Using Arduino with Parts and Sensors – Solar Powered Arduino (part 2)

Using Arduino with Parts and Sensors – Solar Powered Arduino (part 2)

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In the first part, we were able to flicker an LED with Arduino. We learned that it works fine when it’s nice outside, but we weren’t able to do the same thing indoors or when it’s cloudy because the solar panel could not generate enough power to supply Arduino. In this article, we will look at how to store electricity from the solar panel and operate Arduino with more stability.

Today’s Electronics Recipe

Estimated time to complete: 20 minutes
Parts needed:

What is a rechargeable battery?

We use “batteries” every day, from dry cell batteries to manganese, alkaline, rechargeable batteries, storage batteries, and so on. I’m sure you can think of numerous different types of batteries. But the batteries in our cars or motorcycles, are they batteries too?
The batteries we use on daily basis can be divided into 2 large groups: the “primary batteries”(manganese, alkaline) which are thrown away after use, and the “secondary batteries” (NiCad batteries, nickel-metal hydride batteries, lithium ion batteries, lead-acid batteries) which can be reused. (By the way, the battery in our car is a lead-acid battery)
Today, we want to store electricity, but before we do that, let’s look at the characteristics of each battery type.

NiCad battery
  • Can be recharged about 500 times
  • Stable electricity (Stable high output until it gets empty)
  • Easily gets memory effect
  • Large self-discharge
  • Voltage does not go down even during low temperature
Nickel-metal hydride battery
  • Can be recharged about 500 times
  • Larger capacity than the NiCad battery
  • Little self-discharge
Lithium ion battery
  • Can be charged more than 500 times
  • Largest capacity among the 3
  • Voltage does not go down even in low temperature
  • If you leave it fully charged for a long time, it will not last as long
  • Little self-discharge
  • High voltage (3.7V) compared to the other 2 batteries (1.5V)

It seems there are a lot of types of rechargeable batteries.

How to charge rechargeable batteries?

So, we learned that there are many different types of rechargeable batteries and each has its strong points and weaknesses. Now, how do we charge these rechargeable batteries? When we think about it simply, we can just give it electricity. At a closer look, it seems we can charge them by sending electricity at the same voltage as the rechargeable battery (1.5V in the case of AA rechargeable battery). The voltage of batteries drop when we use electricity. Just like water, electricity flows from a high location (charger) to a lower location (rechargeable battery). I looked deeper into the subject and found out that when charging rechargeable batteries, if the voltage is too high or if you try to overcharge (charge a full battery), it puts much load on the rechargeable battery and may cause leaks or explosions. On the battery it says, “Only use the specified chargers when charging the battery”. Hmm, maybe we shouldn’t overlook this. Perhaps we shouldn’t charge it using a solar panel.
Furthermore, depending on the type or manufacturer, specs may vary and the method to check if the battery is full varies as well, so it is quite dangerous. It may be difficult to simply charge a rechargeable battery with a solar panel.

Our savior, the electric double-layer capacitor

So, we hit a wall right from the start. It’s hard to charge rechargeable batteries. so let’s think of a different way. Through my research, I came across something called an “electric double-layer capacitor”. What could this be? It sounds so heavy and powerful. At a closer look, it is also known as a “super capacitor” or an “ultra capacitor.” I don’t know what electric double layer means, but capacitor is quite a familiar term.

According to Wikipedia:

Electric double-layer capacitor – wikipedia

Electric double-layer capacitor, EDLC, is a high capacity capacitor that uses a physical phenomenon known as an electric double layer. Its development began in the late 20th century and it is used in various fields today. With further development, it may replace some batteries. An electric double-layer capacitor has both an anode and a cathode, but that is not why it is called a double-layer. It is named after the physical phenomenon, “electric double-layer” which occurs near the surface of both anode and cathode. Electric double-layer capacitor is also known as an Ultra capacitor or a Super capacitor.

So “it may replace some batteries”. So we learned that it might become a replacement for rechargeable batteries in the future.

Let’s take a quick look at some of the electric double-layer capacitor ‘s characteristics:

  • Can be charged and discharged in a short period of time (due to a low level of internal resistance compared to other batteries)
  • Lasts longer (less damage from charge/discharge)
  • Fast self-discharge
  • Linear change of voltage during charge/discharge
  • Works under low temperatures, unlike other batteries

It may not completely replace rechargeable batteries, but this looks promising.

What is a capacitor?

We’ve been hearing a lot about “capacitors”, but what is it? It actually appeared in our last article too.

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Picture 1. The part in the center and on the left is the capacitor.

Let’s look at Wikipedia.

Capacitor – Wikipedia

A capacitor (Latin: condensare, German: Kondensator) is a passive element that stores and discharges electric energy by capacitance. The unit for capacitance is Farad. A regular capacitor has a Farad of a few pF to a few thousand μF, but when it comes to electric double-layer capacitors, it may even reach thousands. The voltage that can be applied to both sides (pressure-resistant) can range from 2.5V – 10kV.

It says, “stores and discharges electric energy by capacitance.” This is the same thing that a rechargeable battery does, right!? Generally, a rechargeable battery uses electrolytes to combine different metals and creates a chemical reaction to generate electricity. But a capacitor has an insulator between the 2 metals and when electricity flows through them, the load gets stored (both loads pull on each other over the insulator). Unlike a rechargeable battery that creates electricity through chemical reactions, a capacitor stops flowing electricity once it’s full, so we don’t have to worry about checking if the battery is full or overcharging when using the electric-double layer capacitor. A regular capacitor is used as a circuit to turn AC into direct current or cut noise.

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Figure1. How a capacitor works

Let’s try a little experiment to learn more about capacitors.

Making a capacitor

So, a capacitor has 2 metals and an insulator in the middle. So let’s get a big aluminum foil and  some plastic wrap. Put the wrap between the foils so they do not touch each other.
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Picture 2. Aluminum foil and plastic wrap

Then, connect the + and – of the battery to each foil, release electricity, and measure the voltage…

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Picture 3. Let the electricity flow from the battery, then measure the voltage…

It reads 0.036V. So it has a small amount of voltage. The closer the metals are, the stronger the tie. Next, connect the copper wire to both metals and fold them. Make sure they do not touch each other.

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Picture 4. Folding our homemade capacitor

After you’ve folded it, roll it up into a ball.
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Picture 5. Rolling up the homemade capacitor into a ball

After you’re done, fix it in place using a masking tape and use a vise to press the metals together.
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Picture 6. Wrapped with masking tape.

Now we have our homemade capacitor. Now let’s try to store some electricity in it from a battery again.

Caution: When trying to charge it, please use 1~4 batteries. Please do not use AC outlets in your home.

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Picture 7. Charging our homemade capacitor


It discharges quickly, but we can see that it is storing electricity! Through this experience, we learned how a capacitor works.

The difference between a rechargeable battery and an electric-double layer capacitor

Now we understand how a capacitor works, let’s review the difference between a rechargeable battery and an electric-double layer capacitor.

Advantages Disadvantages
Rechargeable battery
  • Large capacity
  • Cheap
  • Takes time to charge
  • Short lifespan (500〜1,000 charges)
Electric double-layer capacitor
  • Quick charge/discharge
  • Long lifespan (100,000~)
  • Linear change of voltage upon charge/discharge
  • Works in low temperatures
  • Expensive
  • Quick self-discharge

Furthermore, when using an electric double-layer capacitor, here are some convenient features and points to consider.

  • You don’t have to worry about overcharging
  • Keep the voltage fixed level when charging
  • Do not use high voltage that exceeds the voltage endurance when charging.

Using an electric double-layer capacitor

Today, we will be using a 1F (Farad), 5.5V electric double-layer capacitor. The capacitors we often see are measured in μF(micro-Farad), nF(nano-Farad), pF(pico-Farad), so it’s easy to tell its size.

1F = 1,000,000μF = 1,000,000,000nF = 1,000,000,000,000pF

As we did in the last experiment, let’s charge the capacitor from a battery, connect the LED, and measure the time. 4 AA batteries will result in 6V. This is ok, but we will use 2 diodes and drop the voltage to 5V when charging.

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Picture 8. Charging the electric double-layer capacitor with a battery

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Picture 9. Electric double-layer capacitor after it’s been charged

If you use a voltmeter, you will see the voltage rises rapidly. We now know that it is fairly charged, so now let’s connect the LED.

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Picture 10. Testing the LED

It lights up. Although I only charged it for a minute, the LED lit up for a few minutes.
Try the following if you want a circuit allowing you to switch the charge and the LED alternately.

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Figure 2. Circuit for electric-double layer capacitor charging and LED flicker

Summary

We learned that electric double-layer capacitors have the potential to replace batteries in the future. Using this knowledge, we will try to charge electricity from the solar panel to an electric double-layer capacitor and create an actual circuit to use Arduino with stability in the next article!