# What does an e-bike controller do? Source: https://pedalandvolt.co.uk/blog/ebike-controller-explained Published: 2026-10-12 Publisher: Pedal and Volt **In short:** The controller is the e-bike's brain. It reads signals from the pedal sensor, brake sensors, speed sensor and display, then decides how much current to send from the battery to the motor. It enforces limits such as the 15.5 mph assist cut-off and the maximum current. On a UK-legal e-bike these settings keep the bike within EAPC rules, so they should not be changed to add speed or power. Every e-bike has a controller, though you rarely see it. On factory e-bikes it is often built into the motor or hidden in the frame. On conversion kits it is a separate box with a bundle of cables. Understanding what it does helps when choosing a kit, diagnosing faults and judging listings that promise more power. This explainer covers the controller's inputs and outputs, the key numbers on its label and how its settings relate to UK law. ## Inputs and outputs The controller combines these signals many times a second. When you pedal and are below the assist limit with brakes released, it sends current to the motor according to the assist level. When any condition fails, it stops. | Connection | What it does | | --- | --- | | Battery | Supplies power, at the pack voltage such as 36 V or 48 V | | Motor phase wires | Carry current to the motor windings | | Motor hall sensors | Tell the controller the motor's position, on many hub motors | | Pedal sensor | Cadence or torque signal that you are pedalling | | Brake sensors | Cut motor power when you brake | | Speed sensor | Wheel speed, used for the assist limit | | Display | Assist level, settings and error codes | | Lights | Some controllers power front and rear lights | What connects to a controller ## Voltage and current ratings A controller label usually shows a rated voltage, such as 36 V, and a current limit in amps, such as 15 A. The voltage must match the battery. The current limit, together with voltage, sets the maximum electrical power the motor can draw at peak. That is different from the motor's continuous rated power, which is what UK law uses. A UK-legal motor is rated at 250 W continuous. It can draw more power briefly at peak, for example pulling away on a hill, which is normal. A controller with a very high current limit paired with a 250 W motor may overheat the motor, and listings that sell 1000 W kits are not EAPC compliant. Our [36 V vs 48 V explainer](https://pedalandvolt.co.uk/blog/36v-vs-48v-ebike-battery) covers voltage in more detail. ## Controller settings and UK law Many controllers, especially on kits, have programmable settings for the speed limit, current, wheel size, sensor type and throttle. To stay within the EAPC rules in Great Britain, the assist must cut out at 15.5 mph (25 km/h), the motor must be rated at 250 W and power must only flow while you pedal, apart from walk assist up to 6 km/h. These rules are summarised on [gov.uk](https://www.gov.uk/electric-bike-rules). Raising the speed limit, enabling a throttle beyond walk assist or fitting a higher-power motor turns the bike into a motor vehicle in law. That needs type approval, registration, tax, insurance and a licence. See [are conversion kits legal in the UK](https://pedalandvolt.co.uk/blog/are-ebike-conversion-kits-legal-uk) and our [throttle rules guide](https://pedalandvolt.co.uk/blog/ebike-throttle-rules-uk). This is general information, not legal advice. [Check a bike against EAPC rules](https://pedalandvolt.co.uk/tools/uk-ebike-law-check) ## Common controller faults Start with the checks in our [pedal assist not working guide](https://pedalandvolt.co.uk/blog/ebike-pedal-assist-not-working) and [motor cutting out guide](https://pedalandvolt.co.uk/blog/ebike-motor-cutting-out). If the controller is at fault, replace it with one designed for your motor and battery voltage, set up for 250 W and 15.5 mph. - Error codes on the display pointing to the motor, sensors or communication. - No assist although the display works, often a sensor or brake cut-off signal. - Motor stuttering, which can be a hall sensor or phase wire problem. - Power cutting out on hills, from current limits, low battery voltage or overheating. - Water damage in the controller box after riding in heavy rain. Note: Controllers carry high currents from a lithium battery. Disconnect the battery before touching wiring, never open a controller that is still connected, and have a shop handle repairs if you are not confident with electrics. ## Choosing a kit or bike with the right controller When buying a conversion kit, check that the controller voltage matches the battery, that it is supplied set to 25 km/h, that brake sensors are included, and that there is no throttle beyond walk assist. Waterproof connectors and a mounting position away from spray help reliability. Our [front vs rear hub kit comparison](https://pedalandvolt.co.uk/blog/front-vs-rear-hub-conversion-kit) covers motor choices. On factory e-bikes, branded systems keep controller settings locked and diagnosable by dealers, which helps with legality and servicing. Our [electric hybrid guide](https://pedalandvolt.co.uk/best/best-electric-hybrid-bikes) compares road-legal complete bikes, and our [replacement battery guide](https://pedalandvolt.co.uk/best/best-ebike-replacement-batteries) explains how to match voltage when replacing a pack. ## Frequently asked questions **Where is the controller on an e-bike?** Inside the motor or frame on many factory bikes, or in a separate box on the frame or rack on kits. **Can I change my e-bike controller settings?** Display settings like units are fine. Changing speed or power limits can make the bike illegal on roads. **What size controller do I need for a 250 W motor?** One designed for that motor and battery voltage, as supplied in the kit. Ask the kit maker. **Can a bad controller stop the motor working?** Yes. It can cause no assist, cutting out or error codes. ## Buying guides on this topic - https://pedalandvolt.co.uk/best/best-electric-hybrid-bikes - https://pedalandvolt.co.uk/best/best-ebike-replacement-batteries - https://pedalandvolt.co.uk/best/best-electric-bikes-under-1000 ## Sources - [GOV.UK: Riding an electric bike](https://www.gov.uk/electric-bike-rules)