Internal resistance is the one electrical property that is both measurable on a live cell and deeply informative: it sets the Joule heating budget, the voltage drop that masks state of charge, the current a cell can accept, and - as it grows with age - its state of health. This paper moves from the simple DC resistance a charger can measure in milliseconds to the frequency-resolved picture of electrochemical impedance spectroscopy, interpreting the NiMH Nyquist signature through the Randles equivalent circuit and showing how each impedance component maps onto a physical process and a charge-control decision. It bridges the electrochemistry group and the estimation group by turning the cell's impedance response into a usable control variable.
The simplest measurement applies a current step and divides the immediate voltage change by current, capturing the ohmic resistance of electrolyte, separator, electrodes and welds; holding the pulse briefly and watching the slower voltage evolution adds the polarisation resistance associated with charge-transfer kinetics and concentration. Hybrid-vehicle practice, for example, uses 100-millisecond DC pulses within a 30 to 70 percent SOC window to characterise the ohmic component under conditions relevant to regenerative charging.
DC resistance is SOC-, temperature- and current-dependent: it rises at the extremes of SOC and sharply in the cold, so a charger that current-limits from resistance must compare like with like - same SOC band, same temperature - or mistake a cold cell for a dying one.
Electrochemical impedance spectroscopy sweeps a small AC excitation across frequency and plots the result as a Nyquist diagram. A NiMH cell follows a Randles-like structure: a high-frequency intercept on the real axis gives the ohmic resistance; a depressed mid-frequency semicircle represents the charge-transfer resistance in parallel with the double-layer capacitance; and a low-frequency near-45-degree Warburg tail reflects solid-state proton and species diffusion. Each feature is a window onto a different internal process.
De-embedding the components lets a researcher distinguish a dried-out separator (rising ohmic intercept) from degraded electrode kinetics (growing semicircle) from slowed diffusion (lengthened Warburg tail) - diagnoses a single lumped DC number cannot make, and the basis for physically meaningful state-of-health inference.
As a cell charges, the charge-transfer resistance and diffusion parameters evolve with electrode phase composition, and they change abruptly as the cell enters the oxygen-evolution regime - offering a research-grade marker of the end-of-charge knee that parallels the voltage and thermal signatures. Over life, NiMH degradation studies consistently identify growth of the activation/charge-transfer resistance and the ohmic component as the dominant impedance signatures of ageing, driven by surface oxidation, alloy corrosion and gradual electrolyte redistribution.
Tracking these features over cycles therefore separates reversible state effects (current SOC, temperature) from irreversible trend (health): a resistance that returns after warming is state; a resistance that grows cycle over cycle at matched conditions is ageing - a distinction central to the degradation group later in the series.
Full EIS requires laboratory instrumentation, but its insights can be compressed into a few frequencies or a single broadband pulse suitable for an embedded controller: a high-frequency or instantaneous component for ohmic resistance, a short-time window for charge-transfer, and an optional longer relaxation for diffusion. An observer can fit a reduced Randles model to such pulse-relaxation data online, updating resistance parameters that simultaneously improve SOC estimation (Paper 16) and heat prediction (Paper 4).
This is how impedance feeds charge control: the updated resistance predicts the Joule heating at a candidate current, allowing the controller to derate current for a high-impedance aged or cold cell automatically rather than using a fixed limit that is either abusive to weak cells or overly conservative for strong ones.
Reliable impedance needs a linear, stable response: the excitation must be small enough not to shift SOC or trigger non-linear kinetics, the cell must be at a defined SOC and temperature, and leads must use four-wire (Kelvin) sensing to exclude contact resistance - critical in consumer holders where spring-contact drift otherwise masquerades as cell ageing. Causality and stability checks (Kramers-Kronig consistency in lab work) validate the spectrum; embedded pulse fits instead rely on redundancy across multiple pulses.
The first figure builds the Randles circuit and maps its Nyquist features; the second shows how ohmic and charge-transfer resistance rise with age at matched conditions, the measurable basis for health-aware charge derating.
Define the resistance metric and exact test conditions (pulse duration, SOC band, temperature) so readings are comparable; set current derating from a validated resistance-temperature map; use resistance trend as a health input and its abrupt change as a fault detector; and reserve full EIS for cell characterisation that calibrates the reduced online model. Weijiang supplies DC-resistance and representative EIS characterisation across SOC and temperature for its grades, giving partners the parameters behind a resistance-aware profile.
Resistance is the bridge between observable voltage and internal state; the next paper combines it with the thermal model to build the coupled electro-thermal estimation that accurate fast charging requires.
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.
