Accrochages

Following our research on rechargeable batteries and the meeting with Bartaku we ran experiments using a simple solar charger.

The circuit

The circuit is pretty easy to build: basically you plug the solar panel straight into NiMH batteries and in your circuit. You need to put a diode between the solar panel and the batteries to avoid the current to flow back from the batteries to the solar panel. To avoid the diode to take a big part of the voltage (silicon diode take 0.7V) preferably use a low consumption diode such as a Schottky (0.2V) or Germanium diode (0.3V). You also need to make sure the solar panel won't damage the batteries by adding a resistor (if necessary). An alternative would be to use a regulator such as LM317T.

Experiment

We ran experiments using two solar panels:

1. Panel A was bought at a local store in Brussels, it is of type "crystalline" (best effective under direct sunlight) and generates 2V max.
2. Panel B was part of a "garden lamp" bought in Montreal (the same with which we made the bird modules, it is of type "amorphous" (which works well even if there is shadow) and generates around 3-4 V in plain sunlight

We also used two types of NiMH batteries:

1. Batteries 1 are 1.2V, 600mA batteries that were taken from those same "garden lights". We know they are pretty bad cause we tried to charge them with a battery charger and it didn't prove very efficient.
2. Batteries 2 are also 1.2V ??? mA, we bought them in Montreal

Results

Best setup is Panel B with Batteries A (we use two batteries ie. 2.4 V). After 1h charging under lamps it provides 1.8 V and the batteries are able to keep a LED alive without light. Measured current in circuit if we cut sunlight is 200 mA.

I was not able to drive a single ATMEL chip circuit with that however.

Conclusion

Good practices:

1. Use amorphous-type solar panels
2. Use solar panels that drive enough current/voltage
3. Use good NiMH batteries

We need to work on the question of current (A) as well as that of voltage (V). Now we get proper voltage but seemingly not enough current to drive our circuits.

References

We suggest you follow the following detailed instructions if you plan to built your own circuits:

• Solar battery charging
• Same with a LM317T regulator