E-bike controllers
The controller is the brain of an e-bike, it switches the motor phases and decides how much current it lets through. It has to match the battery on voltage and, on current, whatever the battery and motor can take, otherwise the BMS protection trips or something burns out. Phase and Hall wires are joined by colour, and if the machine judders or runs backwards once connected, the combination is wrong. A controller of 3000 to 5000 W turns the machine into a vehicle rather than a bicycle, because the limit is 250 W.
E-bike controllers
The controller, or control unit, sits between the battery and the motor and decides everything that happens between them. On the signal from the throttle or the pedal sensor it switches the motor phases and meters the current, watches the battery for undervoltage and cuts the drive when you pull the brake. Units are sold for the 48 to 84 V range with a current around 80 A and a power of 3000 to 5000 W, that is parts for heavy conversions and electric motorcycles, not for a city bike. The label dual can mean two outputs for two motors or two control modes, and the listing usually does not tell them apart, so ask the seller before you buy.
First the voltage and current have to match, because a controller produces no energy of its own. The 48 to 84 V range means the unit handles packs from thirteen to twenty cells in series, but the current you choose has to come out of the battery as well. Eighty amps from a 48 V and 20 Ah battery is four times its capacity per hour, which most packs of 18650 cells will not sustain and the BMS cuts the drive. Before buying, find out how many amps the BMS protection in the battery is rated for, and choose the controller by that, not by the highest number in the listing.
Phases and Hall sensors are three power wires and five signal wires, and their order decides whether the motor turns the right way. Colours on Chinese units usually match, but not always, so after connecting lift the bike first and try spinning the rear wheel free. When the machine judders, hums or turns backwards, you swap two phase wires, or two Hall wires if that fails, and keep trying. There are thirty-six combinations in all, but the right one usually turns up by the third attempt.
Cooling and placement are what gets underestimated most on an eighty-amp unit. The aluminium case is also the heatsink, so the controller is mounted against the metal of the frame or in the airflow, not wrapped in a bag between the battery and the tools. An overheated unit first limits the current and the ride starts surging, and only after that does the output stage go. Route the wires so they do not rub on the frame and so the connectors point downwards, because water gets into them from above.
A controller of 3000 to 5000 W does not turn the machine into a bicycle, rather the opposite. In the European Union a bicycle is only a machine up to 250 W of continuous rated power with assistance up to 25 km/h, so with a unit like this you are dealing with a vehicle, one for private land or for registration. Fitting a stronger controller also voids the type approval on a bike that had one, and with it the insurance cover.