Designing the storage bank for a floating monitor means converting the architecture of the previous paper into concrete numbers and a physical package that survives salt water. The designer must choose how many cells and in what arrangement, how much stored energy buys the target number of overcast days, how the pack supplies motor and radio pulses without sag, how the solar charge controller treats a NiMH bank differently from a lithium one, and how the whole assembly is built to resist the humidity, condensation and corrosion that destroy lesser packs on the water. This second paper works through those decisions for nickel-metal hydride: translating a seven-day autonomy target and a twelve-volt-style rail into a series-parallel pack, sizing the local pulse reservoir, setting a marine-appropriate charge regime, selecting low-self-discharge cells, designing corrosion-resistant and vibration-proof construction, and adding the redundancy and monitoring that let a buoy keep streaming trustworthy hydrology data even when a string or a cell begins to age.
Sizing starts from the autonomy requirement rather than the rail. Take the cycle-averaged load in watts, multiply by twenty-four hours and by the target overcast-day count - seven in many commercial buoys, more in high-latitude or monsoon deployments - to obtain the watt-hours the bank must deliver with negligible recharge, then divide by usable depth of discharge, the per-cell watt-hour and a derating for temperature and calendar ageing. The result is the minimum parallel capacity; the series count is set separately by the system rail.
A nominal twelve-volt bus is naturally built from ten NiMH cells in series (10 times 1.2 V), with parallel strings added for capacity; lower-voltage logger rails use fewer cells with a converter. Because NiMH end-of-discharge voltage per cell is near one volt, the designer verifies the regulator still operates at the bottom of the pack, not at nominal. Building in one or two extra parallel cells' worth of energy is cheaper than a boat trip to rescue a buoy that browns out in its first bad week.
Multi-cell NiMH packs must be assembled from matched cells because series strings are only as strong as their weakest member: capacity and impedance mismatch causes some cells to reach empty first and others to absorb disproportionate overcharge during the solar top-up. Procuring matched, sorted cells and using consistent welded interconnects keeps string behaviour uniform. NiMH is more forgiving of overcharge than lithium-ion, which is a genuine advantage in a simple marine charge system, but matching still sets the achievable cycle life.
Parallel strings increase capacity and provide graceful degradation - if one string weakens the others carry the load - while the series count defines voltage. The pack design documents cell orientation, tab welding, insulation between strings and accessible test points so that ageing strings can be diagnosed during maintenance visits. This modular, matched construction is what lets a buoy bank deliver its rated autonomy year after year rather than only when new.
The average load sizes the energy bank, but the starting inrush of a wiper motor or pump and the transmit pulse of a cellular radio can briefly demand several times that average. As in sensor-node design, a local reservoir - a supercapacitor or low-ESR capacitor bank held at the rail - supplies the instantaneous pulse while the NiMH strings recharge it between events. Sizing uses the pulse energy and the maximum allowable voltage droop, and the reservoir is placed physically close to the load to minimise loop impedance.
This matters more on a buoy than on land because a weak cellular signal at the water's edge makes the radio transmit longer and at higher power, precisely when a cold or partly discharged NiMH pack has its highest impedance. A correctly sized reservoir prevents logger resets and failed uploads without forcing the entire bank to be oversized for a pulse lasting a fraction of the cycle, saving weight and volume in a platform where both are constrained.
A marine charge controller for a NiMH bank differs from a lithium battery management system: there is no need for the tight per-cell voltage clamping lithium-ion requires, but the controller must still limit charge current to the cell capacity, terminate or taper the full charge using -delta-V, temperature rise or a safe timer, prevent night-time reverse current, and manage the frequent shallow top-ups that are the normal condition on the water. A temperature-compensated, conservative profile protects the bank through the heat of a floating enclosure.
Because the platform is repeatedly, shallowly recharged rather than cycled hard, the controller is tuned to keep the bank in its healthy middle band and to treat full charge as an occasional event rather than a daily target. Low-temperature charge lockout is set to the cell's rated limit; notably NiMH can accept some charge below freezing where lithium-ion cannot, an advantage in cold-water deployments that is explored further in the weather-station paper. The controller also reports state of charge and health so the shore platform can flag a degrading bank before it causes a data gap.
The mechanical package is where marine packs live or fail. Cells sit behind the IP68 hull barrier in a dry, drained compartment; interconnects are welded rather than spring-loaded to survive continuous motion; busbars and tabs use corrosion-resistant plating; every connector is rated for the environment and protected against condensation; and the assembly is conformal-coated or potted where appropriate to stop salt creep. Potting also fixes cells against vibration and conducts heat toward the hull.
Thermal placement avoids parking the pack against sun-heated dark surfaces, since sustained heat accelerates NiMH calendar ageing, while still allowing the modest warmth of charge to escape. Humidity barriers, desiccant in the compartment and a vent that equalises pressure without admitting water complete the package. The discipline is to treat corrosion and condensation as design variables from the first layout, not as field problems to be discovered after deployment.
Because a buoy is expensive to visit, redundancy is designed in: parallel strings degrade gracefully, a separate reserve can hold the logger and last-known-data through a fault, and the controller logs per-string voltage and temperature so an ageing cell is predicted rather than experienced. Maintenance intervals are then set from real state-of-health data, with the NiMH bank's long cycle life keeping those intervals long.
The validation plan replays a worst-case scenario on the bench: a programmable source mimics seven dim solar days, electronic loads replay sonde, wiper and radio pulses at cold and warm temperatures, and the pack is required to hold the rail through every upload and to recover fully when insolation returns. Passing that replay - together with the formal marine qualification in the final paper - is what turns a collection of cells into a storage bank an operator can trust on the water for years.
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.
