Supercapacitor vs Battery: Choosing Backup Power

1. Two different jobs

A supercapacitor (EDLC) stores charge in an electric double layer: it delivers huge bursts of power, charges in seconds and survives hundreds of thousands of cycles, but holds very little energy per kilogram. A battery stores far more energy but delivers less peak power and wears out after a few hundred to a thousand cycles.

Supercapacitors are measured in farads (1–3000 F); batteries in mAh or Wh. They are not interchangeable — they solve different problems.

2. The numbers that decide it

Property	Supercapacitor (EDLC)	Li-ion battery
Energy density	~5–10 Wh/kg	~150–250 Wh/kg
Power density	very high (kW/kg)	moderate
Charge time	seconds	1–3 hours
Cycle life	500,000–1,000,000	300–1000
Cell voltage	2.7–3.0 V (needs balancing)	3.6–3.7 V
Self-discharge	higher (days–weeks)	low (months)

Rule of thumb: if the job is energy (run a device for hours), use a battery. If the job is power (a short burst, a backup hold, or smoothing a pulse), use a supercapacitor.

3. How to size an EDLC

Energy stored is E = ½ × C × V². For a backup that must hold a rail from 5.0 V down to 3.0 V, the usable energy is ½ × C × (5.0² − 3.0²), so a 1 F part gives about 8 J — roughly 1.6 W for 5 seconds.

• Derate the cell voltage (use ≤ 2.5 V on a 2.7 V cell) for life
• Series stacks need voltage balancing resistors or an active balancer
• Check ESR: it sets the maximum pulse current (I = V / ESR)
• Account for self-discharge if the hold time is longer than a few hours

4. Where each belongs

• RTC / memory backup and graceful shutdown → supercapacitor
• Peak-power assist for pulse loads (motors, radios) → supercapacitor in parallel with the battery
• Energy harvesting cold-start → supercapacitor
• Hours of runtime → battery

5. How to choose

• Define the hold time and the acceptable voltage drop → compute C
• Check ESR against your peak current
• Decide series/parallel stack and balancing
• If both energy and power are needed, combine: battery + supercapacitor

Related products

• Supercapacitor Series
• Tantalum Capacitor Series
• SMD Capacitor Series
• DC-DC Converter Series