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Brushless ESC Failsafe?

Old 10-08-2014, 09:30 PM
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CLBetten
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Default Brushless ESC Failsafe?

Are all brushless ESCs with BEC designed with a cut out for the motor that leaves the BEC with enough battery power to at least land?
Old 10-08-2014, 09:42 PM
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Rob2160
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Originally Posted by CLBetten View Post
Are all brushless ESCs with BEC designed with a cut out for the motor that leaves the BEC with enough battery power to at least land?
Most of the good ones will allow you to set the failsafe level, usually as Low, Mid or High. This will reduce the power output to the motor while leaving plenty available for the BEC to power the radio gear allowing you to land.

You can usually set the way the ESC reduces power to the motor also. Eg, immediate cut off or gradually. The latter is best for most applications.

Last edited by Rob2160; 10-08-2014 at 09:45 PM.
Old 10-20-2014, 09:35 PM
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CLBetten
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Hey Rob many thanks. I'm working with my first electric plane although I've been flying glow for nearly 20 years. My next question: Is it best for battery longevity to habitually fly to the cut off point then store the battery at that state of charge until the next flying session?
Old 10-21-2014, 12:08 AM
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Originally Posted by CLBetten View Post
Hey Rob many thanks. I'm working with my first electric plane although I've been flying glow for nearly 20 years. My next question: Is it best for battery longevity to habitually fly to the cut off point then store the battery at that state of charge until the next flying session?
It depends on the setting you have chosen for the low voltage cut off in the ESC. Generally if you let it fly until the low voltage cutoff that will take too much out the LiPo.

I usually fly my electrics to a timer and that time is calculated after a bit of trial and error for each model.

For example, I work on not discharging more than 80% of the battery (leaving 20% capacity remaining).

This is very easy on the batteries and they will last years if charged and stored properly.

My chargers will show how much is being put back into the batteries and from this I know if I am flying too long (or can fly longer)

EG. With a new model I will fly for a few minutes, note the time then recharge and assess how much charge was required. From there you can increase flight times until you are using about 80% of the battery's capacity and set my radio timer accordingly. I always try to land when the timer goes off.

Others may have better techniques but that works for me.

Last edited by Rob2160; 10-21-2014 at 09:04 AM.
Old 10-21-2014, 08:07 AM
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CLBetten
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So basically you believe about 20 percent capacity remaining is best for both cut off and storage?
Old 10-21-2014, 09:00 AM
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Originally Posted by CLBetten View Post
So basically you believe about 20 percent capacity remaining is best for both cut off and storage?
Yes, that is going to work. Recommended storage voltage is 3.8 volts per cell, and that equates to about 20% capacity so you're all good.

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Last edited by Rob2160; 10-21-2014 at 09:03 AM.
Old 10-21-2014, 09:54 AM
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CLBetten
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Ok, you lost me I think it's a type error. The cells are only rated for 3.7 volts so 3.8 must not be 20%. Do you mean 1.8 volts or .8?

Last edited by CLBetten; 10-21-2014 at 09:57 AM.
Old 10-21-2014, 03:31 PM
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Fully charged lipo cells are 4.2V.
3.7V is the nominal voltage, and there's little useable power left at that point. The chart Rob2160 provided shows that clearly. Just check the left hand column (1S) for the single cell voltages and percentage of charge remaining.
Pete
Old 10-21-2014, 04:59 PM
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CLBetten
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Ok thanks. I get it. I struggled to enlarge the chart with my phone. I didn't realize the voltage rating was so different from what I'm used to.

Last edited by CLBetten; 10-21-2014 at 05:04 PM.
Old 10-22-2014, 08:17 AM
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When talking about LVC we have to recognize that there is voltage under load and there is resting voltage. Your LVC is set to your voltage under load, what I call the voltage "sag", not resting voltage.

The chart presented above is resting voltage. It suggests that at 3.7V per cell you are down to 10% capacity on the lipo. I don't know the source but that is not what your LVC is responding to.

Your ESC's LVC is more likely set for something between 3.0 and 3.3V per cell under load. If you are running a 3 cell pack and your LVC is set for 3.3V then at 9.9V, under load, the LVC will cut or pulse the motor. When the battery is disconnected and put on a volt meter it will read much higher.

What the LVC sees also depends on how much load. If you go to full throttle your motor may cut out based on the LVC reading of the battery as "sagging" down below the LVC voltage. But at half throttle you may be able to run for some additional time as the load on the motor is not as high so the voltage will not "sag" as much.

A fully charged battery will sag less than one that is nearly empty. And a battery that is running at or close to its rated C limit will sag more than one that is running no where near its rated C limit. This means that a 1300 mah 50C rated battery may actually give you more flight time than a 1300 20C rated battery if you are running your motor at 26 amps. 26 amps is right at the limit of the 20C rated pack so the voltage will sag a lot and sooner. 26 amps is no where near the rated limit of the 50C pack and so that pack will likely sag less under load giving you more useable capacity out of the same 1300 mah rated pack.

Just things to know. You can read more about this in the book below.

EVERYTHING YOU WANTED TO KNOW ABOUT ELECTRIC POWERED FLIGHT

http://www.rcuniverse.com/forum/m_7100376/tm.htm

Last edited by aeajr; 10-22-2014 at 08:23 AM.

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