wire and the battery holder(s). (Like this the ammeter is said to be connected in series with the previous circuit).

If one of the ammeter terminals is marked with a plus sign be sure to connect this to the red wire coming from the PV module. With a current of 350 mA a single battery will charge in 2 hours. With a current of 150 mA a single battery will charge in 5 hours. With a current of 65 mA a single battery will charge in 14 to 16 hours. With two batteries times should be doubled. If you do not have an ammeter, you can instead use the supplied torch bulb and socket as a simple current-detecting device. The socket has two terminals, about which wires can be wrapped.

Open the circuit at any convenient point; for example, disconnect the red wire coming from the PV module from the battery holder(s), and connect the socket between the red wire and the battery holder(s); (the direction of the current through a torch bulb doesn’t matter, so you can connect the socket either way round).

Screw in the 1.5 volt 0.3 amp torch bulb supplied. The bulb filament will just begin to glow when the current reaches about 30 mA, enough to charge a single battery in a matter of 30 hours. The bulb will glow brightly at a current of 300 mA, enough to charge the battery in 2 to 3 hours. The presence of the bulb does not significantly reduce the charge rate of the battery; however, if the bulb fails charging will of course stop until the bulb has been replaced.