When pressing the brake lever on my ebike the bldc motor controller (aka ebike controller) 'cuts power' to the motor until the brake lever is released again/sensor is deactivated. I would love to know how that works/what happens.

I am wondering what causes the power to be 'cut off' when the sensor is triggered by pressing the brake lever. My theory is that the power (5v +) to the throttle and pedal assist sensor are cut off when one of the brake sensors is activated, this in turn causes the user to be unable to 'apply throttle' .

Perhaps my theory is way off, do any of you know how this works? Does it differ per case/controller ? Or is the method (of cutting off the power when one of the brake sensors is activated) quite universal ?

The controller I'm using: https://a.aliexpress.com/e1R175D8X

My battery: 20s 72v 10parallel enter image description here Thank you very much for any input, it is greatly appreciated!

  • This is a pedal-assist setup? Where the controller decides to run the motor according to an algorithm instead of a user-operated throttle?
    – Criggie
    Commented Jan 11, 2020 at 2:07
  • The bike is currently setup to use a twist throttle. Commented Jan 11, 2020 at 2:09
  • 1
    There are a dozen different way this feature could be implemented. Without detailed wiring diagrams it would only be speculation. Commented Jan 11, 2020 at 2:53
  • @DanielRHicks thank you for the info. I'll try to obtain the wiring diagram Commented Jan 11, 2020 at 2:54
  • It could probably be worked out from a better connection diagram. But a Hall sensor could be used for either cadence detection or brake detection, and what on earth is "self-study"?
    – Chris H
    Commented Jan 11, 2020 at 6:09

2 Answers 2


In your current set up, the electric brake lever sensor will protect against the situation where you're emergency braking but forgot to release the throttle.

The brake sensors override the throttle because there's basically no reason to power the motor while braking.

If your bike was configured to use a pedelec or pedal-assist mode, then the controller will detect when your cranks are turning, and the controller then instructs the motor what power levels to be putting out. There's generally some lag, where you have to do some rotation of the cranks before the motor starts to power you. At the other end, the same crank sensor will take a short time to decide you've stopped pedalling before it drops the motor assist.

Its conceivable that you may be braking hard, and have been riding at 60 RPM on the crank. Your feet may turn another half rotation, and then the motor takes a "missing" half rotation to decide to cut the power. That could be an entire second of time where your motor is powering you forward against the braking force, which is 7 metres at 25 km/h or about 2 car lengths.

The electric brake lever sensor should be telling the controller to STOP RIGHT NOW and override those other inputs.

Additionally, electric brake lever sensors can be used to activate secondary electric brakes and scavenge some power back into the battery. Sometimes this simply uses the motor as a generator, and sometimes there are separate coils.

This is relatively rare on ebikes, though electric trail bikes may have this feature.

Side note - electric brakes aren't legal, and the bike still needs mechanical brakes to be on the road.

  • 3
    That's exactly right, and explains why I find pedal assist such a pain - I've rarely ridden e-bikes and from riding normal bikes tend to time gaps when turning across traffic, and change gear to match. The no-force forward pedalling needed to change gear causes the cadence sensor to turn on the motor in that case, unless you apply the brake. In the lever it uses a magnet and a reed switch on the one I've worked on, with thin wires suited to carrying a control signal, not current. The switches from the two brakes were connected together in a junction box and act as an input to the controller
    – Chris H
    Commented Jan 11, 2020 at 6:07
  • 2
    In some controllers, the brake signal is also applied on the hardware level to e.g. the MOSFET gate controller chip. This makes sure it will stop the motor even if the software is malfunctioning.
    – jpa
    Commented Jan 11, 2020 at 12:01

It is just possible to use a Normally Closed (NC) Single Pole Double Pole (SPDT) relay to open/cut the motor current/voltage when the handbrake lever switch is applied (i.e., when the handbrake lever is used to brake the vehicle/E bike/Escooter.

The relay is, effectively, just used to reverse the standard Normally Open (NO) switch in the usual handlebar handbrake lever to a Normally Closed switch but it, the relay, now OPENS the motor voltage/current flow circuit through the relay when the handbrake is applied.

Be sure to use a relay coil voltage which is the SAME voltage as your E bike/Escooter battery and also ensure the relay's internal switching contacts will carry at LEAST twice (or preferably more!) the current in AMPS that the motor normally uses when it is on and working.

Standard Automotive relays will work fine PROVIDED it is Single Pole Double Throw (SPDT). Double Pole Double Throw (DPDT) relays will work just fine but they are a bit of an overkill for the job that is needed. They just offer an extra set of contacts that will not normally re required.

Hope it helps.... Kind regards, Robert, Amateur Radio, ZL2ROB (QTHR).

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