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Author Topic: Extending Battery Leads to a Brushless ESC (why dangerous)  (Read 2494 times)

Paul R

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Extending Battery Leads to a Brushless ESC (why dangerous)
« on: November 24, 2013, 10:15:07 PM »

Hi,
Why is it bad practice to lenghten the pos/neg leads from your lipo batteries to the ESC if you have to.
I keep reading posts in relation to this on other forums that say 'add extra capacitors' and 'dont extend li-po batt leads' as it could kill your esc, but nobody has said or explained why?, why cant you extend batt leads to your brushless ESC, if you do what actually happens. Can someone who is qualified in electrical theory /practice explain why this is a bad idea? 
 
 
 
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nick_75au

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Re: Extending Battery Leads to a Brushless ESC (why dangerous)
« Reply #1 on: November 25, 2013, 10:37:17 AM »

No electrical engineer but the longer leads have higher inductance, as the ESC generates some ripple IE waveform in the leads, the higher inductance causes higher resistance, what happens is the ESC then sees voltage spikes and dips which is what causes the failures. the capacitors "fill in"these variations to reduce the peaks and troughs .


Unless your drawing maximum currents from your batteries this issue is rarely a problem for most general applications. peak performance racing is a different matter.


Its usually OK to extent the motor leads to a certain extent with less effect but your still introducing extra resistance, again only an issue where peak performance is required


Nick
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malcolmfrary

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Re: Extending Battery Leads to a Brushless ESC (why dangerous)
« Reply #2 on: November 25, 2013, 10:49:52 AM »

What nick said - plus, if you are using sensorless motors, the "unused" lead(s) become the sensor.  Longer leads mean more transmission from the lead(s) powering up or down, and more unwanted pickup on the rest.  Depending on design, this is rarely a good thing.
On the battery side of things, you have to remember that resistance increases with length, a longer power wire also needs to be thicker to get the resistance back down where it is needed.  At high currents, the voltage dropped by the line might be difficult to measure using a meter.  The internal circuits of the electronic bits, depending as they do on getting precise timing information from a rising voltage and comparing it with a reference derived from the supply, can and do notice these minute variations.  The provision of a bank of high value capacitors at the power input of the ESC mitigates this in a simple installation.  In a more complex installation, the variation in the voltage of the ground line will have all sorts of interesting side effects.
Then theres the question of how good the joints are.
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Paul R

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Re: Extending Battery Leads to a Brushless ESC (why dangerous)
« Reply #3 on: November 25, 2013, 04:36:16 PM »

Hi,
Thanks for those replies Nick, Malcolm, i now know a bit more about this problem. So essentially extending the batt leads introduces more resistance and creates more inductance within the leads which can create voltage spikes, ok. So for example in a fast electric cat, say 100amp esc continious current, 12awg batt leads series connected (2 x 3s 25v) 4mm bullet connectors and I then extend the batt leads by 15cm, would this really bump up the resistance and cause a problem for the esc?
Thanks
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Norseman

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Re: Extending Battery Leads to a Brushless ESC (why dangerous)
« Reply #4 on: November 25, 2013, 04:40:38 PM »

I just came across this page and remembered your recent question
https://astecmodels.co.uk/brushless-motors.php
Might help?

Dave
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malcolmfrary

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Re: Extending Battery Leads to a Brushless ESC (why dangerous)
« Reply #5 on: November 25, 2013, 05:03:20 PM »

Hi,
Thanks for those replies Nick, Malcolm, i now know a bit more about this problem. So essentially extending the batt leads introduces more resistance and creates more inductance within the leads which can create voltage spikes, ok. So for example in a fast electric cat, say 100amp esc continious current, 12awg batt leads series connected (2 x 3s 25v) 4mm bullet connectors and I then extend the batt leads by 15cm, would this really bump up the resistance and cause a problem for the esc?
Thanks
The voltage spikes will be there anyway - the longer leads just give the resulting magnetic field more ground to work from.  In the presence of a varying field, a longer wire does a better job of picking up unwanted signal.  Since ESCs work by switching current on and off very rapidly, pulses of current produce large varying fields.
When talking about the current carrying ability of wire it is good to remember that there is a value of current that will melt the wire (as in a fuse) and a much lower level that does not result in an unacceptable voltage drop over the length.  Referring to Norseman's link, scroll down to "do's and don'ts", in particular the bit about longer wires.  12AWG sounds good, extra decoupling capacitors should do no harm.  3 of 220ɥF, of appropriate rating, in parallel.
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More Coffee

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Re: Extending Battery Leads to a Brushless ESC (why dangerous)
« Reply #6 on: November 25, 2013, 09:32:03 PM »

I'll bite.
Theres been no mention of twisted pair's or trio's..twisting the conductors, or if your disparate ,using aluminum foil-tape to shield each wire.
Generally, to eliminate transients ,the capacitors make a huge difference...twisting of the conductor's , wrapping each one in aluminum foil tape. just general stuff a guy can do.
 
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