If you need to buffer a 50A motor startup spike or keep an RTC alive for three weeks after main power fails, standard electrolytics will boil and Li-ion batteries will degrade after a few thousand cycles. This is where ultra capacitors (often called supercapacitors or EDLCs) earn their keep. They bridge the gap between the high energy density of chemical batteries and the high power density of traditional dielectric capacitors.
But slapping a 100F can onto a 5V rail without balancing resistors is a fast track to a ruptured seal and a bench covered in corrosive electrolyte. Below is the decision-forward playbook for selecting, sizing, and keeping ultra capacitors alive in your designs.
The Verdict: When to Deploy Ultra Capacitors vs. Batteries
The fundamental rule of energy storage on the bench: Batteries are for energy (Joules); ultra capacitors are for power (Watts). A 100F ultra capacitor at 2.7V stores roughly 364 Joules. A standard 1000mAh Li-ion cell at 3.7V stores over 13,000 Joules. However, the ultra capacitor can dump its entire 364 Joules in under a second without thermal damage, whereas the Li-ion cell would sag, overheat, or trigger its BMS protection under the same C-rate.
| Your Application Need | Choose This Technology | Why |
|---|---|---|
| Days/weeks of backup power for high-draw loads | Li-ion / LiFePO4 Battery | Ultra caps self-discharge too fast (mV/hr) for long-term hold-up. |
| Sub-second ride-through for microcontrollers/SRAM | Ultra Capacitor (Coin/Radial) | Millions of cycles, no charging IC required, instant current delivery. |
| Buffering high-current spikes (motors, solenoids) | Ultra Capacitor (Screw Terminal) | Extremely low ESR handles 50A+ transients without voltage collapse. |
| Energy harvesting (solar/wind MPPT smoothing) | Ultra Capacitor Bank | Absorbs erratic charge profiles that would confuse battery BMS logic. |
Ultra Capacitor Types: Construction and Bench Specs
Not all 'supercaps' are created equal. The market is dominated by three distinct chemistries. While standard capacitors are judged by their capacitance temperature coefficient (tempco), ultra capacitors are judged by their ESR temperature dependence. An EDLC's ESR can easily double when dropping from 25°C to -20°C, which will choke your circuit in cold environments.
| Type | Construction / Chemistry | Capacitance Tolerance | ESR Tempco (-20°C vs 25°C) | Typical Bench Use |
|---|---|---|---|---|
| EDLC (Symmetric) | Activated carbon electrodes, organic electrolyte | -10% to +30% | ESR increases ~200% | High-current buffering, motor startups, regenerative braking. |
| Pseudocapacitor | Metal oxides (RuO2) or conducting polymers | ±20% | ESR increases ~150% | High energy density needs where space is constrained. |
| Hybrid (Li-ion Cap) | Asymmetric: Carbon + Li-ion intercalation electrode | ±10% | ESR increases ~120% | Bridging the gap; higher voltage per cell (3.8V-4.0V). |
Source: Battery University - How does a Supercapacitor Work
Decoding the Can: Reading Markings and Datasheet Codes
Unlike ceramic caps that hide behind cryptic 3-digit EIA codes, ultra capacitors usually print their vital stats directly on the sleeve. However, the format changes drastically based on the physical form factor.
Large Screw-Terminal Cans (e.g., Eaton/Maxwell, KEMET)
These 100F to 3000F beasts use laser-etched alphanumeric strings. Take the industry-standard Eaton BCAP0100 E270 T11:
- BCAP: Product family (Boostcap).
- 0100: Capacitance in Farads (100F).
- E270: Rated voltage (2.7V). Never exceed this; the electrolyte decomposes rapidly above 2.8V.
- T11: Terminal type and physical case size code.
Coin Cell and Radial Cylindrical (e.g., Panasonic, Vishay)
Smaller 0.1F to 5.0F caps used for logic backup use standard part numbering, but the physical print often includes explicit decimal Farads. For a Panasonic EEC-S5R5V105:
- EEC: Panasonic's Electric Double Layer Capacitor prefix.
- S5R5: Voltage series (5.5V). The 'R' stands for the decimal point.
- V105: Capacitance code. Here, 105 means 10 × 10^5 pF, which equals 1,000,000,000 pF, or 1.0F. (Yes, they sometimes revert to the pF multiplier code on the datasheet, but the can itself will usually say '1.0F').
Failure Modes and Visual Autopsy
Ultra capacitors don't fail like ceramics (which short and crack) or electrolytics (which dry out and lose capacitance). They fail primarily due to overvoltage and thermal abuse. Here is what to look for when debugging a dead board.
1. Swelling and Crimp Seal Bulging
Visual Symptom: The bottom rubber crimp seal pushes outward, or the top of the can domes visibly. The cap may no longer sit flush on the PCB. Root Cause: Chronic overvoltage or high ambient heat. Operating a 2.7V cap at 2.8V accelerates electrolyte decomposition, generating gas. The internal pressure pushes against the crimp seal. The Fix: Derate your voltage. If your rail is 2.5V, use a 2.7V cap. If your rail is 3.3V, you must use two 2.7V caps in series.
2. Venting and Electrolyte Weeping
Visual Symptom: A crusty, dark residue around the safety vent on the top or bottom of the can. A sharp, sweet, or acrid chemical smell (acetonitrile or organic carbonate solvents). Root Cause: Catastrophic overvoltage or reverse polarity. The internal safety vent popped to prevent the can from exploding, releasing the liquid electrolyte.
3. ESR Creep (The Silent Killer)
Visual Symptom: The cap looks perfectly fine. However, under load, the voltage sags much deeper than Ohm's law predicts for the circuit. Root Cause: End-of-life aging or continuous operation near the maximum temperature rating (usually 65°C to 85°C). The electrolyte slowly evaporates through the seal over years, increasing the Equivalent Series Resistance (ESR). The Fix: Measure the ESR with a dedicated milliohm meter. If the ESR has doubled from the datasheet's initial value (e.g., creeping from 5mΩ to 10mΩ on a large can), the cap has reached its end-of-life definition and must be replaced.
The Substitution and Sizing Decision Tree
Sizing an ultra capacitor requires abandoning the 'just pick a bigger number' mentality. You must calculate based on the acceptable voltage droop. The governing equation is: C = (I × t) / ΔV.
Example: Your ESP32 draws 50mA during a WiFi transmission burst that lasts 2 seconds. You want the voltage to droop no more than 0.5V (from 3.3V to 2.8V) during this burst.
C = (0.050A × 2s) / 0.5V = 0.2 Farads (200,000 µF).
Follow this decision path to select the exact physical part and topology for your bench build:
Step 1: Determine Topology (Single vs. Series String)
- If your max rail voltage is ≤ 2.5V: Use a single cell. Go to Step 2A.
- If your max rail voltage is > 2.5V but ≤ 5.0V: You MUST use a series string. Two 2.7V cells in series yield 5.4V max. Go to Step 2B.
Step 2A: Single Cell Selection (Low Voltage)
- For < 1.0F (Coin/Radial): Pick the Panasonic EEC-S5R5V105 (1.0F, 5.5V). Yes, it's rated for 5.5V, but using a single 5.5V coin cell on a 3.3V rail gives you massive voltage derating, ensuring it will outlive the rest of the PCB.
- For 10F to 100F (Through-hole/Screw): Pick the KEMET FG0H104ZF (0.1F) for small hold-ups, or step up to the Eaton BCAP0100 (100F, 2.7V) for heavy current buffering.
Step 2B: Series String Selection (High Voltage)
When you put capacitors in series, the total capacitance drops (C_total = C / N), and the voltage rating multiplies. More importantly, leakage current mismatch will destroy the string without balancing.
- The Calculation: If you need 10F at 5V, you need two 20F, 2.7V caps in series (20F / 2 = 10F; 2.7V × 2 = 5.4V).
- The Mandatory Balancing Rule: You must place a balancing resistor across each capacitor. A 10kΩ 1/4W resistor across each 2.7V cap will bleed off excess voltage and ensure neither cell exceeds 2.7V if their internal leakage currents differ. For high-reliability or high-voltage strings, use dedicated active balancing ICs like the Texas Instruments BQ33100.
- Concrete Pick for 5V Strings: Buy a pre-matched, pre-balanced module. The Eaton/Vishay 5V 10F Super Capacitor Module (e.g., Vishay 196 DLC series) comes with internal balancing resistors already potted into the module. It saves you from routing high-current PCB traces and guessing resistor tolerances.
Source: DigiKey - Supercapacitors: Bridging the Gap Between Batteries and Capacitors
Ultra capacitors are unmatched for high-cycle, high-current abuse, provided you respect their strict voltage ceilings and temperature limits. Calculate your ΔV, derate your voltage by at least 10%, add your balancing resistors, and your energy storage will easily outlast the silicon it's protecting.
