Where -delta-V becomes unreliable - warm cells, low currents, mismatched strings - temperature becomes the primary witness to full charge, because recombination heat grows precisely when the voltage signal shrinks. Production NiMH chargers therefore layer three distinct thermal decisions: a rate-of-temperature-rise test, dT/dt, that catches the recombination burst; a rise-over-ambient test, delta-T, that catches a cell finishing unusually hot relative to where it started; and an absolute-temperature ceiling that protects against every fault the incremental tests might miss. This paper specifies each criterion, the sensor and signal chain behind them, and the redundancy logic that makes thermal termination both sensitive and false-trip resistant.
Panasonic and Duracell guidance converges on a maximum permissible temperature-rise rate near 1 degree C per minute during fast charge; below it the cell is warming mainly from resistive losses, above it recombination has become the dominant current sink and full charge has been reached or passed. Charge ICs implement dT/dt by differentiating filtered thermistor readings over a sliding window and comparing against a programmed slope.
Differentiation amplifies noise, so the slope is computed over a window long enough to smooth ADC and thermal-contact noise - the same multi-sample philosophy as -delta-V - yet short enough to react within the overcharge band. A genuine end-of-charge acceleration persists and steepens, whereas a handling or airflow transient decays, and the debounce separates the two.
A cell's absolute temperature depends on ambient, which varies from a cold garage to a warm equipment bay; the rise above its own charge-start temperature is more portable. Delta-T termination trips when T_now minus T_start exceeds a configured allowance (commonly of order 10 to 20 degrees C depending on rate and pack size), catching a charge that is driving the cell anomalously hot regardless of the absolute scale.
Delta-T needs a valid start-temperature sample taken after the cell has rested in the charger and before current warms it, and it must be disarmed or widened if the cell was already warm from a prior discharge, otherwise ordinary charging trips it. Together dT/dt and delta-T cover the two thermal signatures of overcharge - a sudden acceleration and a large cumulative excursion.
An absolute cutoff provides the final, assumption-free backstop: Duracell-cited guidance places it around 60 degrees C, with many designs choosing 50 to 55 degrees C for extra life margin. It does not depend on slope history or a start reference, so it catches sensor-drift faults, blocked ventilation, a shorted cell or a hot ambient that the relative tests could misjudge.
Fast charge is also inhibited at the cold end: below roughly 0 degrees C recombination is sluggish and charging risks pressure and plating-like damage, so controllers withhold fast charge until the NTC reports a permissible window - the bq2002 family's temperature-gated fast-charge initiation implements exactly this cold/hot envelope.
A thermal terminator is only as good as its thermal contact. The NTC thermistor should be mechanically coupled to the cell or pack body - clipped to a cell wall or embedded in the pack - rather than soldered to the PCB where it reads component heat; in multi-cell packs it should sit among the central, least-cooled cells that run warmest. A resistor divider excites the NTC, and the IC compares the resulting Vtemp against thresholds for hot, cold and rate-of-change, as described in NTC fast-charge application literature.
Signal conditioning must reject self-heating in the thermistor (limit excitation current), mains and switching noise, and loose connectors that read as an open-circuit 'cold' fault; a credible design treats a thermistor open or short as a charge-inhibiting fault rather than a permit to continue.
The two channels fail under almost complementary conditions. -delta-V weakens at high temperature and low current; dT/dt weakens only if thermal contact is poor or the current too small to heat the cell, and it strengthens exactly when a warm cell erases the voltage dip. Combining them with maximum-voltage and maximum-time limits yields a termination that covers cool and warm, matched and mismatched, fast and slowing conditions simultaneously.
The first figure layers the thermal criteria as a decision stack and the second compares the timing of the voltage peak and thermal inflection, showing how a well-tuned charger stops at the earlier trustworthy signal rather than waiting for a specific one - the OR-logic that production charge-management ICs hard-wire.
A complete thermal specification lists the valid fast-charge temperature window, the dT/dt threshold and its differentiation window, the delta-T allowance and start-sample rule, the absolute ceiling, the sensor location and coupling, and the fault behaviour for an open or shorted probe. Validation uses instrumented cells in a thermal chamber across ambient, rate and cell ageing, logging the true state of charge at each trip to prove termination lands at full charge without sustained overcharge.
Weijiang supplies cell-specific thermal signatures - bulk-charge slope, end-of-charge inflection and the current that reaches 1 degree C per minute - so partners set thermal thresholds from measured data. Beyond the incremental thermal tests lies a deliberately simple fallback examined next: the zero-delta-V plateau detector and the safety timer that must always be present.
Weijiang Power designs and manufactures nickel-metal hydride cells, matched packs and charging-ready configurations for consumer, industrial, medical and mobility customers, and supports partners with charge-protocol guidance, IEC 61951-2 performance files, IEC 62133-1 safety evidence and charger co-validation. Share your cell format, charge rate, thermal envelope and cycle target and our engineers will specify a cell-and-charge combination that protects both runtime and service life. Review the range on the products page.
