Where to put the capacitor (and why it matters)

What the capacitor is for

Here is the mechanism, because almost every source stops at "add a cap" and leaves you to take it on faith.

Your ESCs do not deliver a smooth current to the motors. They switch — tens of thousands of times a second, they connect and disconnect the battery from the motor windings, in hard, fast steps. Meanwhile, the battery leads between the pack and the ESC are not an ideal wire. Any length of wire has inductance, and an inductor's defining property is that it resists a change in current: force the current through it to change quickly, and it answers with a voltage. The faster the change, the bigger the voltage. That is not a metaphor, it is the wire's job description.

So you have a switching stage demanding violently fast current changes, sitting at the end of an inductive lead that refuses to supply them cleanly. The result is a voltage spike on your power rail — a transient that rings well above the pack's nominal voltage every time the ESC switches, and that gets worse the harder the ESC is working.

Those spikes do two things to you, and both are bad. First, they are electrical noise, and they propagate everywhere your power rail goes: into the gyro's supply, into the video transmitter, into the GPS receiver's front end. Every one of those is a device you have asked to detect something very small, and you have just filled its environment with something very large. Second — and this is the part people miss — a spike can genuinely exceed the voltage rating of a component on the board. A capacitor rated for the pack voltage but not the ringing above it, a regulator, an ESC's own FETs. Parts do not fail only from sustained abuse. They fail from transients.

A capacitor across the power input is a local reservoir. It sits right where the current is being demanded, holds charge, and supplies the fast edge locally instead of dragging it down the inductive lead from the battery. It absorbs the spike rather than letting the rail ring. That is the whole idea. It is not magic, it is not a tuning change, and it is not optional on a build that switches serious current.

Where it goes

As close to the ESC's power input pads as physically possible. On the main battery pads themselves, legs cut short, soldered right there.

This is not a neatness preference. Go back to the mechanism: the thing you are fighting is the inductance of the conductor between the battery and the switching. A capacitor on the far end of two long legs has added an inductive path of its own between the reservoir and the place the reservoir is needed. It cannot supply the fast edge, because the fast edge is exactly what its own leads now resist. You have fitted a capacitor and cancelled most of its benefit in the same operation.

This is far and away the most common mistake, and it is invisible — the capacitor is on the aircraft, the build looks right, and it is doing a fraction of its job. Short legs, on the pads. If your capacitor is dangling on 40 mm of lead somewhere convenient, move it.

Polarity

Electrolytic capacitors are polarised. They have a positive end and a negative end and they are not interchangeable.

Fit one backwards and it will fail. Not "perform poorly" — fail, and it can fail loudly, venting hot electrolyte with a bang, on a bench, near your face. Every electrolytic has a stripe down one side marking the negative leg, and the negative leg is usually the shorter one on an unclipped part. Check the stripe. Check it against the pad. Check it again before you apply the iron, because after you have soldered it you have no further opportunity to be careful.

Wear eye protection when you first power up a build with a freshly fitted capacitor. That is not fussiness; it is the cheapest insurance in the hobby.

What size

Be careful here, and be sceptical of anyone who gives you a single number as universal truth.

Voltage rating first. It must comfortably exceed the maximum voltage of your pack — a fully charged pack, not its nominal figure. And then more, with real margin, because the entire point of the capacitor is to absorb transients that go above the nominal. Sizing the rating to the nominal voltage is sizing it to the one condition it will not be operating in. A marginal rating is a part that will die, and possibly die noisily.

Type second. You want a low-ESR electrolytic, of the sort explicitly intended for switching-supply work. ESR — equivalent series resistance — is the internal resistance of the capacitor itself, and a high-ESR part cannot dump charge quickly enough to be any use against a fast edge. A generic capacitor from a parts drawer may be the right capacitance and the right voltage and still do very little.

Capacitance third, and honestly: it depends on the build. It depends on your ESC, your current draw, your lead length, your pack. This is why sensible ESC manufacturers state a recommendation, and why that recommendation is the right place to start — not a number you read in a forum post about a completely different aircraft. Follow the ESC vendor's guidance. If you have none, follow the guidance for the closest comparable part from the same manufacturer. Do not treat a number from someone else's build as a specification.

What it fixes

• Video with lines or bars that move with the throttle. The most recognisable one. If the interference tracks your throttle stick, it is coming from the power rail, and a properly fitted capacitor is the standard first answer.
• Gyro noise. A quieter supply is a quieter gyro, which means a cleaner trace, which means you can run the D term you actually want instead of filtering your way out of trouble.
• Brownouts and reboots under a hard punch. The moment of maximum current change is the moment of maximum spike and maximum sag. A local reservoir helps both.
• GPS interference and poor fix quality. A GPS receiver is trying to hear a signal from orbit. It is exquisitely sensitive to a noisy neighbour.
• Component life on the power rail. The least visible benefit and arguably the most valuable. Every transient you absorb is one that does not land on a part's voltage rating.

What it does NOT fix

Say this part out loud, because capacitors have become a superstition — a thing people add when the aircraft is misbehaving and they have run out of ideas.

A capacitor will not fix a bad solder joint. It will not fix an undersized or over-long battery lead. It will not fix a pack that is genuinely sagging because it is old, cold, or too small for the aircraft you have bolted it to. It will not fix mechanical vibration — that is a prop, a bearing, or a mount, and no amount of electrolytic will help you. It will not fix a bad build, and it will not fix "my quad is weird".

It is standard good practice on a build that switches large currents. It is not a component you bolt on in the hope that things improve. If you do not know which of the symptoms above you have, adding a capacitor is a guess, and you will learn nothing from it either way.

Mounting it so it survives

A capacitor is a comparatively heavy lump on two thin legs, bolted to an airframe that vibrates continuously. Left to itself, it will swing, the legs will work-harden at the solder joints, and one day it will break off — mid-flight, at speed, with a live power lead flapping around inside your electronics stack.

That failure is considerably worse than never having fitted a capacitor at all. So secure it. Heat-shrink it against the stack, put a fillet of hot glue or an adhesive under the body so the legs are not the only thing carrying it, or strap it down. Whatever you do, the body must be held by something other than its own solder joints, and the legs must not be the structure.

Keep the legs short — which you were doing anyway, for the reason in the second section — and check the joint on any post-crash inspection.

A short checklist

• Low-ESR electrolytic, of a type intended for switching applications.
• Voltage rating comfortably above your fully charged pack, with real margin for the transients.
• Capacitance per the ESC manufacturer's recommendation, not per a forum post.
• Soldered directly on the main battery pads, legs as short as you can make them.
• Polarity checked against the stripe. Twice. Eye protection on first power-up.
• Body mechanically secured — glued, shrunk, or strapped. The legs are not a mount.
• And you know which symptom you are actually treating.