While riding (or charging), the whole bike goes dead: dash off, no drive. After a short wait or a restart it may come back. This is usually the BMS opening the battery's output to protect the cells. The BMS prevents over-charging, over-discharging and overheating (Microchip), and UN Regulation 136 tests battery protection against over-discharge, over-temperature, over-current and short circuit. So a cut-off means the BMS saw something it did not like: a cell voltage too low under load, too much current, too high or too low a temperature, or a fault. A loose swap battery can look the same.
Is it safe to ride?
Do not ride until you know the cause. Cutting out in traffic is dangerous. If the battery is hot, hissing or smells, stop and see Battery pack is hot, hissing or smells.
Quick checks first
- What was the SoC when it cut? Was it low?
- Hard acceleration, steep hill or heavy load at the moment it cut?
- Hot day after a long climb, or a cold morning?
- Bump or pothole at the moment it cut (swap battery loose)?
- Battery hot, smelling or damaged?
Diagnose it step by step
1Is the battery hot, swollen, smelling or hissing?
- Yes: stop. Handle a damaged battery pack.
- No: go to step 2.
2Did it cut on a bump, and is it a swap battery?
- Yes: Check swap battery contacts and latch.
- No: go to step 3.
3Was the SoC low, and did it cut under hard load?
- Yes: under-voltage protection. Charge earlier and ride gently when low. If this happens at a higher SoC than before, the battery is ageing or has a weak cell group: go to step 6.
- No: go to step 4.
4Was the battery very hot (long climb, hot day, charging straight after riding) or very cold?
- Yes: temperature protection. Let the battery reach normal temperature. Cold cells sag more under load (Battery University). See Check cooling of the motor and controller and Poor range or slow charging in the cold.
- No: go to step 5.
5. Is there a code? Read electric drive error codes.
- Over-current or short-circuit code: something in the controller, motor or wiring draws too much. Workshop. Do not keep restarting.
- Cell or BMS code: go to step 6.
- No code: go to step 6.
6. Check the battery's record: Check battery state and health. Hand the bike or battery to the dealer or fleet battery team with your notes (SoC, load, temperature, time). On swap networks, report the battery ID and swap it.
What happens if you keep restarting after repeated cut-offs: you force a battery that is protecting itself, and you may be cut off again in a dangerous place.
Causes, most likely first
| Cause | How to confirm | Fix |
|---|---|---|
| Loose swap battery | Cuts on bumps | Check swap battery contacts and latch |
| Low SoC under hard load | Low SoC at cut | Charge earlier |
| Battery too hot or cold | Weather, climbs | Let battery reach normal temperature |
| Weak cell group or ageing pack | Cuts at rising SoC | Dealer or fleet battery check |
| Over-current from motor or controller fault | Over-current code | Diagnose a controller fault |
| BMS fault | BMS code | Dealer or fleet |