Most owners assume the battery has failed. In reality this is a storage-grade BMS protection threshold mismatch. On the flyover ramps of Lagos, the heavy-freight routes of Pune and the rolling hills around Nairobi, large numbers of converted three-wheelers (Keke / Tuk-Tuk) fitted with non-standard or storage-type packs black out mid-gradient. This is not only a lost shift — a loaded tricycle can roll backwards. This report publishes the full root-cause model, captured on vehicle dynamometers and digital oscilloscopes.
verified100ms sampling window · sized for China and India gradient duty
When a loaded tricycle pulls away on a brick-yard or builders-merchant ramp, motor back-EMF collapses to zero and the armature enters a locked or half-locked rotor condition. The measurement proves that storage-grade BMS boards use a very conservative continuous-current time constant (typically 100ms): the moment a heavy start arrives, the controller applies full duty-cycle PWM and blows straight through that threshold.
Specification notice:The figures above are engineering reference values. They may differ with production batch, configuration and actual duty cycle, and the percentages for life, efficiency, availability and coverage are engineering estimates and bench references. Contact us for the detailed specification sheet.
Most overseas dealers blame capacity and simply order more amp-hours. In fact 92% of cut-offs come from the three electrochemical and BMS control mismatches below.
At low speed under full load, motor back-EMF collapses towards zero and the phase windings conduct heavily. The motor momentarily demands 3–4x its rated locked-rotor pulse (150A~220A).
Storage board: single-stage, 100A / 100ms fuse.
TIANQIU standard: two-stage motive curve, 300A (1s) / 450A (100ms).
Low-rate, high-resistance cells (such as salvaged storage cells above 1.8mΩ internal resistance) driven hard on a long climb heat the module core past 65°C within four minutes through Joule heating (Q=I²Rt).
Thermal probe reading: 72°C localised at the MOSFET bank and tabs.
BMS response: NTC sensing forces a lockout and refuses restart until cooled.
A textbook weakest-link effect. The pack meter may still show 50% remaining, but if one ageing cell in a 24S string is behind, its terminal voltage instantly breaches the 2.5V cut-off floor under high current.
Cell delta dynamics: 15mV at rest ➔ 380mV at full load.
Protection logic: hard cell under-voltage trip.
A cell-level dynamic-sag analyser shows exactly how string 14 collapses non-linearly at 150A.
Why does a 1500W motor still cut out on a 100A lithium pack? Because the controller current ceiling and the inverter efficiency factor were both ignored. This is the on-vehicle calibration matrix behind the mandatory sizing formula.
| Motor specification | Bus nominal voltage | Controller current limit (A) | Loaded uphill transient peak | Wrong selection (market storage pack) | TIANQIU industrial motive standard | Recommended cable (AWG) |
|---|---|---|---|---|---|---|
| 1000W differential motor | 48V DC | 35A ± 1.5A | 75A ~ 95A | 50A board (instant cut) | 80A continuous / 150A transient (5s) | 8 AWG pure copper |
| 1500W heavy passenger (Keke) | 60V DC | 45A ± 2A | 110A ~ 140A | 80A board (thermal trip) | 120A continuous / 240A transient (3s) | 6 AWG silicone |
| 2200W cargo climber | 60V / 72V | 60A ± 2A | 150A ~ 185A | 100A generic board (punch-through) | 160A continuous / 350A transient (3s) | 4 AWG high-temp silicone |
| 3000W two-speed polar motor | 72V DC | 80A ± 3A | 200A ~ 240A | 150A board (frequent UV lockout) | 200A continuous / 450A transient (5s) | 2 AWG / 35mm² |
The selection envelope must cover continuous current and the transient crest. The pull-away surge and the second shock of crossing a speed hump fall inside the same time-constant window; sizing a board on continuous current alone is the single biggest cause of gradient cut-offs. For board and vehicle matching detail see the commercial BMS selection and CAN bus guide.
Specification notice:The figures above are engineering reference values. They may differ with production batch, configuration and actual duty cycle, and the percentages for life, efficiency, availability and coverage are engineering estimates and bench references. Contact us for the detailed specification sheet.
I_bms_peak ≥ (P_motor / (V_cutoff × η)) × K_uphill, where η (motor efficiency) ≈ 0.82 and K_uphill (gradient load factor) = 2.2~2.8.
The BMS rated discharge current (continuous) must be at least 1.5x the controller maximum rated current; the transient trip current must reach 3.0x or more, and the transient delay threshold must be calibrated to 2000ms~3000ms so that speed humps and gradient pull-away torque crests are enveloped rather than tripped.
Below 10°C cell DC resistance multiplies and the sag slope (ΔV = I × R_internal) steepens, so under-voltage protection trips earlier. The compensation strategy for high-latitude and highland packs is covered in the sub-zero charge/discharge and PTC self-heating guide.
Shorting bypasses all semiconductor protection. With the cell-level second line removed, an external short circuit leads directly to fire. Diagnose through the four-step SOP below instead.
When a rider reports "it stops every time I go up a slope", the technician must not open the pack first. Bring a standard multimeter, a clamp meter and an infrared thermometer, and close the diagnosis loop through these four steps.
Put the probes across the battery output busbars, prop up the rear wheel and hold the brake (or sit on a measured 10° gradient), then apply full throttle for 2 seconds:
Verdict A | sag < 4.5V and still cuts out: the cells are healthy and the BMS transient current threshold is simply set too low. No pack replacement needed — re-parameterise or fit a motive board.
Verdict B | sag > 12V collapse: cell resistance is extreme or a string is open. The module is deeply polarised or the cells are gassing; open the pack and repair at cell level.
Open the TIANQIU mobile engineering app and read the first freeze frame recorded at the moment of cut-off: ERR-02: Peak-OCP (hardware overcurrent) · ERR-05: Cell-UVP (cell under-voltage) · ERR-09: MOS-OTP (MOSFET over-temperature). Frame definitions and register mapping are in the BMS selection and CAN bus guide.
Tools: smartphone app / USB-TTL host softwareAfter 100 metres of loaded climbing, lift the seat base immediately and inspect the Anderson connectors for oxidation, looseness or cold-soldered cable lugs. Alarm threshold: above 60°C at a terminal, a poor joint of only 15mΩ will eat 1.5V and dissipate 150W at 100A, dropping the voltage at the controller input below its own cut-off.
Tools: thermal imaging camera / contact thermocoupleIf pack sag and BMS current thresholds are both within specification but the vehicle still cuts out, move to the motor. Heat-induced demagnetisation lowers the torque constant, so the controller raises its current limit to hold torque — which closes a self-reinforcing overload loop.
Tools: scope with Hall signal breakout / resistance bridge
Lagos and Delhi service points keep this bench setup on hand: read the symptom, locate the failure layer, and stop opening packs at random.
The seven questions our after-sales teams in Africa, South Asia and Latin America are asked most often at the service counter.
Send your target motor specification, controller current limit and destination market, and the TIANQIU powertrain institute will reply within 24 hours on working days with a full dynamometer matching study and a custom sample-pack proposal.
Specification notice:The figures above are engineering reference values. They may differ with production batch, configuration and actual duty cycle, and the percentages for life, efficiency, availability and coverage are engineering estimates and bench references. Contact us for the detailed specification sheet.
Vehicle drawings, controller brands and duty-cycle data you submit stay strictly confidential. A mutual NDA can be signed first.
