Nickel-metal hydride (NiMH) is often specified for its safety, cycle life and tolerance of abuse, but relatively few engineers outside the cell industry can explain exactly what happens inside the can while current flows. Understanding the internal chemistry pays back in very practical ways: it explains why NiMH cells deliver a flat 1.2 V, why a charger looks for a voltage dip rather than a voltage peak, why a sealed cell never needs topping up with water, and why temperature — not voltage alone — is the truest measure of a full cell. This article opens the can and walks through the electrochemistry step by step.

The Three Active Components

Every sealed NiMH cell is built from the same three active building blocks.

• Positive electrode — nickel hydroxide. A porous nickel foam or fibre substrate is filled with active nickel hydroxide, Ni(OH)₂, with small cobalt and nickel additives that improve conductivity and charge efficiency. During cycling this material switches between nickel(II) hydroxide and nickel(III) oxyhydroxide, NiOOH.
• Negative electrode — hydrogen-storage alloy. A mischmetal-based AB₅ alloy, or an AB₂ Laves-phase alloy in high-capacity designs, acts as a solid-state "hydrogen sponge". Hydrogen atoms occupy interstitial sites in the alloy lattice, forming the metal hydride MH — no high-pressure gaseous hydrogen is ever stored inside the cell.
• Electrolyte and separator. A concentrated aqueous solution of potassium hydroxide (KOH), sometimes blended with LiOH or NaOH, fills a thin, gas-permeable non-woven separator. Critically, the electrolyte is a mobile ion bridge; it is barely consumed by the net cell reaction, which is why a correctly built sealed cell stays maintenance-free for life.

Discharge: Moving Hydrogen from One Electrode to the Other

On discharge, a hydrogen atom leaves the hydride alloy at the negative electrode, combines with a hydroxyl ion from the electrolyte, and releases an electron into the external circuit:

Negative electrode (discharge): MH + OH⁻ → M + H₂O + e⁻

Electrons travel through the device being powered, while at the positive electrode nickel oxyhydroxide accepts them and is reduced back to nickel hydroxide:

Positive electrode (discharge): NiOOH + H₂O + e⁻ → Ni(OH)₂ + OH⁻

Adding the two half-reactions, the water and hydroxyl ions cancel and the overall process is elegantly simple:

Overall (discharge): MH + NiOOH → M + Ni(OH)₂

In plain terms, discharge is the transfer of hydrogen from the negative alloy to the positive electrode through the electrolyte, with the useful electron current flowing the long way around through the load. The KOH electrolyte shuttles ions but is regenerated in the same cycle — a key reason NiMH tolerates long service life without electrolyte maintenance.

Charge: Reversing the Reaction

Charging drives the same reactions backwards. At the positive, Ni(OH)₂ is oxidised to NiOOH while releasing a proton; at the negative, the alloy re-absorbs hydrogen and becomes the hydride again:

Overall (charge): M + Ni(OH)₂ → MH + NiOOH

The hydrogen is pumped back into the alloy lattice, where it is stored as atomic hydrogen in solid solution. Early in charge the process is highly efficient and close to thermoneutral. Above roughly 80–90 % state of charge, however, the positive electrode begins to run out of hydroxide to convert, and a competing side reaction — oxygen evolution — starts to take over. Managing that transition is the whole basis of NiMH charge control.

Why the Cell Delivers a Flat 1.2 V Plateau

The difference between the operating potentials of the nickel electrode and the hydride electrode works out to roughly 1.2–1.3 V across most of the state-of-charge range, giving NiMH its familiar nominal voltage of 1.2 V per cell. Because both electrode reactions are two-phase solid-state transitions, their potentials stay nearly constant while active material remains on both sides of each reaction. The result is the characteristically flat discharge curve: a fresh cell starts near 1.35–1.40 V under load, holds close to 1.2 V for the great majority of its capacity, and rolls off toward the 1.0 V end-voltage only when genuinely exhausted.

For designers this is useful in two ways. Regulators and motor drives can be sized around a predictable supply voltage rather than a steadily sagging one, and capacity is meaningfully measured to a defined cutoff — conventionally 1.0 V per cell at the rated discharge current.

The Sealed-Cell Trick: Internal Oxygen Recombination

A NiMH cell is permanently sealed, yet charging near 100 % inevitably generates oxygen at the positive electrode:

Oxygen evolution (overcharge): 4OH⁻ → O₂ + 2H₂O + 4e⁻

Rather than letting pressure build until the cell vents, NiMH uses a closed oxygen cycle, enabled by deliberate design choices:

• Starved-electrolyte design — the cell is filled with slightly less electrolyte than it could hold, leaving gas pathways through a partially wetted, gas-permeable separator.
• Capacity imbalance — the negative alloy electrode is built with more capacity than the positive. When the positive is fully charged and starts evolving oxygen, the negative still has free alloy sites, so it never reaches the point of evolving hydrogen in normal overcharge.
• Recombination — oxygen gas diffuses across the separator and reacts directly with the charged hydride at the negative: 4MH + O₂ → 4M + 2H₂O. The oxygen is turned back into water, the alloy is partly discharged and recharged again, and net gas consumption balances generation.

A modest overcharge current is therefore absorbed indefinitely as heat rather than gas — the reason a correctly designed NiMH pack can tolerate a low trickle current and forgiving chargers. The mechanical safety vent remains only the last line of defence against gross abuse.

Reading the End of Charge: −ΔV, Temperature and Pressure

The oxygen-recombination cycle hands charger designers the signals they need to detect a full cell reliably:

• −ΔV (negative delta V) — once recombination dominates, cell voltage stops rising, peaks, and then drops by a few millivolts. That small, repeatable dip is the classic full-charge flag.
• ΔT/Δt (temperature slope) — recombination releases heat, so cell temperature begins climbing quickly exactly when charge efficiency collapses. A fast rise in temperature per minute is a more robust termination signal than −ΔV at slow charge rates, where the voltage dip can be too small to detect.
• Pressure and plateau timing — internal pressure tracks the same transition; combined with timers and absolute-temperature cutoffs (typically around 50–60 °C), these backups prevent sustained overcharge even if a primary sensor fails.

This is why quality NiMH chargers combine −ΔV, ΔT/Δt, maximum temperature and a safety timer, and why trickle charge is kept to a low C-rate: the chemistry is forgiving, but heat is always the signature of wasted current.

What the Electrochemistry Means for OEM Design

• Specify the right termination strategy for the charge rate: −ΔV for fast charging, temperature-slope or timer control for slow and trickle charging.
• Design thermal headroom into the pack — recombination heat at end of charge is normal, but packs that cannot shed it age faster.
• Use matched, graded cells in series strings, so no single cell is driven into reversal by a weaker neighbour.
• Define capacity to the 1.0 V/cell cutoff at the rated current when comparing suppliers; nominal mAh figures without a test condition are not comparable.
• Respect storage recommendations — cool storage at partial charge preserves the alloy lattice and the nickel electrode far better than sitting fully hot-charged.

Weijiang Power: Chemistry Under Control, Batch After Batch

At Weijiang Power, this electrochemistry is not theory — it is how production is controlled. Electrode formulations, negative/positive capacity balance, electrolyte fill volume, formation profiles and grading currents are all set around the reactions described above, so the cells that leave our Huizhou lines charge predictably, hold a flat 1.2 V plateau, recombine gas cleanly and cycle consistently lot after lot. With 16 years of NiMH-focused R&D, certified production and full OEM/ODM customisation from single cells to welded packs, we can match the electrochemistry to your device. Send us your discharge profile, duty cycle and enclosure constraints, and our engineers will recommend the cell or pack design that makes them work.