Designers moving between battery chemistries repeatedly rediscover that charging rules are not portable: a method that is essential for one chemistry is useless or dangerous for another. This paper compares the charge control of NiMH, nickel-cadmium and lithium-ion as complete systems - what variable is controlled, how completion is detected, what sets the fast-charge limit, how full batteries are maintained, and what failure each regime prevents - explaining the roots of the differences in electrode chemistry and showing, concretely, why NiCd chargers under-serve NiMH, why lithium CC-CV cannot charge NiMH, and why a universal 'one charger for all' is a safety compromise. It consolidates the series' recurring chemistry comparisons into a single decision reference.
Both nickel chemistries are charged under current control with state-derived termination, because their end-of-charge is marked by gas and heat rather than a hard potential ceiling (Paper 15); lithium-ion is charged CC-CV, constant current to a precise per-cell voltage then constant-voltage taper, because exceeding that voltage drives lithium plating and cathode damage. The inversion is fundamental: for NiMH/NiCd current is controlled and voltage is observed; for lithium voltage is controlled and current decays. A controller built one way cannot safely run the other.
Concretely, holding a lithium-style fixed voltage on NiMH drives thermal positive feedback as the warming cell demands more current (Paper 15), while applying a NiMH -delta-V termination to lithium would overcharge it past its voltage limit before any such dip occurs.
NiCd shows a large, robust -delta-V (NiMH's dip is only about half as large, roughly 5-15 mV/cell) and tolerates overcharge comparatively well, which is why legacy NiCd chargers use prominent -delta-V with simple timers; NiMH needs smaller thresholds, strong dT/dt backup (its dip vanishes when warm) and tighter overcharge control because its recombination-driven wear is more consequential (Papers 5, 7). Lithium terminates the CC phase at the voltage setpoint and the CV phase at a minimum current, with no -delta-V mechanism at all.
This is why a NiCd charger under-serves NiMH: its larger -delta-V threshold may never be reached by NiMH's shallow dip, leaving the smaller-dip cell to rely on the timer while overcharging, and its higher permissible trickle is too aggressive for NiMH's C/20-class maintenance bound (Paper 9).
NiCd accepts high charge currents and overcharge most robustly; NiMH fast charge is bounded by oxygen recombination and heat, typically 0.5-1C without special cells and careful thermal control; lithium fast charge is bounded by lithium-plating at low temperature and by diffusion, expressed as a current that varies strongly with SOC and temperature. All three derate cold and hot, but the mechanism differs: NiMH pressure from slow recombination, lithium plating risk, NiCd comparatively the most forgiving.
Maintenance also diverges: NiMH needs only light pulse maintenance and is harmed by continuous trickle; NiCd tolerates heavier trickle; lithium requires no trickle at all once the CV taper completes, and continued charging is unsafe. These differences make 'maintenance mode' chemistry-specific.
Each charge architecture exists to prevent its chemistry's characteristic failure: NiMH control prevents recombination heat, pressure venting and the corrosion/drying that overcharge causes (Papers 3, 4, 21); NiCd control manages overcharge it largely tolerates while preventing thermal excess; lithium control prevents overvoltage-induced plating, cathode decomposition and thermal runaway, with per-cell voltage supervision that nickel strings (relying on matched overcharge tolerance) usually lack. Understanding the prevented failure explains every threshold.
Safety architecture differs accordingly: lithium packs require per-cell protection and monitoring electronics as a condition of safe charging, while NiMH relies on electrode capacity balance, the vent and charger termination - a simpler cell-side safety model but one that makes the charger's termination quality decisive.
A safe multi-chemistry charger selects chemistry explicitly (user selection or reliable identification, never voltage inference), then switches both power topology and termination logic to the correct regime (Paper 15); it must never apply NiCd trickle to NiMH, NiMH -delta-V waiting to lithium, or lithium CV to nickel. The first table-figure scores the three chemistries across control, termination, fast charge, maintenance and sensitivity; the second is a compatibility decision flow for a charger encountering an inserted pack.
Where products span generations - a tool line moving from NiMH/NiCd to lithium (Paper 35) - pack identification (mechanical keying or digital ID) is the safety-critical element that routes the pack to its correct charge regime.
NiMH remains compelling where tolerance of partial charge and overcharge, safety, ruggedness and cycle life outweigh lithium's energy-density advantage, and its charge systems reward careful current control and multi-criterion termination - the entire subject of this series. Weijiang supplies NiMH cells with chemistry-specific charge envelopes and supports designers porting products between chemistries with the control implications spelled out. The series' final paper looks forward, mapping the open research questions that define the next decade of NiMH charging science.
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.
