Supercapacitor Runtime Calculator
Calculate how long a supercapacitor can power a load before reaching a cutoff voltage. Returns runtime, usable energy, total stored energy, and charge — using linear constant-current discharge analysis.
How to Use
- Enter capacitance C with engineering suffixes (F, mF). Supercaps are typically 0.1 F to 3000 F.
- Enter the fully-charged starting voltage V<sub>max</sub> — usually the capacitor's rated voltage.
- Enter the cutoff voltage V<sub>min</sub> below which your load can no longer operate.
- Enter the discharge current I (constant-current approximation).
- Results: runtime t = C·(V<sub>max</sub>−V<sub>min</sub>)/I, plus usable and total energy.
Show Work
Formulas
History of the Supercapacitor
The electric double-layer capacitance phenomenon that supercaps exploit was first described by Hermann von Helmholtz in 1853 — he noted that an electrode immersed in electrolyte develops two layers of opposite charge separated by atomic-scale distances, creating enormous capacitance per unit area. For a century the effect remained a curiosity with no practical device behind it.
The first commercial double-layer capacitor was patented by General Electric's H. Becker in 1957, but the technology sat dormant until the Standard Oil Company (SOHIO) produced the first commercially successful version in the 1960s. NEC licensed and commercialized them as "Supercapacitors" from 1971, initially for memory backup in early calculators and computers. Maxwell Technologies and Panasonic drove ratings up through the 1990s and 2000s; modern large-format cells from Skeleton Technologies and others now reach 3000+ farads at 2.7 V.
The performance envelope is distinctive: 10× the energy density of electrolytic capacitors but 10× less than lithium-ion batteries; 10× the power density of lithium-ion; and million-cycle lifetimes versus thousands for batteries. That combination fits niches batteries handle poorly: regenerative braking buffers, very fast backup power, hold-up for SSD write flushes, and starter-assist in stop-start vehicles.
About This Calculator
Enter capacitance, starting voltage, cutoff voltage, and discharge current. The calculator uses the constant-current linear-discharge formula t = C·ΔV/I and returns runtime plus usable vs. total stored energy. The visualization shows the linear voltage ramp from Vmax down to Vmin.
For constant-power loads (most DC-DC converters feeding microcontrollers), this tool slightly underestimates runtime — the real voltage curve is concave-up rather than linear because current rises as voltage falls. For a tighter estimate, iterate with decreasing current at each voltage step. Everything runs client-side; no values leave your browser.
Frequently Asked Questions
How is a supercap different from a regular capacitor?
Supercaps use electrostatic double-layer capacitance (EDLC) instead of dielectric-based capacitance, so they achieve farad-scale capacitance in small packages — values that would be physically impossible with ceramic or electrolytic dielectrics. Energy density is ~10× lower than lithium batteries but power density is 10× higher, and cycle life reaches millions vs. thousands for batteries.
Constant current vs. constant power load?
A constant-current load gives a linear voltage ramp (V drops linearly with time), so runtime = C·ΔV/I exactly. A constant-power load draws more current as voltage drops, so it pulls the cap down faster — runtime is shorter and requires solving a differential equation. This tool uses the constant-current approximation.
Why is the usable energy less than stored?
A capacitor fully charged to V<sub>max</sub> stores ½·C·V<sub>max</sub>². By the time it discharges to V<sub>min</sub>, it still holds ½·C·V<sub>min</sub>². Usable energy is the difference: ½·C·(V<sub>max</sub>² − V<sub>min</sub>²). You can't extract the stored energy below V<sub>min</sub> without boost conversion.
Can I put supercaps in series for higher voltage?
Yes — series stacking gives V<sub>total</sub> = N·V<sub>cell</sub>, capacitance = C/N. Every cell in the stack needs a balancing resistor or active balancer because manufacturing tolerance causes one cell to drift high and overvolt; a balancer bleeds current from the higher-voltage cell to equalize.
What about leakage current?
Supercaps have significant leakage — typically µA to mA depending on size and chemistry. For long-term hold-up, leakage can drain the cap faster than the load does. Datasheets specify leakage at rated voltage; always design with margin.
Common Use Cases
Memory Hold-Up on Power Loss
A 0.1 F 5.5 V coin-cell supercap can keep a µA-level RAM or RTC alive for weeks after main power fails — far longer than a backup battery with equivalent footprint.
SSD Power-Loss Protection
Enterprise SSDs use arrays of supercaps to provide 10–50 ms of hold-up so the controller can flush in-flight data to NAND during a brown-out. Typical values: 10–100 mF at 5 V.
Regenerative Braking Buffer
Transit buses and hybrid vehicles capture brief, high-current bursts from regenerative braking into supercaps, then trickle that energy back to the main battery — protecting cells from damaging charge spikes.
Camera Flash Energy
Large supercaps (1–10 F) can replace electrolytic photoflash capacitors, delivering the same energy in a package that cycles a million times without degradation.
Solar Lamp Bridge
Outdoor solar lights use supercaps to bridge brief cloud-induced dropouts, preventing the LED from flickering during variable illumination.
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