Why are some ESC's only rated to 3s or 4s Lipos ?

Here is a bench test I have done to find out if a 2s to 4s Afro ESC's can run on 6s.

ESC’s  and Label Specs

Afro Slim ESC 20amp 2- 4s battery afro_nfet.hex firmware

Afro Bec ESC 30amp 2- 4s battery afro_nfet.hex firmware

Afro HV ESC 20amp 2- 8s battery afro_nfet.hex firmware (control)

ESC component specs taken from the manufactures data sheets.

Afro Slim 20amp 2-4s:

5v MCU supply regulator LM78L05 Max input 30v Max load 100ma

NFets FDMS8018 Max Voltage 30v, Continuous Drain @ Ta=25°C = 30amps

BC817-40W 6CW max Voltage 45v 500ma

All components are rated up to 150°C

Afro bec 30amp 2-4s:

5v MCU supply regulator LM78L05 Max input 30v Max load 100ma

NFets FDMS8018 Max Voltage 30v, Continuous Drain @ Ta=25°C = 30amps

BC817-40W 6CW max Voltage 45v 500ma

All components are rated up to 150°C

Afro HV 20amp 2-8s:

5v MCU supply regulator LM78L05 Max input 30v Max load 100ma

(But this LM78L05 is fed from two DAR transistors that take the HV voltage and step it down to 12v, their Max voltage is 40v) this is why I think the Afro HV was de-rated from 12s to 8s.

NFets FDMS86540 Max Voltage 60v, Continuous Drain @ Ta=25°C = 20amps

 (If the 5v MCU supply was by-passed and you fed the MCU with 5v from another source the Nfets are rated at 60v) So I would assume the ESC will run at 12s with a new 5v supply for the MCU.

All components are rated up to 150°C 

Test equipment:

Power supply 60amp 24v power station

Amp and Watt meter Turnigy 130A

Tarot 4114 motor with 15 x 5.5 prop fitted

Test meter with temperature probe

Motor stand with weight scale attached for thrust readings

Servo tester for PMW control 



Apply a constant 24v supply to a power meter then to an ESC to measure amps drawn with the esc connected to a standard motor and propeller running at 50 % PWM throttle controlled by a servo tester and run for 10mins whist monitoring temperature, amps, watts and thrust.   


ESC                                                         Amps Draw                         Watts                    Temp                   Thrust 

1st Afro slim 20A                                5.65                                        133                         47.1C                     1114g

2nd Afro BEC 30A                              5.68                                        134                         53.8C                     1160g

3rd Afro HV 20A                                 5.3                                          125                         46.0C                     1111g


It Appears as if the Afro Slim 20amp and the Afro Bec 30amp ESC’s are capable of handling the use of a 6s battery as a power source, according the specs of the components provided by the chip manufacturer,  the temperature of the components was very similar and the amps drawn the same. So if you wanted to hide the ESC’s in the arms of the Multi-rotor (provided cooling holes for air was allowed for) then this will be fine and they will run on 6s power.



Slim ESC wiring was upgraded to suit a higher amp draw and the use of 6s batteries, (although the original wiring fitting is 20amp rated so should be fine for most Multi-rotors) the other component’s on the boards were not investigated, so no information on them is known.

Use this info at your own risk.

Video https://youtu.be/91rc37c5YBQ





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Comment by Paul Bealing on January 31, 2016 at 8:35pm

I have used mostly Afro ESCs and not had any failures I can remember. I (almost) always re-work to improve the heatsink contact and solder wiring direct to the board, removing all connectors. Often the heat sink is not evenly contacting all fets.

One ESC failure I recall from ages ago was a test where the quad dropped from quite high with motors off and restarted for a landing. I suspect the free-wheeling generator effect killed the ESC; probably over-voltage.

It has been my experience that over-voltage easily kills mosfets, but with good heat sinking they are quite robust to brief periods of over-current. Capacitor voltage ratings could be stressed with more cells. I also wonder if all 7805 regulators are made to the same spec. I replaced some mosfets in a gimbal controller late last year and there seemed to be quite a performance difference between the originals and replacements although all had the same part number. It has worked since, so I didn't investigate.

ESC failure possibility does suggest that some sort of fuse between the battery and ESC may be worth considering, at least in fixed-wing where reliability is important and loosing an ESC is recoverable.

Comment by Charles Blouin on February 1, 2016 at 7:51am

Thanks for the test!

Comment by Rustom Jehangir on February 2, 2016 at 9:29am

Peter is right about the LDO power dissipation, but when nothing is connected to the BEC, it only has to provide about 30-40 mA to the microcontroller and other circuitry. On 4s, that's (14.8-5)*0.040 = 0.4 Watts. That's not a big deal.

However, if you run the BEC at the published output of 500mA, then it's (14.8-5)*0.5 = 5 Watts, which will get very very hot. As you push the voltage up, it will get even hotter.

The other thing that provides a limit is the MOSFET "absolute maximum Vgs" which is the maximum voltage between the gate and source. Most FETs (including the SiR158DPs used on the 30A Afros) have a maximum Vgs of +/-20V.

Most ESCs under 40 amps use charge pumps to produce a voltage high enough to turn on the high-side MOSFETs. The charge pump effectively doubles the battery voltage so that Vgs is (2*Vbat - Vbat = Vbat), so, if your battery voltage is over 20V, then Vgs is also over 20V, going over the maximum rating. (See here for the schematic of the BlueESC, which is very similar to the AfroESCs)

That said, I've also taken the Afros well above 20V and they work great. I'm not an EE, so I don't understand what is supposed to happen when you reach Vgs max. Perhaps someone else here can speak to that.


Comment by Alex B on November 9, 2016 at 12:49am

Can you configure those ESCs for reverse (3D or car mode), it think they will burn on 6s..

Comment by Reuben on November 9, 2016 at 2:00pm

No don't think they can be configured for reverse during flight ... But you could add a crossover switch to two of the output wires from the esc to the motor then program to a switch on your transmitter, which you then flip on and off to reverse the motor.

Comment by Alex B on November 9, 2016 at 2:11pm

you can enable reverse in ESC and tell flight controller to use it. In that case PWM 1500 means stop anything above that spins in one direction, anything lower spins the opposite. 

That mode is used in acro tricks with quads. 

I think going from full throttle one way to full throttle the opposite can fry ESCs unless FETs are rated for at least 2x of battery voltage


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