Reflex or 'burp' charging is the most distinctive member of the pulse family: between positive charge pulses it inserts a brief, larger negative (discharge) pulse followed by a rest, claimed to depolarise the electrodes, dislodge gas bubbles and lower internal pressure faster than a passive rest. Patented in the lead-acid world and widely discussed for nickel chemistries, Reflex divides opinion between advocates who credit it with cooler, faster charges and sceptics who see added converter complexity for marginal benefit. This paper separates mechanism from marketing, examines what controlled evidence supports for NiMH specifically, and gives designers a clear-eyed framework for deciding when a four-quadrant pulse profile is justified.
A Reflex cycle consists of a charge pulse of seconds, a very short negative pulse at perhaps two to three times the charge magnitude for tens to hundreds of milliseconds, and a rest before the next charge pulse. The proposed mechanisms are: forced removal of accumulated charge to collapse concentration polarisation rapidly; mechanical and electrochemical 'burping' of gas bubbles adhering to electrode surfaces; and equalisation of charge among series cells by briefly discharging the string.
Compared with the passive-rest pulse charging of Paper 13, Reflex actively drives the cell backward during the negative segment, which requires a bidirectional (four-quadrant) power stage rather than a simple buck - a real cost and control penalty that must be justified by a measured benefit.
A discharge pulse does lower the electrode overpotentials faster than open-circuit relaxation by actively consuming the concentrated species near reaction sites, and the voltage trace shows a quicker return toward equilibrium. For NiMH the relevant polarisation at end of charge is dominated by the oxygen-evolution regime; a brief discharge does temporarily back the positive away from that potential, buying a short window before it is re-approached on the next charge pulse.
Whether this beats a slightly longer passive rest depends on time constants: where diffusion relaxation is slow, an active pulse accelerates it; where it is already fast on the seconds scale, as in many small NiMH cells, the incremental benefit over passive pulsation is small and the energy cycled backward is energy lost to round-trip inefficiency.
Adhered micro-bubbles can block active surface and raise local current density; a current reversal and the associated slight mechanical perturbation can detach them, and traction-cell microscopy literature notes gas management as central to high-rate NiMH. However, in a starved sealed cell the dominant gas pathway is diffusion through the separator to the negative for recombination, which a negative pulse aids only indirectly by briefly consuming charge; there is no large free-electrolyte bubble population to 'burp' as in flooded lead-acid, where the technique originated.
Designers should therefore not import lead-acid burp parameters wholesale; any gas-management benefit in NiMH must be demonstrated against a passive-pulse control with matched average current, rather than assumed from the older chemistry.
The strongest defensible NiMH use case is series strings with mismatch: a brief shared discharge pulse lets the most-charged cell relax most and, repeated over a charge, can reduce the divergence that otherwise makes the first-full cell overcharge while laggards catch up. This is a poor substitute for true per-cell balancing (a later pack-charging paper) but can help in low-cost strings that lack balance circuitry.
It also carries risk: in a mismatched string the weakest, least-charged cell is driven deepest into discharge during each negative pulse, and excessive reversal of a depleted cell damages its electrodes; negative-pulse amplitude and duration must be bounded so no cell is driven below its safe minimum voltage.
Reflex is worth its bidirectional stage only if a controlled study - equal average charge current, equal delivered capacity, matched thermal conditions - demonstrates lower peak pressure or temperature, higher termination SOC, or better cycle retention than passive pulse or multi-stage constant current. Without that control, faster apparent charging can simply reflect different net current, and cooler operation a lower throughput. Much of the favourable Reflex literature lacks this control or transfers lithium/lead-acid results uncritically.
The first figure lays out the charge-negative-rest waveform and its voltage effect; the second scores Reflex against passive pulse and CC across depolarisation, gas management, equalisation, efficiency and cost, clarifying where the technique earns its complexity.
Choose Reflex when instrumented characterisation of the target cell shows passive rests insufficient - typically high-rate, thick-electrode or tightly packed traction cells with slow gas relaxation and meaningful mismatch - and when a bidirectional stage is already present for regenerative operation (as in hybrid drivetrains, where negative 'pulses' arrive free during braking). For small consumer cells, passive pulse or descending constant-current stages usually capture most of the benefit at lower cost and higher net efficiency.
Specify negative-pulse amplitude and duration from measured relaxation, bound minimum cell voltage, and validate cycle life against controls. Weijiang can provide the relaxation and mismatch data needed to make that call. The next paper steps back to compare the three fundamental current regimes - constant current, constant voltage and constant-current/constant-voltage - and why NiMH favours the first.
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.
