# Build a solar panel nimh charger

Today I found this tutorial on how to build a battery/solar power supply for an arduino here. The tutorial explains on how to use NIMH or Lithium-Ion batteries for this. Step 4 explains how to use NiMh batteries which is what I prefer. The tutorial explains I have to connect the solar panel to the battery and to put a diode between the positive wire of the battery and the solar panel. Then the tutorial explains that you have to use a power booster to get the voltage up to 5 volts for the arduino.

Now the part which I don't understand. Where do I connect the power booster? It is not visible in the tutorial (only for the Lithium-Ion battery, which uses a differenf circuit). Do I connect it together with the solar panel to the battery? Wouldn't the battery be charged and discharged at the same time then? Could someone explain this to me?

• Did you look at the block diagram in step 6? Commented Jul 9, 2015 at 13:02
• Carefully read the end of step 4, if you use 2 NiMh batteries then the voltage is too low for the Arduino. The boost converter is used to convert the voltage up to 5V which is enough for the Arduino. Commented Jul 9, 2015 at 13:03
• Technically it's the Arduino's speed and external peripherals that the voltage is too low for. The Arduino's MCU is rated down to 1.8V, but not at full speed. Commented Jul 9, 2015 at 13:05
• @IC_designer_Rimpelbekkie I understood that part, but where do I connect the boost converter? do I connect it together with the solar panel to the battery? Commented Jul 9, 2015 at 13:05
• @IgnacioVazquez-Abrams The diagram in step 6 shows how to connect the lithium ion charger and battery to the solar panel, not the nimh one Commented Jul 9, 2015 at 13:06

I would strongly suggest you do not use the NiMH set-up that this guy shows.

He assumes the diode to make voltages compatible, but with a 4V solar cell and an average diode at rated load you get 3.1V if the diode happens to be of the normal PN type and uses 0.9V at the measely 200mA he calculates. But most diodes at such low current will be hanging closer to the 0.7V mark.

0.7V drop would give 3.3V, which is 1.65V per cell peak, which is not good. Not to mention that at a lower current where they start to fill up, the diode voltage might lower and you'd get 3.5V total, making 1.75V per cell. Which is very extremely bad.

To add to that CC-CV charging a NiMH is not the best way to get the longest life span ever recorded in history. The best, still relatively easy, way is Delta-Peak, for which you need at least an op-amp and a couple of transistors.

The reason this is all of no interest to him, most likely, is that he uses LiIon and a special LiIon charger/conditioner board for power packs.

Although his LiIon set-up isn't great either. He assumes that no defect will ever occur in the solar power collection and that the battery will thus always be recharged, but if for some reason the power drain is more over time than the sun can re-supply over that same time (in the winter, for example) his booster will keep working till the LiIon cell is 0.9V. 0.9V for a Li-Ion means absolute death.

Glossing over all those details, assuming it would be a good idea to use the NiMHs as in that Instructable (which, really, really, really, it isn't), you put the booster in parallel with the battery, so that it can always convert the voltage at the battery into the 5V you need. It doesn't matter if the power comes from the battery or the solar cell, if the sun is shining a little, maybe half comes from the sun and half from the battery, if the sun is shining a lot, all the energy will come from the solar panel and the panel will charge the batteries back up as well. When it is night all the energy comes from the batteries.

Putting the booster, solar and (PROTECTED) batteries in parallel just ensures absolutely continuous power to the Arduino (as long as the batteries are still alive, which may be years, months or weeks as the case of these instructions may be).

A diode added to the solar cell may well be important if the protection circuit doesn't take care of that, standard use-outside-solar-cells will actually often drain current away from your battery if there's no sun when you don't add some form of "one-way traffic" like a diode. Just as a small after-thought.

• Thank you for this really long detailed explanation :) I wanted to use NiMh batteries as I have a lot of rechargeable ones just laying at home and I could use them for this project. Lithium Ion batteries are not that cheap :( So how high does the charging voltage for normal NiMh batteries need to be? Couldn't I just add a couple of resistors to bring the voltage down enough to not damage the batteries? Commented Jul 9, 2015 at 15:38
• @qwertz Nope. Resistors have a voltage drop that depends on current. As the batteries approach the solar voltage they will ask for less current eventually and over time it will balance out to the maximum voltage at no current, just like with the diode. No single-component solution will give you a decent battery life. You need transistors at the least to turn off charging, a chip or well designed circuit to guarantee correct charging curves is the best. Commented Jul 9, 2015 at 15:42
• Thank you. Should I just use some Lithium Ion batteries then? Can I just buy some cheap ones from Ebay or is the risk too high of them starting to burn or damage other components? Also if I connect the battery in paralell to the arduino and measure the voltage and use a relay to disconnect it @ around 3V to prevent damage? Would that be a valid solution? Can I just use 2 Lithium Ion batteries in paralell like I can with just 1? Like with the same charging circuit without worrying? Commented Jul 9, 2015 at 15:47
• @qwertz You could possibly use the arduino to monitor the battery voltage and keep them healthier for longer, but you need to thoroughly test that. You can put two Li-Ion batteries in parallel, provided they are balanced to each other. If you don't know connect them together with a 100Ohm resistor for a day or two. You can use his Li-Ion design without risking fire, but you should really make sure you are alarmed or that stuff turns off completely if they drop below 2.9V, or you could be buying new ones often. Cheap ones will likely not be their advertised capacity, otherwise they're fine. Commented Jul 9, 2015 at 15:56
• Thank you. I was looking @ the Ultrafire BRC 3000 ones, as they seem to be the best rated on ebay and are quite cheap. But don't they get unbalanced during normal usage and loading? Just know from rc planes that you have to balance them on every charge to keep them balanced. Also how would you connect them to each other? Just a 100Ohm from plus to plus and minus to minus? Also how much capacity should I take? I want to use the arduino nano with a couple sensors, which draw like 50ma under load. Also how big should the solar panels be? Is 5W 5.5V enough to keep them charged with 2-4 sun hours? Commented Jul 9, 2015 at 16:46