Components

What parts make up an e-bike system?

Five things have to talk to each other: a battery, a motor, a controller, a display and the wiring between them. The controller reads the pedal sensor or throttle and decides how much current to send the motor; the display sets the assistance level and shows battery state. Mid-drive motors like the Bosch Performance Line and Shimano STEPS systems drive through the chainring and use the bike's gears, so they climb well but wear the chain and cassette faster. Hub motors drive the wheel directly, are simpler and cheaper, and are what most conversion kits use.

Will a component from one system fit another?

Usually not. Proprietary systems from Bosch, Shimano, Yamaha and Brose use closed connectors and firmware, so a display or charger from one is unlikely to work with another even where the plug appears to fit. Open-standard parts built around a generic controller are far more interchangeable, but you still need to match voltage — a 36V battery on a 48V controller will not deliver the power, and a 48V battery on a 36V controller can destroy it. Match the connector type, the voltage and the system before buying anything.

Is it safe to buy a used e-bike battery?

This is the part to be most careful with. Lithium cells lose capacity with every charge cycle and with age regardless of use, so a five-year-old pack that has sat discharged may hold half its stated range even if it looks unmarked. Ask for the cycle count if the system reports one, ask how it has been stored, and treat any pack with a dented, cracked or swollen case as scrap — impact damage to cells is a fire risk and is not repairable at home. Check the mount and the contacts for corrosion, and check the key lock works, since a replacement mount often is not sold separately. If a seller cannot say how old a battery is, assume the worst.

What wears out on motors and controllers?

Hub motors fail at the bearings and, on geared hubs, at the nylon planetary gears — spin the wheel by hand and listen for grinding or a rhythmic click. Mid-drives develop play at the bottom bracket bearing and rattle from a worn drive gear. On any motor, inspect where the cable enters the axle or casing, as that is where water gets in and where the wire fatigues. Check display screens for water ingress behind the glass and for delaminated buttons. Controllers rarely wear out but do fail from moisture; look for corrosion on the pins of every connector, which is by far the most common fault on a used loom.

What is a fair price, and when should I buy new?

Motors, displays and controllers hold value reasonably and are sensible second-hand buys if they are the matching part for your system. Batteries are the exception — a used pack is worth substantially less than its age suggests because you are buying an unknown number of remaining cycles, and a new battery from the original system maker is often close to half the value of the whole bike. Buy new for batteries and chargers, where a mismatched or damaged item is a fire risk rather than an inconvenience, and buy used for mounts, wiring, switches, throttles and mechanical motor parts. If you are pricing a repair against a whole bike, compare with complete electric bikes before committing.