Capacitor Storage

Part of Energy Storage & Batteries

Capacitors store energy in electric fields rather than chemical reactions — delivering or absorbing charge almost instantly, but with far lower energy density than batteries.

Why This Matters

Capacitors fill a gap that batteries cannot. A battery delivers energy steadily over minutes or hours but cannot instantly dump all its stored charge without damage. A capacitor does the opposite — it releases all its stored energy in milliseconds or microseconds. This makes capacitors essential for applications requiring brief, intense power: ignition sparks, flash lamps, radio transmitter pulses, and motor starting.

For a rebuilding civilization, capacitors enable several critical technologies. A large capacitor bank charged by a hand-cranked generator and discharged through a spark gap can ignite fuel reliably — a vast improvement over flint-and-steel. Capacitors in radio transmitters allow pulsed high-power signals from modest continuous power sources. Starting large electric motors requires brief current surges that would damage batteries but are ideal for capacitor delivery.

Understanding capacitor construction, energy storage limits, and safe discharge procedures lets you build and use these devices effectively. Even simple capacitors made from household materials can store meaningful energy for practical applications.

How Capacitors Store Energy

A capacitor consists of two conductive plates separated by an insulating material called a dielectric. When voltage is applied across the plates, opposite charges accumulate on each plate — electrons pile up on the negative plate, and the positive plate becomes electron-deficient. The electric field between the plates represents stored energy.

Capacitance measures how much charge a capacitor stores per volt: C = Q/V, where C is capacitance in farads, Q is charge in coulombs, and V is voltage. One farad stores one coulomb per volt — an enormous capacitance; practical capacitors are measured in microfarads (μF, 10⁻⁶ F) or picofarads (pF, 10⁻¹² F).

Stored energy: E = ½CV² joules. This quadratic dependence on voltage is critical — doubling the voltage quadruples stored energy. A 100 μF capacitor charged to 100 V stores 0.5 joules; charged to 1,000 V, it stores 50 joules.

Voltage rating: Every capacitor has a maximum voltage before the dielectric breaks down, current punches through, and the capacitor fails — often explosively for electrolytic types. Always operate well below the rated voltage.

Homemade Capacitor Construction

Leyden jar type: The simplest capacitor. Take a glass jar, coat the inside and outside surfaces with aluminum foil or metallic paint (silver paint, carbon paint, or wrapped thin sheet metal). The glass is the dielectric. A wire extends from the inner foil through a stopper or lid. Connecting the inner and outer conductors to voltage charges the capacitor. A typical half-liter jar achieves 1–2 nF capacitance.

Flat plate capacitor: Alternate layers of aluminum foil and wax paper or plastic sheet. Each foil layer connects alternately to one terminal — all odd layers to terminal A, all even layers to terminal B. The capacitance scales with the number and area of layers and inversely with layer thickness. For a 10-layer stack with 10 cm × 10 cm aluminum foil and 0.1 mm thick wax paper dielectric, expect roughly 0.1–0.5 μF.

Rolled capacitor: Lay two foil strips with dielectric sheets between them, offset slightly so each foil edge extends beyond one side. Roll the stack tightly. Connect to the extending edges. This construction packs large plate area into small volume.

Dielectric materials and their properties:

• Wax paper: 2.5× relative permittivity, breaks down at ~20 V/μm
• Mica sheets (mineral): excellent — 6–9× permittivity, 100–200 V/μm breakdown
• Glass: 6–8×, moderate breakdown
• Dry paper: 2–3×, low breakdown (~15 V/μm)
• Petroleum jelly (Vaseline)-impregnated paper: improves both dielectric constant and breakdown voltage

Charging and Discharging

Charging time constant: When charging through a resistance R (in ohms) with capacitance C (in farads), the time to reach ~63% of final voltage is τ = RC seconds. To charge to 99%, allow 5τ. A 100 μF capacitor charging through 100 Ω reaches full charge in about 50 ms.

Discharge into a load: Similarly, discharging through resistance R, voltage falls to 37% in time τ = RC. For a spark gap, the resistance is essentially zero and discharge happens in microseconds, creating a high-current pulse.

Safe discharge procedure: Never assume a capacitor is uncharged after disconnecting power. Capacitors can hold charge for hours or days. Always discharge through a resistor (1–10 kΩ is safe for most low-voltage capacitors) before handling. For high-voltage capacitors (above 50 V), treat them as live even after disconnection, use insulated tools, and discharge through a high-resistance load first.

Practical Applications

Ignition systems: A 10–50 μF capacitor charged to 200–400 V from a hand-cranked generator and discharged through a spark gap ignites fuel vapor reliably. This is the basis of capacitor discharge ignition (CDI) used in motorcycles and chainsaws.

Motor starting: Induction motors need a phase-shifted current to start rotating. A starting capacitor (typically 50–200 μF) in series with one winding provides this phase shift. Remove it after the motor reaches speed via a centrifugal switch.

Radio transmitter spark gap: Early wireless telegraphs charged capacitors to high voltage from a transformer, discharged through a spark gap to create damped oscillations in a tuned circuit. This principle works with homemade components.

Energy pulse welding: A large capacitor bank (1–10 farads at 10–20 V) discharged through thin wire melts it instantly, welding the surrounding contact points in spot welding applications.