A radio battery that survives a year of dispatch duty is a small, disciplined piece of engineering: six matched high-rate cells, low-resistance welded interconnects, protection that never starves a transmit pulse, a housing that latches exactly like the original, and a charging strategy that keeps three shifts of operators on the air. This second paper converts the 5-5-90 load profile into a concrete nickel-metal hydride pack design for professional land-mobile radios. It covers the selection and matching of high-rate cells, the mechanical and electrical construction of a drop-in replacement pack, the capacity-versus-weight decision that matches pack to job, the charge management and multi-shift rotation that prevents radio downtime, and the special construction rules for packs used in intrinsically safe radios in explosive atmospheres. It closes with the validation programme that proves talk-time, pulse behaviour and cycle life, setting up the standards and certification campaign of the final paper.
Cell selection starts from the transmit pulse, not the standby current. High-rate NiMH cells are chosen for low internal resistance and proven ability to deliver repeated 1-2 A pulses with minimal voltage depression and limited heating, rather than for the largest capacity number alone. Cells are drawn from a consistent production lot and graded for capacity and impedance so the six-cell series string charges and discharges evenly; a single weak cell in a series pack dictates end-of-charge and end-of-discharge for the whole battery and is the usual cause of premature 'low battery'.
Matching is documented, not assumed: cells are measured on formation and grouped within tight impedance and capacity windows. For high-capacity packs (toward 2600 mAh) the designer balances the larger cell's slightly higher impedance against the runtime gain, confirming by pulse test that the chosen cell still holds the rail at the radio's transmit current at low temperature and after cycle ageing.
Inside the pack, cells are joined by nickel tabs welded directly to the cell terminals - never relying on spring pressure alone for the high-current transmit path - with tab cross-section sized to keep interconnect resistance and heating negligible. The pack includes a thermal cut-out or PTC and, where the radio design requires it, a protection arrangement that guards against external short circuit and over-temperature without introducing enough impedance to sag the transmit rail.
This is a careful balance: over-aggressive protection that trips on a legitimate transmit pulse cripples the radio, while under-protection is unsafe. The design is validated by capturing the rail waveform during long keyed transmissions and rapid key-up/key-down cycles, confirming the voltage stays within the radio's operating window and the pack surface temperature stays within limits. The first animated figure steps through the pack construction layers; the second lays out the design-to-validation flow.
A replacement pack must be mechanically invisible to the user: identical latch geometry, the same belt-clip and contact positions, the same dimensions and a satisfying click into the radio's battery well, because field staff swap packs in gloves, in the dark and under time pressure. The housing uses impact-resistant engineering plastic with ribbing that protects cells from drop energy, and the gold- or nickel-plated contact springs maintain low-resistance contact through years of hot-swapping and vibration.
Ergonomics matter because the pack is worn all shift: weight distribution and the absence of sharp edges reduce fatigue, and a robust latch prevents the pack separating during a fall. Sealing to the radio's ingress rating (many industrial radios are IP67/IP68) requires the pack gasket and moulding to match the original enclosure, so the replacement pack preserves rather than compromises the radio's water and dust protection.
The same radio family is usually offered in several pack capacities, and the right choice is role-specific. A light user - a facilities technician who mostly listens - is best served by a slim, lighter pack around 1200-1700 mAh; a heavy dispatcher or a worker facing a long shift away from a charger benefits from a 2000-2600 mAh high-capacity pack despite its extra mass. Standardising on one capacity per role class simplifies fleet management and spares.
Because NiMH has no meaningful memory problem in modern formulations and tolerates opportunity charging, packs can be topped up between calls without harm - a practical advantage over the old NiCd discipline that required full discharge. Low-self-discharge cells further mean spare packs held on a shelf as emergency backups remain serviceable, reducing the number of batteries a fleet must keep in rotation.
Continuous operations run more shifts than radios, so the battery fleet is sized in multiples: a common rule is three packs per radio - one in the radio, one charging, one ready and cooling - so every shift change is a quick swap rather than a wait at the charger. The charger must be matched to NiMH: controlled current with -delta-V or peak-voltage termination, temperature sensing, a timer backstop and individual bay control, so packs in a multi-bay charger terminate independently.
NiMH's ability to accept some charge in the cold is useful in yard and winter operations, where a lithium pack would have to warm before charging; nevertheless, fast charging is reserved for the rated temperature range to protect cycle life. Marking and pairing packs, rotating them so all see equal use, and retiring them at the cycle-life horizon keeps delivered talk-time consistent across the fleet and makes a failing pack predictable rather than a surprise on shift.
Radios used where flammable gas, vapour or dust may be present - refineries, chemical plants, grain handling - must be intrinsically safe under the IEC 60079 series and ATEX/IECEx schemes, and the battery is a named, controlled element of that certificate. Intrinsic-safe NiMH packs constrain short-circuit current and surface temperature, use specified cells and fusing, and are sealed so an internal or external fault cannot produce an ignition-capable spark or hot surface; the user is required to use only the certified pack, which is why the pack label and construction are tightly controlled.
Changing cell supplier or internal construction in an Ex pack can invalidate the certificate, so an intrinsic-safe programme demands a stable, documented cell source with consistent electrical and thermal characteristics - an argument for partnering early with a battery manufacturer that understands Ex documentation. Validation of any pack, standard or intrinsic-safe, then proceeds to pulse and talk-time testing, thermal and short-circuit tests, cycle life and mechanical abuse, feeding directly into the certification evidence the final paper details.
Weijiang Power designs and manufactures sealed nickel-metal hydride cells and matched industrial packs for remote, off-grid and safety-related equipment, and supports OEM partners with IEC 61951-2 performance files, IEC 62133-2 safety evidence, pulse-load characterisation, wide-temperature testing and charger/pack co-validation. Tell us your duty cycle, peak current, temperature envelope, autonomy target and the standards your product must meet, and our engineers will specify a cell-and-pack combination that protects runtime, reliability and service life. Review the range on the products page.
