A single cell charges in an open tray with air all around; a pack charges inside a constrained volume where interior cells cannot shed heat as readily as surface cells, and where one hot cell can shift the behaviour of the whole string. Pack-level thermal management is therefore a prerequisite for fast charging multi-cell NiMH, not an afterthought. This paper covers the thermal design of a charging NiMH pack - how heat is generated and distributed, how conduction and airflow paths should be arranged, where temperature sensors must sit and how many are needed, and why charge current must always be governed by the hottest rather than the average cell - to keep every cell inside the envelope the single-cell papers define.
Within a pack, heat sources vary: interior cells have higher thermal resistance to ambient and run hotter than edge cells; cells with slightly higher internal resistance generate more I-squared-R heat; and the first cells to reach full charge enter exothermic recombination while others still accept charge, concentrating heat at the locations that are already furthest along. Thermal gradients therefore emerge and sharpen through the end-of-charge band even among nominally matched cells.
A lumped 'pack temperature' masks these gradients; design must reason about the spatial temperature field, which a multi-node electro-thermal network (Paper 18) can predict from geometry and losses before prototyping.
Cells should be arranged so each has a defined heat-escape route: thermally conductive holders or potting that links cells to a chassis or outer wall, metal carrier frames that spread heat, and avoidance of fully insulated interior stacks with no path out. Cylindrical cells in close contact conduct heat between neighbours - helpful for spreading, but capable of propagating a hot spot - so spacing, holder conductivity and contact pressure are design variables with a real effect on peak temperature during charge.
For high-rate packs, thermal interface material to a heat-spreading plate or enclosure wall turns the whole housing into a radiator; the charge current that can be sustained is ultimately set by the thermal resistance of this chain to ambient.
Where convection is available, vent channels should align with the natural buoyancy direction and avoid dead pockets; spacing cells to let air circulate beats a tightly packed block for fast charge even at a modest volume penalty. Forced airflow during charge flattens gradients substantially and permits higher current, but the controller should know fan state - a stalled or blocked fan changes the thermal resistance, and the charge profile must derate to the no-airflow case rather than assume cooling that has failed.
Fan switching also creates dT/dt artifacts (Paper 7), so thermal termination should use filtered measurements and avoid mistaking a fan-induced cooling step for a safe state.
NTCs must measure the cells most likely to be hottest - geometrically interior positions, nearest any external heat source, and (where known) the highest-resistance cells - rather than the easily accessed outer cells. A single sensor is acceptable only for small, near-isothermal packs; larger packs need several, and charge current and termination are governed by the maximum reading: the hottest cell sets the recombination and ageing limit for the entire string, since current is common to all series cells.
Good mechanical coupling (Paper 7) - sensor clipped or bonded to a cell can, not the PCB - and a sensor-fault check complete the sensing design; an unmeasured hot cell is the classic root cause of pack-level overcharge incidents.
The pack controller combines the spatial thermal picture with the per-cell electrical picture: derate current from the hottest cell, terminate the string when the first cells reach the valid stop criteria, and use balancing (the next papers) to reduce the spread that creates hot cells. The first figure contrasts temperature distributions in an unmanaged and thermally managed pack; the second sequences the hottest-cell governance loop.
Predicting the field with a thermal model, confirming with thermocouple-instrumented prototype charges at worst-case ambient and blocked airflow, and validating that no cell exceeds its single-cell envelope is the pack equivalent of single-cell qualification.
Weijiang supports pack integrators with cell-spacing and thermal guidance, single-cell heat-generation data to populate pack thermal models, and matched-cell selection that reduces the inherent spread driving gradients. A thermally sound pack is what allows the multi-cell series and balancing techniques of the next two papers to work at all, because balancing cannot rescue a cell that heat has pushed outside its safe charge envelope.
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.
