When mains power drops for a few hundred milliseconds — a nearby lightning strike, a utility switchover, a brownout — your microcontroller resets, your SSD corrupts its FTL, or your industrial controller loses state. Batteries are overkill for this: they have finite cycle life, need charge management ICs, and hate high-temperature environments. Supercapacitors (also called EDLCs — Electric Double-Layer Capacitors) are the right answer for sub-minute ride-through, offering hundreds of thousands of charge cycles and 10+ year lifetimes.

The circuit topology is straightforward: a charge current limiter (the supercap looks like a short circuit when empty and can pull tens of amps from your 5V rail), an ideal diode or OR-ing MOSFET to prevent the supercap from backfeeding the input, and a boost converter on the output because the supercap voltage sags dramatically as it discharges.

Real-world example: RAID controller cache backup. LSI/Broadcom RAID cards use a supercap module (often 3× 10F in series, ~3.3F effective at 8.1V) to power DRAM long enough to flush it to onboard flash after a power failure. Typical hold time: 30 seconds at ~1W load. This replaced BBU (battery backup units) that needed replacement every 2-3 years.

Sizing calculation. The energy stored is E = ½CV². For a load drawing constant power P over hold time t, you need:

C ≥ 2Pt / (V_start² − V_min²)

Say your 3.3V load draws 500 mW and needs 5 seconds of ride-through. Your supercap charges to 5V and your boost converter still works down to 1V input. Then C ≥ 2(0.5)(5) / (25 − 1) = 0.208 F. A 0.22F, 5.5V EDLC (about $2) does the job.

Watch-outs:

• Series stacking: supercaps have ±20% tolerance, so cells in series need voltage balancing resistors (or active balancers) or the weaker cell will overvoltage and dry out.
• ESR matters: a 1F cap with 100 mΩ ESR loses 100 mV per amp drawn. For pulsed loads, size for ESR-limited voltage, not capacitance-limited.
• Leakage current: 1–10 µA typical, which drains the cap over weeks if the main supply is off. Not a "shelf backup" solution.
• Inrush at power-on: without limiting, a 1F cap will destroy small SMPS controllers. Use a series resistor plus MOSFET bypass, or a dedicated supercap charger IC (LTC3128, LTC4425).

For 100 ms–60 s backup windows, supercaps beat batteries on reliability, temperature range (−40 to +85°C), and maintenance.