Supercap
An electric double-layer capacitor, or EDLC, stores energy for backup power or short bursts of current. Its voltage falls as charge is removed, so a load may need a converter. A supercapacitor is not automatically a battery replacement, and hybrid capacitor technologies have different behaviour.
Porous carbon electrodes contact an electrolyte. Opposite charges gather on the electrode surface and in the nearby electrolyte, forming an electric double layer. The very large accessible surface area and tiny charge separation produce high capacitance. This is not simply the solid dielectric construction of a ceramic capacitor.
In plain terms
Think of a sponge with an enormous internal surface. The pores create room for charge to gather, rather than making one ordinary capacitor plate dramatically bigger.
Why designers use it
- Storing energy for a limited backup interval.
- Supplying a short current burst while a source replenishes the stored charge.
- Combining high capacitance with a charging and balancing circuit suited to the application.
Best for
- RTC backup
- Brief holdup
- Energy harvesting
Key specifications
- Capacitance and usable energy: Use the exact cell's rating and allowed voltage window
- ESR: Creates an immediate voltage drop under load and I²R heating
- Cell voltage: A per-cell limit, not the total series-bank voltage
- Leakage and balancing: Important for standby life and series-connected cells
- Lifetime: Depends on voltage, temperature and the manufacturer's test conditions
When not to use it
- When self-discharge or falling output voltage makes it unsuitable for the required backup duration.
- When the design cannot limit charging current or keep every cell within its voltage rating.
- As an automatic substitute for small high-frequency decoupling capacitors.
Common mistakes
- Applying a bank's total voltage limit to each cell.
- Connecting an empty, low-ESR bank to a supply without considering inrush current.
- Assuming equal series-cell voltages without a suitable balancing design.
- Ignoring polarity, residual stored energy or the required controlled discharge procedure.
- Treating high capacitance as permission to replace a small high-frequency decoupling capacitor.
Where you will find it
- A backup supply can use an EDLC bank to keep a load alive briefly after input power disappears. Size it from the load, allowable voltage drop, ESR and converter behaviour.
- A pulse load can draw energy from a suitably rated bank while a current-limited supply replenishes it between pulses.
A short history
Howard Becker's US 2,800,616 describes porous carbon electrodes immersed in electrolyte, an early route to high capacitance. Modern EDLC application guidance focuses on cell voltage, ESR, temperature, leakage and balancing because those determine whether a useful energy store is also a reliable one.
Good to know
- Stored energy depends on voltage as well as capacitance: the ideal relation is E = ½CV².
- Putting equal capacitors in series raises the bank's potential voltage capability but reduces its equivalent capacitance; each cell still needs protection.