A sealed NiMH cell is a small pressure vessel, and its internal pressure is the most direct physical measure of whether charging is under control. Current that the electrodes cannot store becomes gas; gas that the recombination loop cannot consume accumulates; and accumulated pressure eventually forces the safety vent, losing electrolyte and permanently changing the cell. Pressure is therefore not merely a safety variable - it is the hard ceiling that determines how fast a given cell can be charged. Drawing on instrumented traction-cell measurements and sealed-cell safety literature, this paper traces the pressure trajectory through a charge, separates the hydrogen and oxygen contributions, and shows how pressure-aware thinking sets the current envelope that voltage- and temperature-based termination only approximates.
Two gases can appear. Oxygen is generated at the positive once oxidisable Ni(OH)2 is exhausted, as set out in Paper 1; hydrogen would be generated at the negative if it were driven beyond its hydride storage capacity, which is precisely why the negative is built oversized. In a correctly designed, moderately overcharged cell oxygen dominates and hydrogen is normally absent; hydrogen evolution signals either a grossly overcharged cell or an aged negative whose catalytic storage capacity has faded.
Because oxygen is expected, the cell is engineered to consume it: oxygen crosses the starved separator and recombines on the hydride surface. Internal pressure at any moment reflects the imbalance between the rate gas is being produced and the rate at which this recombination consumes it, a dynamic balance rather than a static value.
Through bulk charge, little gas is produced and pressure stays near ambient. As oxygen onset is passed - near 70 percent state of charge in reported traction measurements - pressure begins to rise; if current stays within recombination capacity it settles toward a modest steady state, but if current exceeds that capacity it climbs progressively, steepening as temperature and generation feed each other. At 1C, studies identify an additional hydrogen-recombination contribution in the final stage associated with the -delta-V event, showing that the pressure curve carries information about both gases.
A controlled C/2 charge of a traction module, with pressure capped near 6.8 atmospheres (about 100 psi), still reached 93 percent state of charge at high efficiency - empirical proof that an explicitly pressure-limited strategy can charge both quickly and almost fully by backing off whenever the gas balance tightens.
Sealed cells carry a resealable or one-way safety vent tuned to open at a design pressure, commonly cited around 10 atmospheres for small cells, before the can is mechanically endangered. Venting releases the gas mixture but also ejects alkaline electrolyte mist and water vapour; the lost electrolyte cannot be replaced, internal resistance rises, the recombination pathways dry out and the cell enters an accelerating decline. A single vent event can remove a meaningful fraction of cycle life.
This makes pressure the true failure boundary: voltage and temperature are convenient external proxies, but the mechanism that actually damages a severely overcharged cell is pressure-driven electrolyte loss and, in the worst case, rupture. Termination design is best understood as an attempt to keep the unseen pressure curve beneath the vent threshold using only the signals available outside the can.
Recombination is transport-limited and has a maximum current set by electrode area, separator gas permeability, alloy catalysis and temperature. Any charge current above that limit in the end-of-charge region accumulates gas almost linearly with time, so the safe fast-charge current is not arbitrary: it is the current whose end-of-charge oxygen generation rate the cell can recombine while rejecting the resulting heat. Doubling charge current does not double the permissible gas-handling rate, which is why 1C is far more demanding than C/2 rather than merely twice as fast.
High-capacity cells with thicker electrodes are especially constrained: they store more energy but offer relatively less recombination surface and longer gas diffusion paths, so a 2800-class AA often tolerates aggressive fast charge less well than a 2000-class long-life cell - another face of the capacity-cycle-retention trade-off.
Consumer cells rarely carry pressure sensors, so chargers infer the gas regime from its correlates: the voltage peak and -delta-V produced when recombination heating overtakes the electrode potentials; the rate of temperature rise dT/dt, which climbs sharply when recombination becomes the dominant sink for current; and absolute temperature as the final backstop. Pack-level and traction designs can add direct pressure transducers or model pressure from current, voltage and temperature, allowing the kind of explicit pressure-lid control demonstrated in the traction study.
The first animated figure shows the staged build from bulk charge to oxygen onset to a recombination-limited plateau, and the second decomposes the gas balance as a production/consumption stack, making visible why a charger that watches only voltage can miss a pressure problem when the cell is warm and the -delta-V disappears.
A defensible fast-charge specification pairs a cell characterisation - oxygen-onset state of charge, recombination-limited current versus temperature, and vent margin - with a charger profile that derates as the high-SOC band is entered and treats rapid dT/dt as evidence that gas generation is outrunning consumption. Conservative designs keep the steady recombination pressure well below the vent threshold rather than merely below it, leaving margin for lot variation, ageing and blocked cooling.
For partners building fast chargers or regenerative packs, Weijiang supplies the pressure and thermal characterisation behind each recommended current step, so the fast-charge ceiling is a measured boundary rather than an assumed one. The next paper turns to the other unavoidable by-product of that recombination balance - heat - and builds the energy balance that predicts how warm a charge will make a cell.
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.
