Supercapacitors as backup power
Supercapacitors (also called ultracapacitors or EDLCs) store far more energy than ordinary capacitors and charge almost instantly. They are used to keep a real-time clock alive, save data when power fails, ride through brief outages or smooth the peaks of a GSM modem. Unlike a battery, their voltage falls steadily as they discharge, so the useful energy depends on the lowest voltage your circuit accepts.
The formulas
- Stored energy: E = ½ C V²
- Usable energy between V1 and V2: E = ½ C (V1² − V2²)
- Constant current load: t = C (V1 − V2) ÷ I
- Constant power load (behind a DC-DC converter): t = C (V1² − V2²) ÷ (2P)
The calculator also works backward to find the capacitance needed for a target time.
Example
A 1 F supercapacitor charged to 5 V supplies a circuit that works down to 3 V:
| Item | Value |
|---|---|
| Total stored energy | 12.5 J |
| Usable energy | 8 J (64%) |
| Hold-up time at 100 mA | 20 s |
| Hold-up time at 0.25 W | 32 s |
| Capacitance for 60 s at 100 mA | 3 F |
A buck-boost converter that runs down to 1 V or less makes much more of the stored energy usable: 96% from 5 V to 1 V.
Practical points
- ESR: the capacitor’s internal resistance causes an immediate voltage step of I × ESR when the load starts. Keep it small compared with V1 − V2.
- Cell voltage: a single supercapacitor cell is usually rated 2.5 to 3 V. Higher voltages need cells in series, which divides the capacitance and requires balancing resistors or a balancing circuit.
- Tolerance and aging: capacitance is often −10% to +30% when new and falls with time, heat and voltage. Add 20 to 30% margin.
- Leakage: supercapacitors self-discharge over days, so they suit short outages rather than long storage.
- Inrush current: an empty supercapacitor looks like a short circuit; limit the charging current.
Supercapacitor or battery?
Use a supercapacitor for seconds to minutes of backup, frequent cycling, very cold temperatures and high pulse currents. Use a battery for hours of runtime or when space is tight, since batteries store 10 to 100 times more energy per unit of volume.