Scope and operating assumptions
This application note addresses a narrow but field-critical subsystem in light electric vehicle (LEV) traction packs: the cell-voltage sense (balance) harness and the interpretation logic that sits behind it inside a battery management system (BMS). It applies generally to series-connected lithium-ion packs of the kind used in electric scooters, motorcycles, and similar two-wheelers — including vehicles of the class produced by manufacturers such as VINFAST, where the pack is integrated into the chassis and exposed to sustained road vibration. Nothing here describes a specific GOTION JTM product, model, chemistry, or threshold; verified JTM product figures are not present in the underlying evidence and therefore none are asserted.
The note assumes a BMS that measures each series group through thin sense leads, compares those measurements against protection limits, and switches charge or discharge paths (typically via MOSFETs on the negative leg). It further assumes the reader is an integrator or service engineer working from logs, connectors, and de-energized measurements only — no pack opening, bypassing of protection, or live wiring work is described.
Working principle and control logic
The control chain is causal and traceable:
- Measured input: each series group's terminal voltage, sampled through the balance harness at relatively low current (microamps to milliamps), plus pack current and temperature where the BMS supports those inputs.
- Decision: the BMS compares the highest and lowest group voltages against overvoltage, undervoltage, and imbalance criteria, and combines the voltage picture with current-sense and temperature data to decide whether charge and discharge paths may remain closed.
- Output/state change: when a single group appears to exceed a limit, the BMS does not need a real cell fault to act — it acts on the reading. A charge halt, discharge inhibit, or a persistent imbalance flag can therefore be produced either by genuine electrochemical behavior or by a defective sense path.
- Release/recovery: some protections clear automatically once the offending measurement returns inside bounds (for illustration, after a rest period or charger attachment); others latch and require an explicit fault-flag clear through the BMS interface or a defined recovery event. Which behavior applies is firmware-dependent and remains model-specific.
The central mechanism worth understanding is the ghost voltage. A sense lead with a broken conductor, a cold solder joint, or a connector pin damaged by vibration presents an open or high-impedance path. The BMS input then floats or is pulled by leakage to a value that does not correspond to the actual cell group. Depending on the input architecture, the floating reading may appear implausibly high (triggering premature charge termination) or pinned low (mimicking undervoltage or imbalance). The pack itself can be healthy while the BMS repeatedly refuses to complete a charge or permit full discharge.
A related mechanism is measurement-induced error: because sense leads carry almost no current, a high-resistance joint does not heat or fail obviously; it only distorts the reading. A simple model makes this explicit. If the sense input impedance is R_in (ohms) and the joint adds series resistance R_j (ohms), the reading error for a group at true voltage V_g is approximately:
V_read ≈ V_g × R_in / (R_in + R_j)
with V_g and V_read in volts. The error becomes significant only when R_j approaches R_in; the assumption is a purely resistive divider and negligible input bias current. This is why a marginal joint can pass a continuity check with a low-impedance meter yet still shift the BMS reading.
Parameters and interfaces
- Sense lead conductor: typically fine-gauge wire whose failure mode is fatigue at the connector or crimp, accelerated by chassis vibration in two-wheeler duty.
- Connector pinout: the sequence in which sense taps are arranged is architecture-specific. Mis-sequencing adjacent taps applies the voltage span of two groups across one input and can damage BMS input conditioning. No universal pinout exists; the pack's own documentation governs.
- Common-port vs split-port topology: in some architectures the BMS switches only the negative leg while the positive path runs direct; in others charge and discharge negatives are separated. This determines whether a tripped protection blocks charging, discharging, or both.
- Communication interface: smart BMS variants expose group voltages, fault logs, and protection flags over a serial or wireless link. Reading the log is the preferred first diagnostic step because it identifies which group and which limit triggered the decision.
Verification and fault diagnosis
Safe diagnosis proceeds from evidence outward, de-energized wherever possible:
- Read the fault log first. A repeated flag on one series group, persistent across otherwise normal cycles, is the signature of a sense-path problem rather than a cell problem.
- Compare per-group readings at rest. One group reading near zero, near full-scale, or frozen at an unchanging value while others drift normally indicates a floating or shorted input, not a real cell state.
- With the pack de-energized and isolated per service procedure, probe the harness side of the balance connector: adjacent pins should differ by one group voltage. A pin pair showing zero difference indicates a broken lead or bridged taps on the harness side — a wiring fault, independent of the BMS.
- Voltage-sag asymmetry under a controlled, instrumented load indicates a real high-resistance element in the power path (interconnect or joint) rather than a sensing artifact; sensing faults do not change actual terminal voltage.
- Recovery testing: observe whether the fault clears on charger attachment or rest (auto-release) or only after an explicit flag clear (latched). Both are normal firmware behaviors; their meaning must be read from the specific BMS documentation.
Limitations
- No protection thresholds, pinouts, identifiers, or recovery times are specified here; these are model-specific and must come from the applicable pack documentation. A missing exact threshold cannot be replaced by assumption.
- This note does not describe any GOTION JTM product, chemistry, certification, or compatibility with any vehicle brand. A vehicle named as context establishes only the operating environment, not a fit or authorization.
- Diagnosis stops at the connector and log level. Any intervention inside a pack enclosure, repair of welded interconnects, or work on live high-current paths belongs to qualified service personnel with the manufacturer's procedures.
- Destructive testing and protection bypass are excluded; where confirmation beyond logs and measurements is required, qualified-lab evidence is the appropriate route.