Sometimes, it's so much better when you can see a video of what is happening.
After watching your video, it appears to be a battery problem
(or to be more specific, its BMS) as the battery voltage is fluctuating so smoothly
and regularly.
I have checked the specifications for the AfterPartz 36v 10.4ah batteries and here is what I have discovered:
Specification :
Cell Brand ?Samsung ICR18650-26F With Underwriters Laboratories Certification
Nominal voltage: 36V (Mind the voltage of your controller?
Really Rated capacity: 10.4Ah
Operating temperature: -20 °C to 60 °C
Charge temperature: 0 °C to 45 °C
Storage temperature: -20 °C to 45 °C
Operating current: 10A
Peak current: 15A
They produce three styles of battery packs
(Fish, Flasche and Mini DIY) but all three have the same cells and current specification.
I don't know whether you have changed the current settings in the controller, but normally the
Main Battery Current (A) is set to
25A by default. This means that the current being drawn by the MP5 is nearly 70% more than the battery's peak current, and 150% more than its continuous current rating.
Theoretically, this should not have caused a problem with the battery, as they also clearly state that it has "
Over current protection" which should automatically limit the current to prevent the battery from being overloaded.
Obviously, the battery is not able to supply a Magic Pie with the required current, but what I don't understand is why it has only just developed this pulsing problem after a month of use.
Perhaps the over current protection is simply not robust enough for intensive use?
Also, the wheel spinning in the video should be drawing a lot less than 10 Amps as there is no load on it, so it should not be triggering the current protection function of the battery's
BMS, which in my opinion, is producing the pulsing effect.
I am now convinced that this is a battery/
BMS fault and definitely
not a motor or controller issue, as the battery voltage was noticeably sagging while the motor rpm decreased
(with no load applied) which is the exact opposite of what would happen if the throttle was held in the maximum position and the controller was regulating the power/speed of the motor.
I am not sure if the battery's
BMS has simply failed (greatly reduced current output) because it has been working too hard over the last month, or whether the regen current may also have had a detrimental affect upon it.
Either way, I looks like you now need to find a more suitable battery as a replacement.
Alan 
