Schematic for battery and solar cell in parallel

battery-chargingschematicssolar cell

I have a thermometer, that the sender uses 2 batteries AA (2 x 1.2V). I would like to add a solar cell to use it to load the battery. I tried to connect both (battery and solar cell in parallel) with a diode in front solar cell to avoid any reverse current into the solar cell. Somehow it didn't work, it means, the thermometer stopped to send information to the base. As soon as i removed the solar cell, it started back to send information to the base. so I would like to ask if there is any schematics that I can use as example.

Best Answer

Using a solar cell to charge batteries used to be done like that in the old days, but nowadays we use charge regulators (not necessdarily expensive). There are a few problems with your approach :

  1. The voltage of a solar panel is not constant.
  2. The voltage of the battery will prevent optimal performance of the solar cell.
  3. The voltage of your battery will be different under load and when charging.

If you want to avoid using a regulator, you are going to need to get your multimeter out and check things are like they should be.

What is the charging voltage of the battery? The charging voltage and the operating voltage of the battery are not the same. If you are running your solar panel in parallel to your battery in parallel to your load, can your load handle the voltage difference? A charge controller will give you a (more or less) constant voltage reference.

What is the open-circuit voltage of the solar panel? You say you are measuring temperature, is it a particularly hot environment? If so, you'll need to check the voltage at the operating temperature of the solar panel. Even so the voltage of your solar cell will change as a function of light intensity and temperature.

What is the voltage drop across your diode? Your solar panel will operate at the charging voltage of the battery plus the diode voltage, about 14.6 V. You need to check that you are getting reasonable current out of your solar panel at this voltage, given temperature and light intensity. For reference check your manufacturer data sheet for a graph similar to this.

If you are confident that your solar panel is the right size for your battery you will need to connect the positive terminal of the solar panel to the positive terminal of the battery, with the diode preventing current from entering the positive terminal of the solar panel. The most reliable solution though is to buy a charge controller.

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