Motor error or battery problem? How to tell the difference on your e-bike
You hop on your electric bike, ready for your commute or a ride in the countryside. You start pedaling, the motor seems to kick in for a moment, then... silence. Or the assistance suddenly disappears as soon as you start climbing a hill. The frustration is great, but the confusion is often even greater. Is the battery dead? Is the motor burned out? Or is it just a loose connector?
Diagnosing problems with an e-bike can be complex, as the symptoms of a faulty battery and those of a malfunctioning motor or controller are often similar. In this article, we delve into the technical aspects to help you tell the difference. We cover voltage drops, BMS, mechanical noises, and faulty connections.
Written by: Hanna Stevens | January 30, 2025 | Reading time: 5 minutes
More about the author: Hanna Stevens
Hanna is an avid cyclist who loves exploring new routes and sharing her experiences to inspire others to get on their bikes more often.
Key findings
- Most "engine problems" are actually battery problems.
- Noises and resistance almost always point to the engine.
- A systematic approach avoids unnecessary costly replacements.
The battery
Nearly 70 to 80% of electric bike problems that resemble engine failure originate in the battery or its management. Batteries are chemical factories that degrade over time.
The phenomenon of "voltage sag" (voltage drop)
A common scenario: the bike starts up perfectly and the screen shows that the battery is 100% charged. However, as soon as you start pedaling or riding uphill (when the motor is under load), the system immediately shuts down or the indicator suddenly drops to 0% or 50%.
This rarely indicates an engine failure. It is a classic battery problem. When stationary (without load), a battery may display a perfect voltage, for example 52 V or 48 V. But if the pack contains aging or defective cells, they can no longer supply the required current (in amps). As a result, the voltage drops as soon as you demand power. The battery management system (BMS) then intervenes and cuts off the power supply to prevent damage.
The BMS
Every modern lithium-ion battery is equipped with a BMS. This system protects the cells from overcharging, excessive discharge, and overheating.
The BMS failure
Sometimes the battery itself is still fine, but the BMS is faulty or "out of sync." If it mistakenly believes that a group of cells is dangerously low, it cuts power to the motor.
Symptoms
The battery no longer recharges, or the bike switches off even though it has just been removed from the charger. Sometimes, resetting the battery (leaving it connected to the charger for a long time) can help to rebalance the cells.
Physical battery signals
Carefully examine the case. Is the battery swollen or deformed? If so, stop using it immediately. This indicates internal gas formation due to chemical degradation and poses a serious fire hazard. Similarly, corrosion on the contacts (the pins connecting the battery to the bike) can cause poor contact and intermittent power supply.
The engine
Although motors are generally more robust than batteries, they can also develop problems. Motor failures often manifest themselves differently: they are more often audible or noticeable.
Unusual noises and resistance
A battery usually dies silently. A motor, on the other hand, often makes itself heard. Do you hear a rubbing, rattling, or buzzing noise while riding? This could indicate worn bearings, damaged internal gears (on geared hub motors), or loose internal wiring.
A motor problem can also create physical resistance. If the motor runs heavily, even when the assistance is turned off, this may indicate resistance in the bearings or a short circuit in the motor phases.
Hall sensors and phase cables
The motor communicates with the controller via sensors (often Hall sensors). If one of these sensors is defective, the controller no longer knows the position of the motor.
Symptom
The engine stalls when starting, makes a strange noise without providing power, or does nothing at all while the screen remains on. This differs from a battery problem, where the entire system (including the screen) often shuts down due to lack of power.
Overheating
Does the motor housing become extremely hot after a ride? This may mean that the motor is being overworked (for example, a motor that is too weak for a heavy cyclist in mountainous terrain) or that there is internal friction. If a motor overheats regularly, the insulation on the copper windings may melt, causing permanent damage.
The controller and connections
Often, the motor or battery is blamed when the problem actually lies with the controller or the connections. The controller determines the amount of current sent from the battery to the motor.
The role of the controller
If your bike turns on but the motor does not respond at all to pedaling or the throttle, the controller may be defective. A faulty controller may sometimes give off a burning plastic smell. It may also go into safety mode if the current demand (in amps) exceeds what the controller or BMS can handle.
Connectors and wiring
Vibrations while riding can loosen connectors. A poorly connected plug between the motor and controller can cause intermittent assistance.
Verification
Follow the cable coming out of the motor. Is the connector properly inserted? Are the pins clean, undamaged, and free of greenish corrosion marks caused by moisture?
Screen contacts
On some brands (such as Gazelle), a removable screen may make poor contact due to corrosion or moisture. If the screen does not transmit any signal, the motor will not work.
Overview of symptoms and causes
| Symptom | Probable cause: BATTERY | Probable cause: ENGINE / OTHER |
|---|---|---|
| The bike cuts out under load (climbing, starting). | Voltage sag. Weakened cell group or BMS intervention due to excessive current demand. | Overheating controller that activates safety mode and cuts off assistance. |
| The system does not turn on (no screen, no light). | Deeply discharged battery (BMS in standby mode)or blown battery fuse. | Screen malfunction or cable to the screen cut (the system is not receiving the "ON" signal). |
| Screen on, but no motor assistance. | Rarely the battery (unless it is almost empty). | Defective pedaling sensor (PAS),blocked brake sensor,broken cable to the motor. |
| Abnormal noises (squeaking, rattling). | No battery-related causes. | Mechanical defect: worn bearings,damaged gearsor magnets in the motor. |
| Reduced battery life (battery drains quickly). | Degradation: aging cells that have lost capacity. | Excessive resistance: brakes that rub, tires soft, ordefective engine bearings that consume energy. |
| The battery does not charge. | Defective charger, damaged charging port,or BMS blocking charging due to imbalance. | N/A |
| The engine is misfiring (jerky operation while driving). | Poor contact in the battery holder(battery moves around). | Faulty Hall sensor in the motoror poor phase connection to the controller. |
Step-by-step diagnostic plan
Want to try it yourself? Follow these steps carefully. Lithium batteries contain a lot of energy.
- The "wiggle" test: Turn on the bike and gently move all cables and connectors, especially around the motor and handlebars. If the motor cuts out or the screen flashes, you have found a broken cable or a loose connection.
- Voltage measurement: If you are comfortable using a multimeter, measure the battery voltage at the contacts (where it connects to the bike). A fully charged "48V" battery should be around 54.6V. If you measure much less (e.g., 40V) immediately after charging, a group of cells is likely defective.
- Check the charger: Does the charger indicator light come on? If possible, try another identical charger. Sometimes the charger is defective and the battery is simply empty.
- Inspect the battery mount: On bikes where the battery snaps into a rail (like many batteries on luggage racks), some play may develop. A small piece of foam can help keep the battery securely in place and prevent vibration while riding.
- Error codes: Look at the screen. Does it display an error code? Consult the manual. Codes are often specific (for example, "Error 30" often relates to communication, "Error 03" to the motor's Hall sensors).
Conclusion
The difference between an engine failure and a battery problem often lies in when the problem occurs. Does the bike shut down during intense effort? First, check the battery (cell condition). Do you hear noises or feel abnormal resistance? Focus on the motor. Does the bike do absolutely nothing even though everything seems to be connected properly? Check the display and controller.
If in doubt, or if you notice physical damage to the battery (swelling, burning smell), do not experiment yourself. Lithium battery fires are extremely dangerous. In case of serious battery or motor problems, always consult a specialist or the manufacturer for safe repair or replacement.
By analyzing the symptoms in a structured manner, you save time and avoid replacing expensive components that are not actually defective. Good luck with the diagnosis!
FAQ
How can I tell if my e-bike battery is worn out?
If it appears full but collapses immediately under load, this is a strong indication.
Can an engine break down without making any noise?
It's possible, but rare. Usually, you hear or feel something.
Is a drop in blood pressure dangerous?
Not directly: the BMS protects it. But it means that the battery is at the end of its life.
Can a poorly connected screen shut down the engine?
Yes. Without a signal, the system assumes you are not driving.
Is repairing always cheaper than replacing?
For batteries, sometimes yes. For engines, it depends greatly on the extent of the damage.