A discharge curve is the single most informative graph in a NiMH datasheet — and the most frequently misread. It plots cell voltage against delivered capacity as a battery empties, and its shape changes with current, temperature and age. Engineers who can read these curves correctly choose better cells, set smarter cutoffs and avoid the runtime surprises that come from comparing mAh numbers quoted under different test conditions. This article explains what a NiMH discharge curve is made of, how load current reshapes it, and how to use that knowledge in product design.

The Anatomy of a NiMH Discharge Curve

A freshly charged NiMH cell starts near 1.35–1.40 V under load and passes through three recognisable regions:

• Initial dip and recovery — voltage drops briefly when load is first applied as internal electrochemical polarisation establishes, then settles within seconds.
• The flat plateau — for most of the capacity range, voltage holds close to 1.2 V. This signature flatness comes from the two-phase solid-state reactions at both electrodes, whose potentials stay nearly constant while active material remains on both sides of each reaction.
• The knee and roll-off — once active material is exhausted, voltage bends downward and falls quickly toward the standard 1.0 V-per-cell end voltage. Continuing to discharge below this point yields little energy and stresses the cell.

The wide, flat plateau is exactly why NiMH powers devices so predictably: a regulator sees an almost constant supply for 80–90 % of runtime.

C-Rate: The Language of Discharge Current

Discharge current is expressed as a C-rate, normalised to the cell's rated capacity. The reference current Iₜ equals rated ampere-hours divided by one hour, so a 2,000 mAh cell has Iₜ = 2,000 mA:

• 0.2C (400 mA for this cell) is a gentle, five-hour discharge — close to the condition at which datasheet capacity is often rated.
• 1C empties the cell in roughly one hour.
• 3C–10C is high-drain territory for tools, motors and pulse loads.

How Current Reshapes the Curve

Increase the C-rate and two things happen simultaneously. The whole plateau shifts downward because current flowing through the cell's internal resistance Rᵢ creates an instantaneous voltage drop of I × Rᵢ — visible as the larger initial dip and the lower running voltage. And the knee arrives earlier: at high current, less of the theoretical capacity can be extracted before voltage reaches the cutoff, so delivered mAh falls.

A cell that delivers a full 2,000 mAh at 0.2C might deliver 1,900 at 1C and 1,700 at 3C. Two suppliers quoting "2,000 mAh" cells are not comparable unless the rating current and cutoff voltage are identical — always ask which C-rate the capacity was measured at.

The Peukert Effect: Why Capacity Depends on Current

The loss of usable capacity at high current follows the Peukert relationship, which describes how a battery's effective capacity shrinks as discharge current rises. The Peukert exponent k quantifies the effect: the closer k is to 1, the more current-independent the capacity. NiMH performs well here, with k typically around 1.05–1.15 — far better than lead-acid batteries (roughly 1.3–1.6) and close to ideal. In practice this means NiMH holds its capacity unusually well under heavy load, one reason it remains competitive in power tools despite lithium's higher energy density.

Temperature Reshapes the Curve Too

Internal resistance rises sharply in the cold because ion mobility through the electrolyte slows. At −20 °C the plateau sags and the knee moves early even at modest current, which is why wide-temperature and high-rate NiMH grades use modified electrolytes and thin, high-surface-area electrodes. Warmth lowers resistance and lifts the curve, but sustained heat accelerates aging — design for a moderate operating temperature rather than relying on heat to mask undersized cells.

Reading Curves for Product Design

• Size for worst-case current — pick capacity using the curve at your highest sustained C-rate and lowest operating temperature, not the 0.2C rating.
• Plan regulator headroom — a motor or DC-DC input that must work down to 1.0 V per cell will extract far more capacity than one cutting off at 1.15 V; set low-voltage thresholds with the sag at pulse current in mind.
• Separate pulse from continuous — NiMH tolerates brief high-current pulses well; the curve for a pulse train sits above the curve for the same current held continuously.
• Compare suppliers on one test — demand curves at the same current, temperature and 1.0 V cutoff; matched test conditions beat marketing mAh figures.
• Watch curve evolution with age — as cells cycle, rising internal resistance lowers and shortens the curve; bench-testing aged samples predicts real end-of-life runtime.

Weijiang Power: Cells Whose Curves Hold Their Shape

Weijiang Power grades every NiMH cell on controlled charge/discharge channels and supplies discharge curves at the currents and temperatures your application actually uses — from consumer 0.2C grades to high-rate 5C–10C cells with optimised electrodes and low internal resistance. Tight grading means the cells inside one pack follow matched curves across their whole service life. Share your load profile, cutoff voltage and operating temperature, and our engineering team will recommend the grade that delivers the runtime your product promises.