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Why an FPV ESC needs an input capacitor

An FPV ESC rapidly switches large motor currents. The battery cannot respond instantly because the battery leads have inductance. A capacitor across the ESC battery input acts as a nearby energy reservoir:

  • It supplies short current pulses to the ESC.
  • It absorbs regenerative current from the motors.
  • It reduces voltage spikes caused by battery-wire inductance.
  • It reduces electrical noise reaching the flight controller and video system.
  • It helps protect the ESC MOSFETs from excessive input voltage.
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Battery ───── long inductive wires ───── ESC
                                      + │
                                   capacitor
                                      - │

Mount the capacitor directly across the ESC's BAT+ and BAT− pads. Long capacitor leads add inductance and reduce its effectiveness.

Capacitor polarity

An aluminum electrolytic capacitor is polarized. Connect its positive lead to BAT+ and its striped negative side to BAT−. Reversed polarity or a voltage above its rating can cause the capacitor to vent or fail.

Battery connection and inrush current

When the battery is connected, an empty input capacitor briefly draws a large charging current. This can create a connector spark and stress the connector or power switch. See Inrush Current in FPV Power Systems for calculations and mitigation methods.

Why voltage spikes happen

Battery wires behave like a small inductor. An inductor resists rapid current changes:

\[ V_L = L\frac{dI}{dt} \]

If the ESC rapidly changes tens of amperes, even a small wire inductance can generate a large voltage spike.

The capacitor accepts or supplies this current locally:

\[ I = C\frac{dV}{dt} \]

Rearranging gives a useful first sizing equation:

\[ C \geq \frac{\Delta I \times \Delta t}{\Delta V} \]

Where:

  • \(C\) is the required capacitance.
  • \(\Delta I\) is the current change.
  • \(\Delta t\) is the time the capacitor must support that current.
  • \(\Delta V\) is the acceptable capacitor voltage change.

Example capacitance calculation

Assume:

  • Current changes by 50 A.
  • The capacitor must support it for 20 µs.
  • The acceptable capacitor voltage change is 2 V.
\[ C \geq \frac{50 \times 20 \times 10^{-6}}{2} \]
\[ C \geq 500\ \mu F \]

The next common size would be approximately:

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680 µF or 1000 µF

This is only a starting estimate. The ESC's current waveform, switching frequency, wiring, motor braking, ESR, and existing onboard capacitance all affect the real requirement.

ESR can matter more than capacitance

A real capacitor has equivalent series resistance, or ESR. A sudden current produces an immediate voltage step:

\[ \Delta V_{ESR} = \Delta I \times ESR \]

For a 50 A current change and a 20 mΩ capacitor:

\[ \Delta V_{ESR} = 50 \times 0.020 = 1\ V \]

If the ESR were 100 mΩ:

\[ \Delta V_{ESR} = 50 \times 0.100 = 5\ V \]

Therefore, a physically large capacitor with poor ESR may perform worse than a smaller low-ESR capacitor. Increasing capacitance alone cannot remove the ESR voltage step. See TI's capacitor transient analysis.

Look for:

  • Low ESR
  • High ripple-current rating
  • 105 °C temperature rating
  • Short, thick leads
  • A reputable capacitor series

Ripple current heats a capacitor according to its ESR, and excess internal temperature substantially reduces capacitor life. See TDK's electrolytic capacitor guidance.

Voltage rating

Use the battery's maximum fully charged voltage, not its nominal voltage:

\[ V_{battery,max} = cell\ count \times 4.2\ V \]
Battery Fully charged Practical capacitor rating
3S 12.6 V 25 V
4S 16.8 V 25 V or 35 V
6S 25.2 V 35 V minimum; 50 V gives more margin
8S 33.6 V 50 V
12S 50.4 V 63 V or higher

The voltage rating must also tolerate spikes. Do not operate an electrolytic capacitor above its rating.

Practical FPV starting points

When the ESC manufacturer does not specify a value, these are reasonable experimental starting points, not universal calculations:

Setup Starting capacitor
Small 3S–4S build 470–680 µF, 25–35 V
Typical 4S build 680–1000 µF, 35 V
Typical 6S build 680–1000 µF, 35–50 V
High-current or long battery leads 1000–2200 µF, suitable voltage

Two capacitors in parallel can be useful:

\[ C_{total} = C_1 + C_2 \]

Parallel capacitors also reduce combined ESR and share ripple current, assuming similar parts and a good physical layout.

Engineering selection procedure

  1. Start with the ESC manufacturer's recommended capacitor.
  2. Calculate the fully charged battery voltage.
  3. Select a low-ESR, 105 °C capacitor with adequate voltage margin.
  4. Estimate capacitance using:

[ C \geq \frac{\Delta I\Delta t}{\Delta V} ]

  1. Check the maximum acceptable ESR using:

[ ESR \leq \frac{\Delta V_{ESR}}{\Delta I} ]

  1. Check that the capacitor's ripple-current rating is sufficient.
  2. Install it directly at the ESC battery pads.
  3. Measure the input voltage with an oscilloscope using a short ground spring.

A long oscilloscope ground lead can create a false voltage spike, so measurement technique matters.

For most ordinary 4S–6S FPV builds, a genuine 680–1000 µF low-ESR, 35–50 V, 105 °C capacitor mounted directly on the ESC is a sensible starting point. Exact sizing ultimately requires measuring the real ESC input waveform.