Introduction to Supercapacitor Charging

Supercapacitors, also known as ultracapacitors or double-layer capacitors, are high-capacity electrochemical capacitors with capacitance values much higher than other capacitors. They store 10 to 100 times more energy per unit volume or mass than electrolytic capacitors, can accept and deliver charge much faster than batteries, and tolerate many more charge and discharge cycles than rechargeable batteries.

Supercapacitors are used in applications requiring many rapid charge/discharge cycles, rather than long term compact energy storage — in automobiles, buses, trains, cranes and elevators, where they are used for regenerative braking, short-term energy storage, or burst-mode power delivery. Smaller units are used as power backup for static random-access memory (SRAM).

To charge a supercapacitor efficiently and safely, a proper charging circuit is required. This guide will cover everything you need to know about supercapacitor charging circuits, including:

• How supercapacitors work
• Supercapacitor charging methods
• Charging circuit design considerations
• Example charging circuits
• Frequently asked questions

How Supercapacitors Work

A supercapacitor is a electrochemical capacitor that has an unusually high energy density when compared to common capacitors, typically thousands of times greater than a high-capacity electrolytic capacitor.

Supercapacitors don’t have a conventional solid dielectric. The capacitance value of a supercapacitor is determined by two storage principles:

1. Double-layer capacitance – Electrostatic storage of electrical energy achieved by separation of charge in a Helmholtz double layer at the interface between the surface of a conductive electrode and an electrolyte. The separation of charge is of the order of a few Angstroms (0.3-0.8 nm), much smaller than in a conventional capacitor.

2. Pseudocapacitance – Electrochemical storage of electrical energy achieved by faradaic redox reactions with charge-transfer between electrode and electrolyte. This is accomplished through electrosorption, reduction-oxidation reactions, and intercalation processes.

The electrolyte forms an ionic conductive connection between the two electrodes which distinguishes them from conventional electrolytic capacitors where a dielectric layer always exists, and the so-called electrolyte (e.g., MnO2 or conducting polymer) is in fact part of the second electrode (the cathode, or more correctly the positive electrode). Supercapacitors are polarized by design with asymmetric electrodes, or, for symmetric electrodes, by a potential applied during manufacture.

Supercapacitor Charging Methods

There are several methods for charging supercapacitors:

Constant Current Charging

In this method, a constant current is applied to the supercapacitor until it reaches its rated voltage. The main advantage of this method is simplicity. The disadvantage is that as the supercapacitor approaches full charge, the charging current must be reduced to prevent overcharging, which complicates the charging circuit.

Constant Voltage Charging

In this method, a constant voltage equal to the rated voltage of the supercapacitor is applied. An advantage of this approach is that there is no danger of overcharging, regardless of how long the supercapacitor is connected to the voltage source. A disadvantage is that the initial charging current is very high, limited only by the internal resistance of the supercapacitor and the current capacity of the voltage source, requiring a current-limiting circuit.

Constant Power Charging

This method combines the advantages of the constant current and constant voltage methods. It involves an initial stage of constant current charging until the supercapacitor voltage reaches a predetermined level, followed by a constant voltage stage for topping off the charge. This allows for fast charging without danger of overcharging.

Charging Circuit Design Considerations

When designing a supercapacitor charging circuit, several factors must be considered:

Charging Current

The maximum charging current is determined by the supercapacitor’s internal resistance and rated voltage. Exceeding this current can cause overheating and damage. A common rule of thumb is to limit the charging current to the supercapacitor’s rated current.

Charging Voltage

The charging voltage should not exceed the supercapacitor’s rated voltage, to prevent overvoltage damage. A voltage regulation circuit is necessary.

Charge Balancing

When multiple supercapacitors are connected in series, voltage balancing is required to prevent any cell from going over-voltage. This is typically done with a balancing resistor in parallel with each supercapacitor.

Temperature

Supercapacitors can be damaged by high temperatures. The charging circuit should monitor the supercapacitor temperature and reduce or stop charging if overheating occurs.

Example Charging Circuits

Here are some examples of supercapacitor charging circuits for different applications:

Simple Constant Current Charger

This circuit provides a constant current to the supercapacitor until it is fully charged. The current is set by the resistor R1.

Constant Power Charger

This circuit charges the supercapacitor with a constant current until it reaches 80% of its rated voltage, then switches to constant voltage mode for the final stage of charging.

Solar Charger

This circuit charges a supercapacitor from a solar panel. The LT3652 is a complete monolithic step-down battery charger that operates over a 4.95V to 32V input voltage range.

Frequently Asked Questions

1. How long does it take to charge a supercapacitor?

The charging time depends on the capacitance value and the charging current. Generally, supercapacitors can be charged much faster than batteries, typically in seconds to minutes.

2. What is the maximum voltage a supercapacitor can be charged to?

A supercapacitor should not be charged above its rated voltage, which is typically 2.5-2.7V for single cells. Higher voltages can be achieved by connecting cells in series.

3. How many times can a supercapacitor be charged and discharged?

Supercapacitors can typically withstand hundreds of thousands to millions of charge/discharge cycles, far more than batteries.

4. Can supercapacitors be charged with a variable power source like solar or wind?

Yes, but a power conditioning circuit is needed to regulate the voltage and current supplied to the supercapacitor. Maximum power point tracking (MPPT) is often used to optimize power extraction from variable sources.

5. What safety precautions are needed when charging supercapacitors?

Supercapacitors can deliver very high currents, so precautions are needed to prevent short circuits. Overcharging must also be prevented to avoid damage. Appropriate circuit protection devices like fuses and voltage regulators should be used.

Conclusion

Supercapacitor charging circuits are an essential component in systems that utilize the unique properties of supercapacitors. Proper circuit design ensures efficient and safe charging, maximizing the performance and lifetime of the supercapacitors.

Whether using constant current, constant voltage, or constant power charging, care must be taken not to exceed the rated specifications of the supercapacitors. Monitoring of parameters like voltage, current, and temperature during charging is recommended.

With their high power density, fast charging capability, and long cycle life, supercapacitors paired with well-designed charging circuits will continue to enable new applications in energy storage and power delivery.