Supercapacitor
Capacitor
An electrochemical capacitor with enormous capacitance — whole farads in a coin-sized can. Sits between capacitors and batteries: charges in seconds, delivers brief bursts of power, survives a million cycles.
Schematic symbol
Capacitor (polarised)
How supercapacitor appears in a circuit diagram.
Types & variants
Key specs
Capacitance
F0.1 F to thousands of farads.
Voltage rating
VVery low per cell — typically 2.7 V (5.5 V for stacked coin types).
ESR
ΩSets the burst current it can deliver and charge efficiency.
Leakage current
µAHow fast it self-discharges — matters for backup use.
Laws that govern it
- Law of capacitance — capacitance formula Q = CVQ = C × V, i = C·dv/dt, E = ½CV²
E = ½CV² is the whole spec sheet: 1 F at 5.5 V holds about 15 J — enough for an RTC backup, nowhere near a battery.
- Ohm's lawV = I × R
ESR × current is the voltage you instantly lose on load, which is why supercaps deliver big currents but sag as they discharge.
Each law is stated in full — with its diagram, variables and worked meaning — on the electrical laws page.
Markings
Capacitance and voltage printed on the can; polarity stripe on the negative lead like an electrolytic.
Standard values
0.22 F, 1 F, 10 F at 2.7 or 5.5 V are common hobby sizes.
How to choose
Size capacitance from the energy needed (E = ½CV²) and check ESR supports your peak current. Never exceed the cell voltage — series stacks need balancing resistors or a balancer IC.
Pinout & package
2 leads, polarised. Coin types for PCB backup, radial cans for power buffering.
Example circuits
- RTC/SRAM backup when main power drops
- Energy buffer for a GSM module's transmit bursts
- Small solar energy harvester storage
Common failures
Overvoltage kills them quickly (venting, capacitance collapse); high ESR growth with age and heat. Series stacks fail when one cell over-charges without balancing.
How to test
Charge to rated voltage through a resistor, disconnect, watch self-discharge with a voltmeter. Capacitance meters can't measure them — time the charge through a known resistor instead (τ = RC).
Substitutes
A small rechargeable battery (slower charge, more energy) or a big electrolytic (less energy, higher voltage) depending on which property you actually need.