Supercapacitor Solar Box




Introduction: Supercapacitor Solar Box

About: I've studied electronics back in the days in my high school. Today it is simply a hobby.

This is an idea which I had for more than 2 years. Here are the people from youtube who inspired me:

As of today they both have new projects with supercaps, check them out!

Please take your time to read the long introduction. It took me a lot of time to gather all parts, and during my tests I observed interesting facts.

The main idea is - to make a device similar to solar powered power banks, but instead of Li-Ion batteries, use supercapacitors. It shall have a USB output, LED light and status measurement.

There are many projects involving solar charging li-ion or lead-acid batteries. Here I decided to use supercapacitors, because they feel more "comfortable" with non-stop charging/discharging cycles with differents currents and unstable parameters. For sure they will last longer than any battery, and this device can be used for a very long time. They also charge faster than batteries, because they are smaller in capacity and have bigger power efficiency - almost 100% of charging power is stored.

Disadvantages, of course include higher price per mkwh than batteries, higher volume and weight, and less capacity.

As you can see, the device has 4 important parts: Solar Panel, Charging Electronics, Supercapacitors, Discharging Electronics. I will put some details on each one of them:


The heart of this device consists of 6 supercapacitors. I decided to use D-cell sized supercaps, becasue they are easy to find, and cheap to buy. They claim 500F 2.7V each. 6 is good, becasue they fit in my small box, balancing between capacity and device size (and money of course).

Let's make some calculations, for supercapacitors:

6 * 500F = 3000F

0.5 * 3000F * 2.7V *2.7V = 10935 Joules (theoretical max energy they can hold)

For comparisson a single 2500mah AA rechargable battery has:

2.5Ah * 1.2V * 3600 = 10800 Joules

A 800mAh Li-Ion battery has:

0.8Ah * 3.6V * 3600 = 10368 Joules

These are of course very rough calculations, but you can see that 6 big supercaps barely reach the capacity of a small battery.

Becasue we have a USB port, we aim for 5V output. Which means that we can boost our 2.7V to 5V, but after a lot of experimenting I found out that it is better to have 2*2.7 = 5.4V , because it gave me about 40% more energy on the output. The reasons for that are: input voltage is closer to the output voltage, so the booster has higher efficiency at least in the beginning, AND my booster cut off ~0.5V input, but having 2 supercaps in series means it drains them down to 0.25V each instead of 0.5V. Unlike batteries you can discharge supercaps down to 0V, they are OK with that. And you HAVE to do it, in order to squeze all their energy.

So my design includes 2 packs in series having 3 supercaps in paralell each. Harder to charge, but with this small capacity every mAh on the output matters!

* Interesting facts about supercaps I observerd - they do not self-discharge so much , after your charge them several times, and keep them charged. Some of them stayed in my wardrobe for months and kept just over 2V. I also observed other interesting fact written by other bloggers - supercaps resist charge/discharge. When you keep them discharged and charge them, they self-discharge quickly, but only the first 1-2 times. If you keep them charged for a long time, and then you discharge them, they start building voltage little by little again, slowly. It feels like energy comes from nowhere. Like they want to stay in their last stable state. May be I am wrong.

Charging Electronics

Probably the hardest part of this project. It took me a lot of time to test different circuits. I am open to suggestions in the comments section.

1. Zener diode - I thought a simple zener diode can cut off charging when 2.7V is reached. However, it appears zener diodes for such small voltages - 2.7V, have very curved V/A diagram. Which means, they leak current at 2.6-2.0V, and during the night supercaps discharge through them, in the morning they are down to 2V. I was surprised to see they do not self-discharge so much, it was the zener!

2. Buck-boost circuit with fixed 2.5V output - I thought whatever voltage comes from the solar panel the buck-boost will make it 2.5V and charge the capacitors. It was working, but with very low efficiency, and it was not suitable for the 2 pack design. I tried different IC boards from Pololu, but they were not OK.

3. Direct connection - connecting the solar panel directly to the supercaps gave the best results. However you must always do it with schotky diode !!! Otherwise when the sun is down, the supercaps discharge through the solar panel. So I couldn't find any other suitable way and bought a standard board which limits the voltage on every pack to 2.68. Some of them limit to 2.5V, although it is written 2.7V so be careful and measure them. It works pretty well!

The disadvantage of this approach is that when the supercaps are fully charged, short circuit goes through the solar panel, or at least for small amounts of time - I have to measure.

I could not find and other circuit, which charges normal batteries and adopt it for the 2pack.

Actually the best way to charge supercaps from solar panel is by using ZSPM4523. This chip is optimized for this purpose and can be configured. Has built in MPPT charger. However perhaps I need 2 of them in some configuration for the 2 packs. They are not very expensive - may be 3$, but I can't solder SMD. The KIT is too expensive. I just saw they reached "end of life" !!?? so soon! They say ZSPM4561AA1R will replace it, but I doubt...

Solar Panel

I chose a solar panel 5.5V (it gives more on direct sunshine), but 6V is OK too. It should be able to charge both supercapacitor banks up to 2.7V (2*2.7=5.4V). Normally it reaches 5.2V when charged. Then I selected a size big enough to cover the box cover, and it is ~300mA.

Discharhing Electronics

Here we need two things:

1. A board that makes 5V for the USB socket.

I found U1V10F5 from Pololu - it makes 5V output, and can take as low as 0.5V for input, so you can drain all the power from the supercaps. On the other hand it can take input voltage even slightly higher (5.5V) than its output.

2. A board that maked 3.3V for the LED (I don't want to use a resistor, to minimize power loss)

Like above, the board from Pololu is U1V10F3. It can also take 5.5V input, and step it down.

After I build the project it seems U1V11A is a better choice - you can regulate brightness/power consumption.

-=CAUTION=- - both boards behave little bit strange sometimes, they blink their output voltage. Still have no explanation for that. Open for suggestions.

It is important to be able to switch them on and off, because even their standby current is very low, it drains down the supercaps.


For LED light, I chose just a pile of cheap strawhat (wide angle) LEDs from e-bay. It seems they are not very bright, on 20ma, and when you put them on 3.3V they drain quite a lot of current. Warm white, I prefer.

I also decided I need to see the charge level of the supercaps. That is why I bought a small voltmeter. Unfortuantely it can work only on voltages above 2V. But it is OK. I could not find one without LED, with matrix LCD like calculator. It is important to know that although the voltage of the device varies between 0V and 5.4V, the middle (like 2.7V) does NOT mean 50% charged. The formula is 0.5*C*V*V. So 50% charge is around 4V.

TIP - my voltmeter has only 2 wires. You can use a voltmeter with 3 wires to measure voltages below 2V, and power it from the output of U1V10F5 mentioned above!!!

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Step 1: Parts

1. Project box - got mine from LIDL

2. 6x 500F 2.7V supercapacitors - the item exists no more on AliExpress, the key term was "low leakege", probably they have no more than 300F..... not sure, they were ~5$ each

3. Small voltmeter 0.28 inch Red - - item changed

4. Solar Panel 1.6W 5.5V 150x86mm -

5. 33 micro Farad 16V capacitor - you can skip that -

6. Schottky diode - got mine from a local store

7. USB socket - salvaged mine

8. 5V regulator - U1V10F5 -

9. 3.3V regulator - U1V10F3 (better use U1V11A for 3V) -

10. Supercapacitor balance board -

11. Buttons and switches - again from Pololu, but they are rather small

12. Strawhat LEDs -

13. Solder, wires, mini drill, hot glue, soldering iron, file, strong glue

Total price - ~60$

Step 2: Solar Box Cover

1. Solder a Schottky diode to the panel. I prefer the minus (-).

2. Solder 2 wires to the panel. In my project white is munis, brown is plus.

3. Put the panel over the transparent box cover, and choose 2 dots to drill holes for the wires. Mark them plus/minus.

4. Drill holes.

5. Put the wires through the holes, and use a generous amount of glue on the top, like shown.

6. Put the solar panel over the cover and use pegs. It has to stay for some time. Your solar cover is ready.

Tip - be sure to have enough space for the diode. My cover has a gap. Otherwise you stick on the internal side of the cover.

In the end you have 2 free wires.

Step 3: Supercapacitor Banks

As we said before we need 2 banks with 3 supercaps each. To achieve 5.2-5.3V.

Tie 3 supercaps with 2 zip ties. Then connect them in parallel with wires as shown. In the end put 2 longer wires to connect them to the other electronics


Step 4: LEDs

1. Use a ruller and mark the holes for the LEDs on the box. In my case - 11.

The distance between them is 1cm, because the leads are short.

2. Drill holes. I had to use 5mm drill.

3. Put the LEDs in order, so that they have their long legs in a row. Use glue, they don't want to stay in the hole tight.

4. Solder them to each other, each one legs to the next one. You can use a clip to help you.

5. Use a 3V coin cell to test them in the end.

Step 5: Drill More Holes

1. Drill holes for the 2 switches and the push button.

2. Drill a rectangular hole for the USB port, Use a file to make a precise size, so your USB socket fits tight.

Step 6: LEDs Circuit

1. Solder the 3V regulator output directly on the last LED leads. Seems elegant but not very serviceable.

2. Connect the other wires to the switch and the 3rd pin as shown, and use the Scheme.

3. In the end you have to longer wires to solder to the supercap board.

4. Put the supercap banks inside.

Step 7: USB Circuit

1. Mount the USB socket in the hole. Use hot glue, or other glue. It needs to be very stable.

2. Solder two wires from the USB to the 5V regulator output. I used a small capacitor recommended by Pololu. Just in case because you will often plug/unplug USB devices, and it is like a protection. It will work without it too.

3. Use the scheme to connect the wires to the swich and the regulator.

In the end you have again 2 free wires.

Step 8: Voltmeter Circuit

Connect the voltmeter using the scheme. It is through a push button. I could not find a voltmeter without LED. It would save a lot of power, and probably measure voltages bolow 2V.

This one can show only over 2V. There is a way to fix it, if you are interested - mentioned in the description above.

I glued the voltmeter on the inside using moment glue, I was tired of drilling holes. It is very nice!!

Please note the max voltage is ~5.3V, BUT 50% full is ~4V (not 5.3 / 2)

In the end you have again 2 free wires.

Step 9: Connect Everything Together

Use the scheme to connect the 2 supercapacitor banks on their respective places on the balance board - the big holes.

All other circuits, including the solar panel are soldered in the same place - all plus wires (brown) on the positive plate, all minus wires (white) - on the negative.

Put the board in the box, so you can close it.

I could NOT :) :) :)

Step 10: Test It !

350mAh @ 5V is not bad.

Its equal to ~470mAh Li-Ion battery.

Charges for 4 hours sunshine. (Note - after the panel gets hot, the efficiency drops).

LEDs - shine for 2 hours if you regulate them for 20mA each. In my case they are too bright, because 3.3V is too much.

USB - gives 10% to my Samsung (2000mAh battery). Most phones will treat it like a computer USB and charge at 500mA or less. You can upgrade it. Search for resistors configuration.


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    7 Discussions


    3 months ago

    I am so much impressed with your own project. Whatever it may be it seems your very much effort as for me. Wish you good.
    One thing i request you that I am unknown of any kind of electronics. Can I have one from you assembled for me for cost? If it is so it could be more and more helpful to me brother.


    1 year ago

    Okay you do realize when you place capacitors in series you're reducing the capacity. You only get 6,000 farad's if you're have all 6 in parallel! Since you have 3 in parallel that would be 1500 F but you have the other group of three in series with the first group... So you have to reduce it by 2...750 F TOTAL!


    Reply 1 year ago

    " when you place capacitors in series you're reducing the capacity" - not exactly, but I know what you are talking about. See the calculations again. Reducing the capacity does not mean less energy, because you double the voltage. Yes you're right it is 1500/2 = 750 F, BUT I have 5.2V which is 5.2*5.2*750/2=10140 joules of energy. In the other case I have all 6 in paralell - 6*500=3000Farads, but only 2,6V which is 2.6*2.6*3000/2=10140 joules. The same amount of energy. But if they are all in paralel the booster is not effective, and I have less energy overall. I did an experiment, read the info!


    1 year ago

    SuperCaps will last 3-4 times than Lipo battery. I don't know for the circuitry. i'm thinking for a LifePo4 Battery, than super caps..


    2 years ago

    I love super caps. However, the size and price is what always gets me.

    While lithium isn't the best solution, it is pretty simple to find and inexpensive for a large battery.

    What about rechargeable AA batteries?


    Reply 2 years ago

    Yes, AAs are good and cheap. I like Eneloop. But I don't think any kind of battery is OK with the everyday charge/discharge cycles. My point is to make you forget about battery maintenance and wear. I had to make it not so big and expensive, but yet with enough energy to be useful for some purpose. I guess 400F good quality C-size supercaps will give like +30% energy.


    2 years ago

    Hmm kept this to have a good look at .There are powerbanks on ebay for
    $18 that seem to do all this with Lithium Polymer batteries in them . I
    used my LiPo bartteries for some thing else so have all these carcasses
    of solar panel,charge and discharge circuits all ready to go . LED light and switch all there.