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.

  • \$\begingroup\$ What nominal voltage/wattage is your solar panel? How much current (roughly) does the thermometer draw? If you have the Model number or specs for it this would be useful. Also, what exactly do you mean by "didn't work" - did it fail to turn on at all or...? \$\endgroup\$
    – Oli Glaser
    Nov 9, 2011 at 18:00
  • 1
    \$\begingroup\$ I don't have the expertise to really summarize this well and I'm not just going to drop a link in an answer - but there's a lot information here - robotroom.com/Solar-Recharging.html \$\endgroup\$
    – rfusca
    Nov 9, 2011 at 18:11
  • \$\begingroup\$ Please provide a schematic of what YOU have done. What you describe sounds like it should work so you need to draw out what you are doing. PV (= photovoltaic = solar cell) voltage MUST be > battery voltage to charghe. Is it? Charging miusgt be done in sunlight - inside light has almost zero energy. Where is the PV panel you are charging the battery?? \$\endgroup\$
    – Russell McMahon
    Nov 9, 2011 at 21:47
  • \$\begingroup\$ @rfusca It's almost what I want, but wouldn't be easier to just use the solar cell to load the battery and use the whole time the battery as power source? \$\endgroup\$
    – VP.
    Nov 9, 2011 at 23:31
  • \$\begingroup\$ See additions to my answer. \$\endgroup\$
    – Russell McMahon
    Nov 10, 2011 at 0:08

2 Answers 2


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.


Please explain what you mean by didn't work.
With no rudeness intended - the connection is simple enough that supplying a diagtram is still no guarantee it will "work" - see below. You appear to have all the parts and appear to have connected them correctly. As it does not work and should, the answer consists of finding out what is wrong.

Please provide a schematic of what YOU have done.
What you describe sounds like it should work if you have done as you said so your equipment may be wrong or you may be doing something wrong. ng.

Charging MUST be done in sunlight or direct daylight
Inside light has almost zero energy.
Where is the PV panel located when you are charging the battery??

PV cell (= photovoltaic = solar cell) voltage MUST be > 2 x AA battery voltage to charge. Using voltmeter place panel UNLOADED in direct daylight
Is it?
For 2 x AA NimH VPV must be > ~= 3.5V unloaded and more ios betterFor 2 x Alkaline AA for Vpv to take over from batteries Vpv unloaded needs to be 4V+ and more is better.

Solar cell addition sanity check

Adding the solar cell to the system when it is running from batteries should either
- Provide extra energy from the solar cell or
- Do nothing
Stopping it working is not an option ! ;-).
If it DOES stop it working and the two x AA batteries are still there then the diode is reversed or faulty.

NimH batteries are about 1.2V nominal when working but are say 1.3V when fully charged AND need up to 1.45V per cell in charge mode to fully charge them.
If a Schottky diode is used it will drop about 0.3V. It a silicon diode is used it will drop about 0.6v.
So 2 x NimH + Schottky diode need about 1.45 + 1.45 + 0.3 = 3.2V
That's loaded. Open circuit voltage will be about 1/3 higher = say 4.2V if you want full charging current when battery is almost charged. My suggested 3.5V minimum was a compromise - with more being better.

To get some idea of PV panel performance

  • Place in bright daylight on a sunny cloudless day - ideally pointing at the sun at midday.

  • Measure unloaded voltage = Voc.

  • Short panel with an ammeter (multimeter on say 200 mA range).
    Measure short circuit current = Isc.

Actual achievable power will be ABOUT Vpp x Isc x 0.5

  • \$\begingroup\$ as usual you are always helpful :-) what I mean with "it doesn't work". With the solar cell, the sender didn't communicate with the base. I removed the solar cell and it worked, it means it sent the temperature to the base. I will check in the sun how much voltage my cell is able to provide. BTW a rechargeable battery (AA) here (Germany) has something around 1.2V. So my cell should generate ~= 2.4V, right? \$\endgroup\$
    – VP.
    Nov 9, 2011 at 23:24

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