“The term supercapacitor and its possible use in electric vehicles, smartphones and IoT devices has been widely considered recently, but the idea of supercapacitors themselves dates back to 1957, when General Electric first attempted to increase its storage capacity capacitors. Supercapacitor technology has improved significantly over the years, and today it is used as battery backup, solar battery packs and other applications that require short power boosts.
Author: Aswinth Raj
The term supercapacitor and its possible use in electric vehicles, smartphones and IoT devices has been widely considered recently, but the idea of supercapacitors themselves dates back to 1957, when General Electric first attempted to increase its storage capacity capacitors. Supercapacitor technology has improved significantly over the years, and today it is used as battery backup, solar battery packs and other applications that require short power boosts.
In this article, we will learn how to safely charge such a supercapacitor by designing a simple charger circuit, and then use it to charge our supercapacitor to check its ability to hold energy. Similar to battery cells, supercapacitors can also be combined into capacitor power banks, but the charging method is different, which is beyond the scope of this article. A simple and common 5.5V 1F coin supercapacitor will be used here, which looks similar to a coin cell battery. We’ll learn how to charge a coin-type supercapacitor and use it in a suitable application.
A vague comparison of supercapacitors to batteries, supercapacitors have lower charge densities and poorer self-discharge characteristics, but supercapacitors outperform batteries in terms of charge time, shelf life, and charge cycles. Depending on the availability of charging current, a supercapacitor can be charged in less than a minute, and if handled properly, it can last for more than a decade.
Compared to batteries, supercapacitors have very low ESR (equivalent series resistance) values, which allow higher values of current to flow into or out of the capacitor, allowing it to charge faster or discharge at high currents. But because of this ability to handle large currents, supercapacitors should be safely charged and discharged to prevent thermal runaway. There are two golden rules for charging supercapacitors. When charging the capacitor, the polarity should be correct and the voltage should not exceed 90% of its total voltage capacity.
Supercapacitors on the market today are typically rated at 2.5V, 2.7V or 5.5V. Just like lithium batteries, these capacitors must be combined in series and parallel to form a high voltage battery pack. Unlike batteries, capacitors connected in series will add up to each other’s total voltage rating, so it’s necessary to add more capacitors to make a decent value battery pack. In our case, we have a 5.5V 1F capacitor, so the charging voltage should be 90% of 5.5, which is close to 4.95V.
energy stored in supercapacitors
When using capacitors as energy storage elements to power our devices, it is important to determine the energy stored in the capacitors to predict how long the device can be powered. The formula to calculate the energy stored in a capacitor can be given by E=1/2CV 2 .So in our case, for a 5.5V 1F capacitor, when fully charged, the stored energy will be
E = (1/2) * 1 * 5.5 2
E= 15 joules
Now, using this value, we can calculate how long the capacitor can power the device, for example if we need 500mA at 5V for 10 seconds. The energy required by the device can then be calculated using the formula Energy = Power x TIme. Here the power is calculated by P=VI, so 2.5 watts for 500mA and 5V power.
Energy = 2.5 x (10/60*60)
Energy = 0.00694 Wh or 25 Joules
From this we can conclude that we need at least two of these capacitors in parallel (15+15=30) to get a 30 joule power bank, enough to power our device for 10 seconds.
Identify the polarity of supercapacitors
When it comes to capacitors and batteries, we should be very careful about their polarity. Capacitors with opposite polarities are likely to heat up, melt, and sometimes burst in the worst case. The capacitors in our hands are coin-shaped, and the polarity is indicated by a small white arrow, as shown in the figure below.
I’m assuming the direction of the arrow indicates the direction of the current flow. You can think of it as, current always flows from positive to negative, so arrows start from positive to negative. Once you know the polarity and if you want to charge it, you can even use the RPS to set it to 5.5V (or 4.95V for safety) and then connect the positive lead of the RPS to the positive pin and the negative lead to to the negative pin, then you should see the capacitor charging.
Depending on the current rating of the RPS, you can notice that the capacitor will charge in a few seconds, and once it reaches 5.5V, it will stop drawing more current. This fully charged capacitor can now be used in suitable applications before self-discharging.
Instead of using the RPS in this tutorial, we will build a charger that regulates 5.5V through a 12V adapter and uses it to charge the supercapacitor. The voltage of the capacitor will be monitored using an op amp comparator, and once the capacitor is charged, the circuit will automatically disconnect the supercapacitor from the voltage source. Sounds interesting, so let’s get started.
LM317 Voltage Regulator IC
BC557 PNP transistor
The complete circuit diagram of this supercapacitor charger circuit is given below. The circuit was drawn using Proteus software and the same simulation will be shown later.
The circuit is powered by a 12V adapter; we then use an LM317 to regulate 5.5V to charge our capacitors. But this 5.5V will be supplied to the capacitor through the MOSFET which acts as a switch. The switch will only close when the capacitor’s voltage is below 4.86V, as the capacitor charges and the voltage increases, the switch will open and prevent further charging of the battery. This voltage comparison is done using an op amp and we also use a BC557 PNP transistor to light up the LED when the charging process is complete. The circuit diagram shown above is divided into several sections for explanation below.
LM317 Voltage Regulation:
Resistors R1 and R2 are used to determine the output voltage of the LM317 regulator according to the formula Vout = 1.25 x (1+R2/R1). Here, we use the values of 1k and 3.3k to regulate the output voltage of 5.3V, which is close enough to 5.5V. You can use our online calculator to calculate the required output voltage based on the resistor values you provide.
Op amp comparator:
We have used the LM311 comparator IC to compare the voltage value of the supercap with a fixed voltage. This fixed voltage is supplied to pin 2 using a voltage divider circuit. Resistors 2.2k and 1.5k drop a voltage of 4.86V from 12V. This 4.86 volts is compared to the reference voltage (capacitor voltage) connected to pin 3. When the reference voltage is less than 4.86V, the output pin 7 will reach a high level of 12V through a pull-up 10k resistor. This voltage will then be used to drive the MOSFET.
MOSFET and BC557:
The IRFZ44N MOSFET is used to connect the supercapacitor to the charging voltage based on the opamp’s signal. When the op amp goes high, it outputs 12V on pin 7, which turns on the MOSFET through its base pin, similarly when the op amp goes low (0V), the MOSFET turns on. We also have a PNP transistor, the BC557, which turns on the LED when the MOSFET is off, indicating that the capacitor voltage exceeds 4.8V.
Supercapacitor Charger Circuit Simulation
To simulate the circuit, I replaced the battery with a variable resistor to provide a variable voltage to pin 3 of the opamp. The supercapacitor is replaced with an LED to show if it is powered. Simulation results can be found below.
As you can see when using the voltage probe, when the voltage on the inverting pin is lower than the non-inverting pin, the op amp boosts 12V on pin 7, which turns on the MOSFET, which is the capacitor (yellow LED )Charge. This 12V also triggers the BC557 transistor to turn off the green LED. As the voltage of the capacitor (pot) increases, the green LED will turn on because the op amp will output 0V as shown in the image above.
Supercapacitor charger on hardware
The circuit is very simple to build on a breadboard, but I decided to use a Perf board so that I can reuse the circuit every time I try to charge a supercap in the future. I’m also planning to use it with a solar panel for a portable project, so try to build it as small and sturdy as possible. My complete circuit after soldering on the dotted board is shown below.
Capacitors can be charged by tapping the two female Begger bars with alligator pins. The yellow LED indicates power to the module and the blue LED indicates charging status. The LED will be on when the charging process is complete, otherwise it will remain off. Once the circuit is ready, just connect the capacitors and you should see the blue LED go off and after a while it will go high again indicating the charging process is complete. Below you can see the board in charging and charging state.
The full work can be found in the video at the bottom of this page, if you have any questions you can post them in the comments section or use our forum for other technical issues.
The circuit design given here is rough and serves its purpose; some mandatory improvements I noticed after building are discussed here. The BC557 gets hot because the voltage between its base and emitter is 12V, so a high voltage diode should be used instead of the BC557.
Second, when the capacitor is charging, the voltage comparator measures the change in voltage, but when the MOSFET turns off after charging, the op amp senses the low voltage gain and turns the FET on again, this process is repeated a few times, and then the op amp will completely closed. A latch circuit on the output of the op amp will solve this problem.
The Links: PM300RSD060 QM75D1X-H