aszarith
New Member
- Region
- Australia
I have an NCM T7S e-bike that I absolutely love, but I’m dealing with an intermittent no-power fault and I’d appreciate some technical input before the battery goes to the Australian importer for assessment.
The bike is less than six months old and the fault has now occurred twice.
What happens:
Both failures have followed a similar pattern:
• Battery has been ridden down to roughly 25%.
• It has then been charged to full using the original charger.
• The battery/bike has remained connected for several days after charging.
• The battery indicator shows that it is fully charged, but the bike will not power on at all.
• Cleaning/checking the external battery contacts and connections doesn't restore power.
• Eventually the fault disappears and the bike works normally again.
The second time this happened, the bike went to the retailer. By the time they tested it, it was working again. They swapped batteries, checked the connections and test-rode it, but both batteries were already close to fully charged, so the circumstances leading up to the fault weren't reproduced.
The retailer doesn't have the equipment to diagnose the battery internally, so it is now going to the Australian importer for assessment.
Current fault theory:
One possibility raised is a weak or imbalanced cell group causing the BMS to enter protection near the top of charge, or alternatively an intermittent BMS protection/latching/sleep-wake fault.
My understanding is that simply checking that the battery currently produces the correct voltage won't necessarily rule this out.
During an actual failure, I'd expect the output measurements to help narrow it down:
• Normal battery voltage at the terminals (~50–54.6 V for this system) but a completely dead bike would point further downstream toward the controller/display/wiring/wake circuitry.
• Zero or abnormally low output voltage while the battery's own indicator says it is charged would strongly suggest the BMS has shut down or latched into protection.
• Correct open-circuit voltage that collapses when a load is applied could indicate a weak cell group, internal connection problem or the BMS tripping under load.
• If connecting the charger causes the output to suddenly return, that would also seem relevant to a possible BMS sleep/wake or protection-reset issue.
What I want the importer to test:
Rather than simply charging it, checking the voltage and confirming that it currently works, I'm thinking they need to:
1. Check individual cell-group voltages and balance, particularly at full charge.
2. Check for a weak cell group reaching the BMS high-voltage threshold before the rest of the pack.
3. Test the battery under load and monitor whether the output voltage collapses or the BMS disconnects.
4. Check the BMS fault/protection behaviour and whether it can become latched or fail to wake correctly.
5. Ideally reproduce the conditions: discharge the battery to around 25%, fully charge it, leave it connected for several days, and then test it before doing anything that might reset/wake the BMS.
6. If the fault occurs, measure the battery's terminal voltage and behaviour under load before reconnecting the charger or otherwise resetting it.
7. Confirm whether the BMS records protection events/fault codes that can be interrogated.
The difficult part is that this is intermittent and the battery behaves perfectly normally between failures.
For those with experience diagnosing e-bike batteries/BMS faults: does that testing approach make sense, and is there anything specific I should ask the importer to measure while they have the battery?
I'm particularly interested in whether the pattern of working normally → discharged to ~25% → fully charged → left connected → fully charged indicator but no output/power is consistent with a cell-balance/BMS protection issue or BMS wake/latching fault.
The bike is less than six months old and the fault has now occurred twice.
What happens:
Both failures have followed a similar pattern:
• Battery has been ridden down to roughly 25%.
• It has then been charged to full using the original charger.
• The battery/bike has remained connected for several days after charging.
• The battery indicator shows that it is fully charged, but the bike will not power on at all.
• Cleaning/checking the external battery contacts and connections doesn't restore power.
• Eventually the fault disappears and the bike works normally again.
The second time this happened, the bike went to the retailer. By the time they tested it, it was working again. They swapped batteries, checked the connections and test-rode it, but both batteries were already close to fully charged, so the circumstances leading up to the fault weren't reproduced.
The retailer doesn't have the equipment to diagnose the battery internally, so it is now going to the Australian importer for assessment.
Current fault theory:
One possibility raised is a weak or imbalanced cell group causing the BMS to enter protection near the top of charge, or alternatively an intermittent BMS protection/latching/sleep-wake fault.
My understanding is that simply checking that the battery currently produces the correct voltage won't necessarily rule this out.
During an actual failure, I'd expect the output measurements to help narrow it down:
• Normal battery voltage at the terminals (~50–54.6 V for this system) but a completely dead bike would point further downstream toward the controller/display/wiring/wake circuitry.
• Zero or abnormally low output voltage while the battery's own indicator says it is charged would strongly suggest the BMS has shut down or latched into protection.
• Correct open-circuit voltage that collapses when a load is applied could indicate a weak cell group, internal connection problem or the BMS tripping under load.
• If connecting the charger causes the output to suddenly return, that would also seem relevant to a possible BMS sleep/wake or protection-reset issue.
What I want the importer to test:
Rather than simply charging it, checking the voltage and confirming that it currently works, I'm thinking they need to:
1. Check individual cell-group voltages and balance, particularly at full charge.
2. Check for a weak cell group reaching the BMS high-voltage threshold before the rest of the pack.
3. Test the battery under load and monitor whether the output voltage collapses or the BMS disconnects.
4. Check the BMS fault/protection behaviour and whether it can become latched or fail to wake correctly.
5. Ideally reproduce the conditions: discharge the battery to around 25%, fully charge it, leave it connected for several days, and then test it before doing anything that might reset/wake the BMS.
6. If the fault occurs, measure the battery's terminal voltage and behaviour under load before reconnecting the charger or otherwise resetting it.
7. Confirm whether the BMS records protection events/fault codes that can be interrogated.
The difficult part is that this is intermittent and the battery behaves perfectly normally between failures.
For those with experience diagnosing e-bike batteries/BMS faults: does that testing approach make sense, and is there anything specific I should ask the importer to measure while they have the battery?
I'm particularly interested in whether the pattern of working normally → discharged to ~25% → fully charged → left connected → fully charged indicator but no output/power is consistent with a cell-balance/BMS protection issue or BMS wake/latching fault.
