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We bought a 0.5 watt solar panel with 4.5 V, a 4 AA battery holder and a 15 ampere Schottky diode 45V , and 4 lithium ion rechargeable batteries.

We connected the red positive solar wire to the diode. We assumed the end of the diode with the silver line is the cathode and connected that end to the solar red wire. We then connected the red wire on the battery holder to the opposite end of the diode that was black. The black wire on the battery holder was placed with the black end of the solar panel.
We placed it in the sun to charge but my daughter said it did NOT charge the 4 lithium batteries.

Please let me know where I went wrong.

Thanks so much for such a quick response! You engineers are amazing, kind and helpful people. This world would be a much better place with people like you in government....but I guess that's why you are engineers.

I heeded everyone's comments.

I still used the 0.5 watt solar panel with 4.5 V, a 4 AA battery holder and a 15 ampere Schottky diode 45 V, and changed to 4 Energizer Recharge Power Plus 2300 mAh NiMH AA. I changed the direction of the diode. Guess what!?! It worked. Thank you for all the warnings about the lithium CoO2/graphite Tenavolts 1.5V AA High-Capacity Rechargeable Batteries with 2775 mWh. Out of curiosity, because we used a 4.5V solar panel and the lithium batteries were 1.5Volts (x4) the total voltage need to charge would 6V solar panel. By using a solar panel that was less than the required voltage, would the batteries get charged but not to full capacity? Would that decrease the risk of fire? How does time of charging come into the equation?

Any books or websites recommendation that are not too esoteric that can explain this to my 12 year old daughter and me would be greatly appreciated.

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    \$\begingroup\$ A photo of what you did might help, but it sounds like you connected the diode backwards. The side with the line is the cathode, but the red wire of the solar panel is probably the one where current will come out, so you want to connect that to the diode's anode. \$\endgroup\$
    – The Photon
    Dec 30, 2020 at 7:12
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    \$\begingroup\$ We need the schematic, and the the part numbers of all components to really tell you what happened. ..But I highly suspect that you cannot safely charge the batteries in this way. Also I agree with photon the diode is probably backwards. \$\endgroup\$
    – Drew
    Dec 30, 2020 at 7:15
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    \$\begingroup\$ Diode is the wrong way - cathode should be towards batteries, but hold on. First of all, is the solar panel voltage even enough to charge batteries, as lithium ion batteries should have voltage of up to 4.2V per cell, requiring over 16V to charge them? Second of all, lithium batteries are extremely dangerous, charging them wrong can damage them and they can explode and start fires, so please bear that in mind, for safety reasons I would not attempt to charge lithium batteries using any home-made solutions, unless you and your daughter have enough experience and really know what you are doing. \$\endgroup\$
    – Justme
    Dec 30, 2020 at 7:20
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    \$\begingroup\$ You need to tell us the battery model and brand. | Also battery chemistry - ie you mention Lithium. Are these lithium ion or ...? . . | 4.5 v will charge a single lithium ion cell. Or 3 X NiMH cells in series. || Using the panel without a diode to operate a low cost TP4056 charger module and to charge 1 x LiIon cell should give you excellent results. \$\endgroup\$
    – Russell McMahon ♦
    Dec 30, 2020 at 7:56
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    \$\begingroup\$ "Please let me know where I went wrong." - where you went wrong is you don't know the first thing about electronics, and yet thought you could throw some random bits together and expect them to work. But don't despair - now is an excellent time to start learning... \$\endgroup\$
    – Bruce Abbott
    Dec 30, 2020 at 8:33

2 Answers 2

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  1. You have the diode backwards. Connect the black end of the diode to the red wire from the solar panel, and the end of the diode with the silver band to the red wire of the battery holder.
  2. A 4.5V solar panel won't generate enough voltage to charge the batteries. Lithium ion cells have a voltage of 3.6 volts and higher. Four such cells in series means a charging voltage of at least 14.4 volts. Your solar cell can't produce that voltage.
  3. This is probably a bad idea, any way. Lithium ion batteries tend to catch fire if improperly charged. A proper charger is not a solar cell and a diode.

You could use your solar cell and diode to charge nickel cadmium cells or nickel metal hydride cells. The crude charger will damage them over time, but they won't explode or catch fire. The voltage of the cells also fits better with the available voltage from your panel - the nominal cell voltage for NiCad and NiMh is around 1.2 volts, so four of them in series could get a decent part of a full charge from your solar panel.

Your cheapest bet is to switch to NiMh cells. They are readily available, and you can charge them with your solar panel.

If you want to stick with lithium ion cells, then you'll need to look into getting a proper battery management IC or module. That can get expensive and complicated for a science fair project.

The Tenavolts batteries you mention in the update contain safety circuits and regulator circuits.

They probably wouldn't go up in smoke if you tried to charge them with your solar cell - but there's no guarantee they would charge at all.

Tenavolts is using lithium ion cells with a buck circuit to provide a constant 1.5V output until the cell reaches its discharge limit - at which point it just shuts off.

It probably also has a charge circuit built in, but Tenavolts doesn't provide any information about that.

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Please pardon me if I am being rude, but its coming from someone whose worked a lot on solar and lithium chargers and I would advise, please treat lithium batteries with respect, unless they have protection circuit built in.

Can you choose readymade IC/Parts? In case you can choose the parts, please get these two below

  • TP4056 module with protection circuitry, and all pins are self-explanatory. TP4056 needs 5v to function, and unless solar cell is fully exposed to sunlight and sitting at perfect angle, would not produce 5v.

TP 4056 Link

  • A 5V boost module. Inputs connect to solar and output connects to Input of TP4056. It has LED's to show the status.

Boost Module Link

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  • \$\begingroup\$ No one is being rude in their answer...no insult taken. I am just trying to help my 12 year old with this project and quite aware of my rudimentary knowledge of electrical science. If she had done a cooking experiment with chemistry involved I could have been more helpful. She requested a lithium battery but I will change to a NiCad. I will switch the direction of the diode. Any other comment will be greatly appreciated! Any source to read about this more will also be appreciated! \$\endgroup\$
    – Christine Chen
    Dec 30, 2020 at 15:52

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