Thread Tools
Nov 12, 2008, 05:01 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP

too long battery wires can kill ESC: causes, precautions, solutions & workarounds

Last update July 28, 2020. I have also copied (parts of or links to) useful posts in this thread into this opening post.
A big thank you to those contributors

... therefore, lengthen the motorwires if possible, not the battery wires.
However, if you cannot but extend the battery wires ...

  • 'Watery' analogon
  • Two problems
    ◦ Voltage spikes higher than battery voltage: transistor/FET/chip breakdown.
    ◦ Current through input capacitors too high: overheating/popping/exploding.
  • Example four costly controllers consecutively cremated
  • Solution I: simply lengthen the motor wires instead
  • Solution II: add extra capacitors, rules of thumb
    Calculation spreadsheet
  • Capacitor type!
  • Capacitor polarity/orientation!
  • How & where (not) to add extra capacitors
    As close to or on controller.
  • DIY capacitor pack pictures
  • Capacitor & pack suppliers
  • Expert/manufacturer opinions & their rules of thumb
    They all say the same ... noise_reduction_101, switching_power_supply_design_101.
  • Cause, explanation, water hammer/knock analogy&video, theory
  • Measurements & scope traces

'Watery' analogon
For a better understanding of the problem a video about water hammer pressure pulses

Two problems.
Conclusion from the links below, all controller manufacturers say the same:
Too long battery wires will kill your ESC over time!
There are two fail mechanisms at work.
  • Voltage spikes higher than battery voltage. The spikes will destroy discrete transistors and integrated circuits through breakdown.
  • Higher current flowing through the standard input capacitors already on ESC (large colourful barrelshaped). Every ESC manufacture has already installed the right size caps for the leads on their ESC, The capacitors will get hotter because they have to work harder and will be destroyed over time. They can overheat and pop/explode.
Using thicker wire will not help, it's a wire inductance problem, not a resistance problem. See Lenz's law. Inductance is what makes sparkplugs spark, 15-25kilovolt.

However, lengthening the motor wires may lead to radio interference. Give the three of them a twist to prevent this, or tie them together.

Exploded electrolytic capacitors.
Click to enlarge.

Four costly controllers consecutively cremated
ESC Failure and Overheating - RCG

Solution I: Keep battery wires short, lengthen the ESC-motor wires.
That's hardly critical because there's already a lot of wire in the coils in the motor itself. If the motor-ESC wire eventually gets too long, it will not harm motor and/or controller. May cause interference though, give the motor-ESC wires a twist. Always a good idea to do that anyway.
Also, motorside wire-inductance is a good thing, it takes care of high frequency Pulse Width Modulation harmonics motorside, by smoothing the current.
Assuming you're not using a core-/iron-less motor (= low inductance, needs high PWM frequency).
Motorwire induction (coils) is good, necessary even battery wire induction is bad

The freewheel/flyback diodes in the power FETs also help in smoothing motorcurrent.
Even though the FETs switch off, the diodes keep the current flowing through the motor and motor↔ESC wires.

Motorside: voltage chopping and smoothing of current.
Click to enlarge. From → course

Click to enlarge.

Some ESC manufacturers advise against lengthening the wires motorside. But that's only because motor wires have a tough insulating varnish/resin coating that has to be removed before soldering. Several methods for several types of insulation. E.g. the aspirin trick mentioned in opening post:
(Re)winding and building motors (sticky)

Noise from the motorwires is hardly an issue. Motorside induction is good, in combination with and passive/active freewheel diodes and PWM frequency it actually smoothes the current.

But what if you can not extend the motor wires ...?

Solution II: add extra capacitors
Controllers already have capacitors on-board to compensate for 'standard' length battery wire inductance. But that may not be enough capacitance when adding extra battery-wire.
If you have to lengthen the battery wires, for whatever reason, add extra electrolytic capacitors in parallel with ESC, never in series with ESC. As a rule of thumb, for (but nut on!) every 4inch/10cm extra length/distance between battery and ESC, add 220uF extra capacitance near the controller (electrolytic condensators, voltage the same as the capacitors already installed, low ESR type) (Ludwich Retzbach, German e-flight author&editor, the 'R' in LRK).
When using two battery packs in series the extra length of wire between the packs counts as well.

Radial and axial e-caps/elcos.
Click to enlarge.

Better to use several smaller caps (in parallel) instead of one biggie. Smaller caps can shed more heat more easily and their total inductance will be lower (inductance per cap is lower and those inductances are paralled to boot). See attached pictures below.
Keep the positive and negative wires as close to each other as possible by taping them together (twisting the wires will increase wire length a bit). When the wires are close to each other then the series inductance will be reduced, because the current is going in opposite directions in each wire (and therefore producing opposite polarity magnetic fields which cancel out). For example, 13AWG wires separated from each other by 1 inch have about 4 times higher inductance than if they are bound together (thanks Bruce Abbott).
Or even remove insulation of one of the wires??? And/or thinner insulation? The less distance between the two copper cores, the lower the inductance.

Calculation spreadsheet.
Extra capacity based on current and wire length, calculation spreadsheet

Influence of ESC max.current rating on choice of capacitance

Capacitor type!
The main spec you need is low impedance and low ESR (Equivalent Series Resistance), otherwise capacitors will get too hot (P=I²Resr) I think the only thing you will find at Radio Shack c.s. will be general purpose caps, not low ESR. The ESR value of a cap is not printed on it, you will have to look up the manufacturers spec sheet. The Rubicon ZL series mentioned in the Schulze instructions is a good one and is available from newark/farnell. The Panasonic FM series is another good low impedance cap and is available from digikey

You would want the voltage rating on the caps to be significantly higher than the battery voltage. Same voltage rating (or higher) as the manufacturer installed caps. Higher voltage rating is no problem. (thanks jeffs555).
If you try it with longer wires and no extra low esr caps it may work for a while, but the longer wires put an extra load on the original cap. The extra load shortens the life of the original cap and it will eventually fail, probably catastrophically. (thanks jeffs555, from

Capacitor polarity/orientation
Electrolytic capacitors, like batteries, have a (+) and (-) lead! Solder them in the wrong way and they will got hot, pop open or even explode. Nasty fumes and stains from the liquid. Don't ask ...

Exploded electrolytic capacitors.
Click to enlarge.

How & where (not) to add extra capacitors!
The capacitors compensate for the effect that wire length (induction) has on the ESC. Therefore all capacitors as close as possible to or on the ESC board, direct across plus and minus power input terminals. More examples in attached pictures below.

Correct and wrong location.
Click to enlarge.

Do not distribute the extra capacitors along the battery wires! (noise_suppression_101, switched_power_supply_101).
By distributing them along the wires between battery and ESC they will be less efficient, or even useless.
It would only add extra weight, take up extra space and it would introduce extra points of failure.
Capacitors near/on battery are useless, the battery itself already acts as a huge capacitor. (English)
YGE controllers, extra capacitors and their location, installation manual, nice pics, click to enlarge: → products → accessories

DIY pictures
Capacitor & pack suppliers
CapacitorsComplete packs
Expert opinions & their rules of thumb
They all say the same ...
Cause, explanation, water hammer/knock analogy & video, theory
First a watery analogy: water running in a pipe and through a tap. Now turn off the tap quickly. You may hear a loud knock/shock sound (voltage spike) in the pipe (wire). The water (current) wants to continu flowing, but it can't, for a moment the water pressure (voltage) is much higher than the static water pressure (voltage). It's the same for a current that's switched off, because of the inductance it wants to keep on flowing, voltage gets higher. This is also what causes sparks (brush fire) in a brushed motor.
See also

Water hammer video
I like the instantly-stopping-a-moving-freight-train-at-the-front analogy in this video.
Also notice the location of the anti-surge device.

The controller is like a watertap that's switched on and off very fast (8,16, 32kHz PWM voltage chopping) to get the desired current. Turning off the current, in combination with the battery wire inductance, causes voltage spikes because the current wants to continue on its course (ref. inertia of the moving watercolumn). These voltage spikes are higher!!! than the battery voltage. The input capacitors (cylindrical aluminium 'barrels') take care of these spikes (they reduce the wire inductance). The longer the wires, the higher the voltage spikes induced in the wires, the harder on the input capacitors. They will get warmer, heat up and explode and the rest of the controller will feel the full brunt of the voltage spikes. This is caused by the wire inductance, not by wire resistance. Therefore, using thicker wire will not help much, it's not a bad idea either, but extra capacitors are the solution, thus reducing/compensating the wire inductance. Or longer motor wires instead of long battery wires.

More on voltage spike and switched inductive load
PWM chopping - Wikipedia
Lenz's law - Wikipedia
Flyback diode - Wikipedia

1 - Full throttle, no PWM--2 - Part throttle, PWM chopping
Click to enlarge. Scope traces from

References (thanks Panther3001, post #685)
    Shows that energy stored is proportional to inductance, L, and proportional to current squared, I²
    Shows that the inductance of a straight wire is proportional to the length of the wire times the natural log of the length of the wire ...which is close to being linearly proportional.

Measurements & scope traces

Vriendelijke groeten Ron
• Without a watt-meter you are in the dark ... until something starts to glow, or blow •
e-flight calculatorswatt-metershigh power motor tips&tricksCumulus MFC

cap caps capacitor capacitors heat heating overheating hot hotter pop popping burst bursting melt melting molten explode explosion explosive
Last edited by Ron van Sommeren; Yesterday at 07:02 AM.
Sign up now
to remove ads between posts
Nov 12, 2008, 06:58 AM
Crash Master
Gene Bond's Avatar
Odd that in the industrial drive world we use input reactors to assist knocking the spikes from the incoming line. I've yet to see this on any ESC, but I have seen it on servos and BEC's.

Is there any manufacturer who adds or suggests a torroid on the input lines?
Nov 12, 2008, 08:58 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
That would only increase the problem because induction in the lines causes the spikes. Industrial controllers are AC voltage fed, our controllers convert DCinto some kind of AC by chopping. The chopping is the cause of the long battery wire problem.

What are 'input reactors'?

Vriendelijke groeten Ron
Last edited by Ron van Sommeren; Aug 11, 2015 at 12:32 PM.
Nov 12, 2008, 10:23 AM
Reduce the drama...
rick.benjamin's Avatar
Here's a PDF from a manufacturer
Nov 12, 2008, 10:25 AM
Reduce the drama...
rick.benjamin's Avatar
And a presentation here
Says they are essential for proper operation of Variable frequency drives
Nov 12, 2008, 10:27 AM
Reduce the drama...
rick.benjamin's Avatar
And finally one more article
Nov 12, 2008, 07:10 PM
Registered User
Industrial drives are used on 3 phase usually.The rest are usually AC of some kind.

Ours are DC. That may be the change point. Chopping DC into the 3 phases.

Industry is trying to stop excessive inrush currents. Use a inductor.

DC.... capacitors...... slow down a voltage rise on the cheap drive parts used.
Nov 13, 2008, 03:25 AM
Crash Master
Gene Bond's Avatar
Originally Posted by cyclops2
DC.... capacitors...... slow down a voltage rise on the cheap drive parts used.
Which a series inductor would do, along with the capacitor across the input... An 'L' filter.

In a VFD, most large drives use a DC choke / reactor in series with the bus capacitors to help filter the bus voltage ripple and reduce the reflected voltage notching on the incoming line due to current pulses.
Nov 14, 2008, 06:35 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
The RCGroups links were dead, fixed it.

Prettig weekend Ron
Nov 14, 2008, 09:22 AM
aka: A.Roger Wilfong
gnofliwr's Avatar
Depends on how you look at it. There are different ways to look at how capacitors and inductors work.

Generally, capacitors resist changes in the voltage across them. But they can also be thought of as passing AC current but blocking DC. Used as a filter as across the input to an ESC, the capacitor(s) acts to resist changes in voltage - both up and down.

Inductors resist changes in current flowing through them. In other words they pass DC and block AC. Inductors used in traditional power supplies are used to block AC caused by changes in the input voltage. This is normally ripple, as in a 50/60 Hz mains power supply, or switching spikes from a switch mode supply. The inductor works by storing energy in a magnetic field at high current portions of the cycle and releasing the enery during low current portions. Wires are also inductors (inductors are simply wires wrapped in circles or around ferrous material to increase the magnetic field and interaction between the fields created by adjacent turns. But a straight wire also has inductance. The longer the wire, the greater the inductance.

With our ESCs, the traditional use of inductors as filter is turned on its head. In a traditional power supply, the load is constant relative to the supply. That is the variations in supply voltage are at a much higher frequency than changes in the load. So, the filters (particularly the inductor) are reacting to changes in the input.

With an ESC, the source voltage is constant relative to the load. That is, the load is changing at a much higher frequency than the supply. This causes large changes in current flow through the inductor (the leads). Since inductors resist changes in current, the result is that the voltage at the ESC end of the wires drops as the as the ESC switches a coil in the motor on and the current instantaneously increases. The capacitors can only compensate for so much voltage sag - which doesn't have a negative effect on them, but can cause the electronics in the ESC to reset or glitch. The problem with the capacitors failing occurs when a coil is switched off. The instantaneous drop in current flow causes the magnetic field in the inductor to rapidly collapse, resulting in a voltage spike into the ESC. The capacitor absorbs this spike by passing the current - passing current causes the caps to heat up shortening their life. But if the spike is too large, it can exceed the voltage rating of the capacitor. This can cause the capacitor to internally short. In either case (heat or short) the cap is going to eventually fail.

Adding more caps in parallel distributes the does two things. First it increases the capacitance limiting the change in voltage - reducing the posibility of a short. And, second it distributes the current from the spikes across several capacitors - reducing the heating.

In short - follow the manufacturer's recommendations on battry lead length.

- Roger
Nov 14, 2008, 09:32 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
Excellent Roger, I added a link to your post to my first message, for copy/pasting the text in the future.

Prettig weekend Ron
Nov 14, 2008, 04:26 PM
Registered User
Sabrejock's Avatar
Originally Posted by Ron van Sommeren
This goes for all makes, they all use the same principle (except , they use sinusoidal commutation instead of trapezoid, they tested 70 meters without capacitors).]
Ron: The MaxCim line of motors [unfortunately no longer in production ] did not have any known limitation on battery-side wire length. Whether the fact that they are sensored makes that difference, I don't know. But for this reason, I'll never part with any of mine. Tex.
Nov 15, 2008, 07:08 AM
Crash Master
Gene Bond's Avatar
Well put Roger.

I know we fight the 'distributed inductance' of long line and load leads with AC-sourced VFD's routinely. The typical 1st level solution is to concentrate a large inductance close to the drive, thus limiting the current change... but again, this is for AC. In the DC section, an inductor is used in conjunction with the capacitors to limit ripple currents.

But, of course, AC and DC circuits have different problems and solutions
Nov 15, 2008, 07:16 AM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
Originally Posted by Sabrejock
Ron: The MaxCim line of motors [unfortunately no longer in production ] did not have any known limitation on battery-side wire length. Whether the fact that they are sensored makes that difference, I don't know. But for this reason, I'll never part with any of mine. Tex.
The switching (coils off) causes the problem, this is not influenced by sensored or sensorless.

At WOT, life is much easier on the ESC and the input capacitors, than at mid-range. There's not so much switching (PWM chopping) going on in the ESC.

Prettig weekend Ron

ESC output at WOT

ESC output at partial load
Last edited by Ron van Sommeren; May 27, 2019 at 09:24 AM.
Nov 23, 2008, 01:54 PM
homo ludens modellisticus
Ron van Sommeren's Avatar
Thread OP
Hacker on long battery wires: (English)
-> Safety and operating instructions (German)
-> Sicherheits- und Betriebshinweise

Vriendelijke groeten Ron
Last edited by Ron van Sommeren; May 27, 2019 at 09:23 AM.

Quick Reply

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Capacitor for long wires between BL ESC and battery DeanW Power Systems 14 Oct 03, 2013 02:40 AM
How long is too long for battery wires for Phoenix 60 Xnaron Power Systems 4 May 02, 2011 05:11 PM
Discussion Is there something that can be done to allow long battery wires? Lynxman Power Systems 5 May 17, 2007 02:46 AM
long motor leads, or battery wires? Phreakish Power Systems 19 Sep 21, 2004 08:48 PM
Long battery leads on brushless ESC Keithw Power Systems 3 Mar 01, 2002 07:11 PM