Paper A of this series established the battery load profile inside marine AIS and GMDSS handheld terminals: long standby windows, short high-current transmit pulses, and a statutory primary battery that must be ready after years in a locker. This Paper B moves from principles to selection and pack design. It is written for the OEM engineer and the marine equipment buyer who must specify a rechargeable battery pack and charging interface for a daily, on-board or training handheld VHF radio or portable AIS device without confusing that product with the sealed, dated distress battery that the Global Maritime Distress and Safety System (GMDSS) actually requires. The duty-cycle arithmetic, capacity figures and voltage curves in this paper are worked engineering examples clearly marked illustrative; the standards and endurance anchors are cited to the source documents, and the certified value on any real appliance always appears in its type-approval certificate.
Key fact 1. IMO Resolution MSC.149(77) and IEC 61097-12 size the survival-craft two-way VHF primary battery for eight hours at the 1:9 duty cycle - six seconds of transmission, six seconds of reception above the squelch opening level and 48 seconds of reception below it - at the highest rated power. UK MSIS guidance recommends at least eight hours and preferably 24 hours.
Key fact 2. The AIS-SART is governed by IEC 61097-14 aligned with IMO MSC.246(83), using ITU-R M.1371 on AIS channels 161.975 MHz and 162.025 MHz; representative commercial AIS-SART samples transmit for about 96 hours on sealed primary lithium, and personal AIS-MOB samples run for about 48 hours at 1 W.
Key fact 3. A sealed NiMH cell has a nominal voltage of 1.2 V, so six cells in series give 7.2 V and eight cells give 9.6 V; pack capacity is sized from the duty-cycle average current and the cold end-of-life voltage, not from the transmit peak current alone.
Key fact 4. Sealed portable NiMH cells are specified by IEC 61951-2 for performance, IEC 62133-1 for nickel-system safety and UN 3496 for transport, with IATA special provision A199 confirming that NiMH is not subject to the lithium dangerous-goods restrictions; marine handheld radios are commonly sealed to IPX7 under IEC 60529 (1 m immersion).
Key fact 5. The statutory, dated primary-lithium distress battery cannot be replaced by a rechargeable NiMH pack. For example, the Icom BP-234 is a 9.0 V, 3300 mAh primary-lithium pack rated IPX7 when installed, and it must be used with the IC-GM1600E to meet GMDSS. Sealed NiMH serves the daily, on-board and training radios that live in charging cradles.
The single most important selection decision happens before any capacity calculation: deciding which battery the pack actually is. The sealed distress appliance - the survival-craft two-way VHF carried for the liferaft, the AIS-SART mounted in its bracket, and the AIS-MOB beacon on the lifejacket - is governed by a statutory performance standard and is shipped with a sealed, dated primary-lithium battery carrying a witness or non-replaceable seal. That battery must be ready after two or more years of stowage for the VHF and typically six to eight years on representative AIS beacons, and it is replaced on its expiry date. It is not rechargeable, and a rechargeable NiMH pack must not be marketed as its substitute.
The legitimate rechargeable application is everything around that sealed appliance: the daily handheld VHF used on the bridge, deck and workboat, the portable AIS carried for pilot transfer and mooring, the training fleet, and the "full kit" GMDSS handhelds that manufacturers ship with a rechargeable pack and a cradle alongside the sealed primary. These radios are charged, exercised, swapped and re-used every day, and the GMDSS harmonization guidance positively encourages frequent hands-on use. Sealed NiMH is well suited to this role because it is tolerant of abuse and shallow discharge, gives a flat, predictable pulse response, contains no lithium for transport purposes, and is recoverable after the cold soak that would deeply trouble a chemistry with a higher impedance. The table below fixes the boundary before the design work begins.
| Device and role | Battery chemistry and design basis |
|---|---|
| Survival-craft two-way VHF (sealed distress unit) | Sealed primary lithium; witness seal and date; 8 h at 1:9 per MSC.149(77)/IEC 61097-12; multi-year shelf; e.g. Icom BP-234 9.0 V / 3300 mAh, IPX7 |
| AIS-SART and AIS-MOB statutory beacons | Sealed primary lithium; about 96 h SART and 48 h MOB samples; per IEC 61097-14, MSC.246(83), ITU-R M.1371; multi-year stowage |
| Daily / on-board / training handheld VHF | Rechargeable sealed NiMH or Li-ion; cradle charged, cycled and exercised; sized to the shift duty, not to years of stowage |
| Portable / handheld AIS for bridge and workboat | Rechargeable sealed NiMH or Li-ion; vessel or cradle powered in service; same pulsed standby-to-transmit profile as the survival radio |
| Alkaline / dry-cell fallback case | Many radios accept a sealed AA case (e.g. LR6 x 5); useful emergency fallback but not a substitute for the dated statutory primary |
A handheld VHF draws three very different currents. In transmit it must deliver the current behind the RF output - about 1 A from a representative 2 W handheld - plus the logic and audio loads. In receive above the squelch opening level it may draw roughly 120 to 200 mA, and in the quiet, below-squelch or standby state it may draw only 20 to 50 mA. The IEC 61097-12 survival-craft duty frame is a convenient, conservative model for the daily radio as well: six seconds transmit, six seconds receive above squelch, and 48 seconds below, repeating every 60 seconds. The duty-cycle average current is the weighted sum of these states over one frame:
I_avg = (6 s x I_tx + 6 s x I_rx + 48 s x I_stdby) / 60 s
Using illustrative values of 1.0 A transmit, 0.15 A active receive and 0.03 A standby, the average is (6 x 1.0 + 6 x 0.15 + 48 x 0.03) / 60 = 8.34 / 60 = 0.139 A, or about 139 mA. The gross charge the pack must deliver is then the average current times the endurance target. The statutory VHF target is eight hours, while the UK MSIS guidance for operational survival radios is at least eight hours and preferably 24; a sensible daily-pack designer uses the 24-hour figure as the preferred target, because a daily radio that is not always returned to its cradle should still survive a long watch. At 139 mA, eight hours needs about 1.11 Ah of gross charge and 24 hours needs about 3.34 Ah. These are pre-derating numbers; the cell capacity chosen in Step 3 will be larger.
Figure 1. The six-step selection and sizing flow for a rechargeable marine handheld pack (illustrative design sequence, not a manufacturer's procedure).
Before choosing capacity, the series string length has to match the radio's voltage window and its low-battery cutoff. A sealed NiMH cell is nominally 1.2 V, reaches about 1.42 V at the end of a fresh charge and is regarded as discharged near 1.0 V. Most commercial marine handhelds are built around one of two voltage classes. A 7.2 V class radio, which is the same nominal voltage as a two-cell Li-ion pack (the Icom BP-265 is a 7.2 V Li-ion example), maps to six NiMH cells in series; the six-cell pack runs from about 8.5 V fresh down to about 6.0 V empty. A 9 V class radio, including designs whose statutory primary is the 9.0 V Icom BP-234, maps to eight NiMH cells in series, running from about 11.4 V fresh down to about 8.0 V empty. Parallel strings are then added only for capacity and current sharing, not for voltage.
| NiMH configuration | Nominal / fresh / empty voltage | Typical radio class |
|---|---|---|
| 6S (6 cells series) | 7.2 V / 8.5 V / 6.0 V | 7.2 V class handheld VHF (common 2S Li-ion equivalent); cutoff must sit below the cold 6S pulse voltage |
| 8S (8 cells series) | 9.6 V / 11.4 V / 8.0 V | 9 V class radios and 9 V primary replacements (e.g. 9.0 V BP-234 class) |
| Add parallel strings (nP) | Voltage unchanged; capacity and pulse current shared | 2P, 3P used to reach the shift capacity and to halve per-cell pulse current |
The pack designer must verify the radio's actual cutoff rather than assume it. A cutoff that is too high will declare the battery dead while usable capacity remains; a cutoff that is too low risks deep-discharging cells during a long cold transmit. Cells in the string should be matched from one production batch, and the pack should be built so that one weak or reversed cell cannot be driven into deep reverse during a long shift.
The gross charge from Step 1 is not the nominal cell capacity, because the pack must deliver that charge at the end of its life, on a cold deck, with a transmit pulse sagging the voltage and a design reserve held back. Applying illustrative factors - about 0.85 usable capacity at -20 degrees C, about 0.85 at the rated end-of-life point, and a 1.15 design reserve - gives a combined usable fraction of roughly 0.63. The eight-hour target of 1.11 Ah therefore needs a nominal pack of about 1.8 Ah, and the preferred 24-hour target of 3.34 Ah needs about 5.3 Ah. In practice a designer reaches these with standard high-rate AA cells or larger sub-C and SC cells: a single six-cell string of high-rate AA cells (about 2.0 Ah) only marginally meets the eight-hour figure after cold and life derating, so a 6S2P AA pack (about 4.0 Ah) is the comfortable eight-hour design, and a 6S3P AA pack (about 6.0 Ah) or a 6S2P high-capacity sub-C/SC pack meets the preferred 24-hour figure.
| Endurance target (illustrative) | Gross charge at 139 mA | Nominal after ~0.63 factor | Representative pack |
|---|---|---|---|
| 8 h (statutory VHF floor) | ~1.11 Ah | ~1.8 Ah | 6S1P high-rate AA marginal; 6S2P AA (~4.0 Ah) comfortable |
| 24 h (preferred operational) | ~3.34 Ah | ~5.3 Ah | 6S3P AA (~6.0 Ah) or 6S2P high-capacity sub-C/SC |
Transmit pulses are the second reason to oversize. On a 1 A, 7.2 V string, a single 2.0 Ah AA cell is being asked for roughly 0.5 C in a transmit burst; that is within the capability of a high-rate cell but produces a visible voltage dip at low temperature, and the dip is deepest at the end of the shift, when the state of charge is already low. The designer therefore specifies high-rate, low-internal-resistance cells, adds a parallel string to halve the per-cell pulse current, and verifies that the pack voltage at the end of the coldest, end-of-life transmit pulse still sits above the radio cutoff. The illustrative voltage curve below shows the pack slowly declining across a 24-hour watch with periodic transmit dips, while remaining above the cutoff; the real curve is radio- and cell-specific and must be measured rather than assumed.
Figure 2. Pack voltage across a 24-hour shift with periodic transmit pulses; the pack is sized to stay above the cutoff on the coldest, end-of-life pulse (illustrative, not measured).
The charging strategy is part of the battery product, not an afterthought. NiMH full-charge detection relies on the characteristic -delta V voltage drop and the dT/dt temperature rise, backed by a safety timer; for a marine cradle, where reliability matters more than speed, the conservative method is a controlled 0.1 C overnight charge (roughly 14 to 16 hours) with a maintenance trickle of C/20 to C/40 thereafter, which is gentle enough to leave the radio docked without overheating the cells. Charging should be inhibited outside roughly 0 to 45 degrees C, so a pack that has come in from a frozen deck or a hot liferaft container is allowed to return to ambient before charging; the pack normally carries a thermistor so the cradle can enforce this. The cradle accepts the ship's 12 V or 24 V DC through a charging module, uses corrosion-resistant or gold-plated contacts that will not seize in salt air, and maintains the IPX7 seal when the radio is docked.
Inside the pack, cells use welded tabs rather than spring clips, matched cells, heat-crinkled sleeves and shock isolation so the drop and vibration tests of IEC 60945 and IEC 61097-12 do not open a joint; a thermal fuse or resettable protective device guards against external short and overcurrent, and the enclosure uses a gasket, a venting path and robust cell retention. The external envelope is the same as the survival radio: operating range of -20 to +55 degrees C, immersion to the IPX7 requirement of IEC 60529 (1 m for 30 minutes; the survival set itself meets 1 m for at least 5 minutes under IEC 61097-12), salt-mist resistance, and operation by personnel wearing immersion-suit gloves. Every one of these features is specified so the daily radio behaves like the safety appliance it backs up.
A rechargeable NiMH pack for daily marine radios follows a clear but deliberately limited compliance path. The cells themselves are specified to IEC 61951-2 for performance and dimensions of sealed portable NiMH cells and to IEC 62133-1 for safety of nickel-containing sealed cells and batteries; the equipment envelope is IEC 60945 for environment and EMC and IEC 60529 for the IP code. Transport is straightforward: NiMH batteries ship under UN 3496, and IATA special provision A199 treats them as not subject to the lithium dangerous-goods rules, provided terminals are protected against short circuit - a significant practical advantage over lithium for an OEM that air-freights radios and spares. The statutory distress appliance itself, however, carries the MED wheel mark under the European marine equipment Directive 2014/90/EU, and that approval is attached to the complete sealed appliance with its dated primary battery. A rechargeable NiMH pack does not extend or replace that wheel-marked distress source, and the two must not be conflated on a drawing, a data sheet or a marketing claim.
| Aspect | Reference for the rechargeable NiMH daily pack |
|---|---|
| Cell performance and dimensions | IEC 61951-2 (sealed portable NiMH cylindrical, prismatic and button cells) |
| Cell and battery safety | IEC 62133-1 nickel-system safety; transport testing aligned with IEC 62281 practice |
| Environment and ingress | IEC 60945 marine environment/EMC; IEC 60529 IPX7 (1 m / 30 min); salt mist, -20 to +55 degrees C |
| Transport | UN 3496; IATA special provision A199 - not subject to lithium restrictions; protect terminals |
| Statutory distress boundary | MED wheel mark under Directive 2014/90/EU stays with the sealed appliance and its dated primary lithium; not replaced by NiMH |
No. The sealed survival-craft two-way VHF is required by IMO MSC.149(77) and IEC 61097-12 to carry a dated primary battery that delivers eight hours at the 1:9 duty after multi-year stowage, typically a sealed primary-lithium pack such as the Icom BP-234 (9.0 V / 3300 mAh, IPX7 when installed). Sealed NiMH is designed for the daily, on-board, training and full-kit radios that are recharged in cradles and exercised regularly, not for the statutory sealed distress source.
Start from the duty-cycle average current over one 60-second frame: I_avg = (6 x I_tx + 6 x I_rx + 48 x I_standby) / 60. With illustrative currents of 1.0 A, 0.15 A and 0.03 A this is about 139 mA. Multiply by the endurance target (8 h, or preferably 24 h per UK MSIS guidance), then divide by the combined cold-temperature, end-of-life and design-reserve factor, which is about 0.63 in the worked example. This yields a nominal pack of roughly 1.8 Ah for 8 h or 5.3 Ah for 24 h, before choosing a standard cell configuration.
Each sealed NiMH cell is nominally 1.2 V. Six cells in series give 7.2 V (about 8.5 V fresh, 6.0 V empty) and suit a 7.2 V class handheld, the same nominal class as a 2S Li-ion pack. Eight cells give 9.6 V (about 11.4 V fresh, 8.0 V empty) and suit a 9 V class radio such as the 9.0 V primary-BP-234 class. Parallel strings are added only for capacity and pulse-current sharing, and the radio's low-battery cutoff must be checked against the cold end-of-life pack voltage.
Yes, with deliberate derating. Low temperature raises internal resistance and lowers the available capacity and the end-of-pulse voltage, so designers use high-rate, low-resistance cells, oversize the pack, add parallel strings to reduce per-cell pulse current, and confirm the radio never hits its cutoff during the coldest, end-of-life transmit. Packs should be charged only after returning to roughly 0 to 45 degrees C, not while frozen.
Not for the statutory function. AIS-SARTs under IEC 61097-14 and MSC.246(83) are designed for about 96 hours of transmission and personal AIS-MOBs for about 48 hours, after years of unexercised stowage, which is why commercial samples use sealed primary lithium (for example McMurdo SmartFind S5A, AMEC PLOMO-500 and SAILOR 5052). A rechargeable chemistry cannot guarantee that readiness; rechargeable AIS is confined to powered, vessel- or cradle-charged portable transponders.
Use IEC 61951-2 for sealed portable NiMH cell performance, IEC 62133-1 for nickel-system safety, IEC 60945 for marine environment and EMC, IEC 60529 for IPX7 ingress, and UN 3496 with IATA special provision A199 for air transport, which does not impose the lithium dangerous-goods restrictions. The complete distress appliance itself is MED wheel-marked under Directive 2014/90/EU; that approval is not carried over to the rechargeable pack.
Standards and performance anchors: IMO Resolution MSC.149(77) and IEC 61097-12 (survival-craft two-way VHF; 8 h at 1:9; primary shelf life); IMO Resolution MSC.246(83) and IEC 61097-14 (AIS-SART); ITU-R M.1371 (AIS technical characteristics, 161.975 / 162.025 MHz); IEC 61108 (GNSS); IEC 62287 (shipborne Class A/B AIS); IEC 60945 (marine navigation and radio equipment environment and EMC); IEC 60529 (IP code, IPX7); IEC 61951-2 (sealed portable NiMH cells); IEC 62133-1 (nickel-cell safety); UN 3496 and IATA special provision A199 (NiMH transport); European Directive 2014/90/EU (marine equipment, MED wheel mark); UK MSIS Chapter 12 (8 h minimum, 24 h preferred guidance).
Representative product data used to anchor the chemistry boundary: Icom BP-234 (9.0 V, 3300 mAh, primary lithium, IPX7, for use with IC-GM1600E), Icom BP-265 (7.2 V Li-ion), Icom BP-291 (LR6 AA x 5 case), McMurdo SmartFind S5A AIS-SART (about 96 h, multi-year storage), AMEC PLOMO-500 AIS-SART, and SAILOR 5052 AIS-SART. All capacity, current and voltage calculations in this paper are worked engineering examples marked illustrative; the actual certified values for any appliance are those in its type-approval certificate and data sheet.
Weijiang Power manufactures sealed NiMH and Li-ion battery packs and charging cradles for OEM marine and industrial handhelds, including rechargeable packs for daily and training handheld VHF and portable AIS radios. We size each pack from the duty-cycle average current, match the 6S or 8S series string to the radio's voltage window, specify high-rate cells for cold transmit pulses, and design the cradle around -delta V/dT-terminated or 0.1 C charging, IPX7 sealing and salt-air resistance, following the IEC 61951-2, IEC 62133-1 and UN 3496 (IATA A199) path. We do not replace the sealed, dated primary-lithium distress battery required by the GMDSS; we build the rechargeable hardware that keeps crews trained and the everyday radio fleet working. Send us the radio band, transmit power, duty cycle, voltage window, temperature range and endurance target, and we will size the pack and charging interface together. In Paper C we turn to the type testing, certification and transport regime that the finished pack and its appliance must pass.
