Scope and operating assumptions
This note addresses the discharge-side electrical behavior of multi-cell lithium-ion UAV traction batteries, with emphasis on three coupled mechanisms: loading voltage sag driven by pack internal resistance, progressive internal resistance growth as a state-of-health indicator, and passive cell balancing during charge. It is written for engineers integrating a smart battery with a flight controller, and for technical service teams interpreting pack telemetry from flight logs or ground control station data. The discussion is limited to general lithium-ion pack behavior; no cell chemistry, setpoint, or protocol value specific to any GOTION JTM product is claimed here, and no figures from third-party packs may be transferred to a JTM battery. Where a platform is named, it serves only as an applicability boundary: a DJI-class commercial UAV, for illustration, typically exposes a smart-battery data stream to its flight controller, and the same integration logic applies to any aircraft that consumes cell voltages, current, temperature, and fault flags over a serial or CAN link. Model-year and regional variants differ, and the named brand neither implies nor excludes any battery supplier.
Assumptions throughout: the pack is a series string of cells (possibly with parallel groups), managed by a battery management circuit that measures per-cell (or per-group) voltage, pack current, and at least one temperature point; the aircraft performs mission planning against reported state of charge; and validation occurs through data capture, not through energized probing, pack opening, or bypassing protection.
Working principle and control logic
Voltage sag under load. When the propulsion system draws current, terminal voltage falls below open-circuit voltage by the ohmic and polarization drop:
V_terminal = V_ocv(SoC, T) − I × R_total
where I is pack current (A), R_total is the sum of cell ionic, electrode, and interconnect resistance referred to the pack terminals (Ω), V_ocv is the open-circuit voltage at the present state of charge and temperature (V), and V_terminal is the measured pack or cell voltage (V). Sag therefore has three causal drivers: load current, internal resistance, and the position on the open-circuit-voltage curve. A high-current climb event produces sag that is normal and reversible — voltage recovers when current drops — whereas sag that deepens at constant, ordinary load indicates rising R_total, which is an aging or fault signature, not a control anomaly.
Control decisions from sag. The protection logic compares the lowest measured cell voltage against an undervoltage threshold. Under load, the weakest or highest-resistance cell reaches that threshold first; the management circuit then reduces or terminates discharge regardless of remaining pack energy. Causal chain: measured low-cell voltage at high current → undervoltage decision → discharge cut or throttle-limiting state change → release only when voltage recovers above a hysteresis band and the load condition is re-evaluated. This explains the classic field observation of a pack that "returns with charge left": energy remains in stronger cells, but the pack is limited by its lowest cell. Exact thresholds are pack-specific and must come from the battery's own documentation, not from another product's values.
Passive balancing. Manufacturing spread means series cells differ slightly in capacity and self-discharge. During charge (or rest at high voltage), the management circuit dissipates small amounts of energy from above-average cells through bleed resistors, narrowing the cell-to-cell voltage spread. Passive balancing redistributes slowly and thermally; it cannot rescue a severely mismatched or defective cell, and bleed heat is one reason balancing occurs preferentially near end of charge rather than at high load.
State of charge estimation. A smart battery typically estimates SoC by coulomb counting — integrating pack current over time — corrected against voltage at known reference points such as charge termination or low-current rest:
SoC(t) = SoC(0) − (1/C_rated) × ∫ i(t) dt, corrected by V_ocv lookup
where C_rated is rated capacity (Ah) and i is positive for discharge. Coulomb counting drifts with sensor offset and unmodeled self-discharge; voltage correction bounds that drift. Return-to-home decisions depend on this estimate, so estimation error translates directly into either premature mission abort or deep discharge — both field-visible as "the aircraft came back early" or "it did not make it back" without any hardware fault.
Parameters and interfaces
- Cell/group voltages (V): the balancing and undervoltage inputs; the minimum cell, not pack average, governs protection.
- Pack current (A): input to coulomb counting and to overcurrent protection; sign convention should be confirmed per interface.
- Temperature (°C): cold raises internal resistance and depresses capacity, so cold-weather sag is expected behavior; elevated temperature accelerates resistance growth and may trigger derating or cutoff before any voltage limit.
- Internal resistance (mΩ): a derived or periodically measured quantity; a rising trend across cycles at comparable temperature and SoC is the useful signal, since absolute values vary with measurement method.
- Communication link (UART, I²C, CAN, or SMBus as generally used in smart-battery implementations): carries cell voltages, SoC, current, temperature, cycle count, and fault flags. CAN suits electrically noisy airframes; I²C suits short, low-noise runs. Interface choice is an integration-layer decision that must be fixed before firmware work begins.
Verification and fault diagnosis
- Compare loaded vs. resting cell voltages from flight logs: recovery after load removal confirms ohmic sag; persistent depression suggests a weak cell.
- Track minimum-cell voltage and current together over successive flights; deepening sag at similar current and temperature indicates resistance growth, typically grounds for retirement review rather than a protection-circuit defect.
- Verify balancing indirectly: cell-voltage spread at charge termination should narrow over successive cycles; a spread that grows or one cell that never converges is abnormal.
- Cross-check SoC against a full-charge or known reference event; divergence between estimated and reference SoC points to current-sensor offset or capacity fade.
- Pre-flight logic such as refusing to arm on low SoC, out-of-range temperature, or an active fault flag is normal protection behavior and should be read from logs, not overridden.
- Destructive testing, energized wiring work, and pack opening are excluded; use data logs and qualified-lab evidence for anything beyond observational diagnosis.
Limitations
No GOTION JTM-specific thresholds, chemistries, capacities, or communication definitions are asserted here; only product-specific documentation and verified JTM records support such figures. General mechanisms described apply to series lithium-ion packs of any brand but do not establish compatibility with any aircraft or OEM ecosystem. Internal-resistance absolute values are method-dependent and not comparable across instruments. Remaining unspecified setpoints, recovery hysteresis values, and derating curves must be treated as unknown rather than borrowed from analogous products.