Most Maritime Shipping Battery Propulsion Studies Are Already Obsolete

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Most maritime battery studies are already obsolete. That is not a criticism of the researchers who wrote them. It is a recognition that their assumptions were grounded in the battery costs and energy densities available at the time. Several of the most detailed recent merchant shipping studies modeled battery system costs in the $300 to $500 per kWh range and, in at least one prominent industry study, installed battery room densities of 30 to 50 kWh per cubic meter.

In 2025, large Chinese BESS tenders were clearing near $65 per kWh at auction and containerized DC blocks are reaching roughly 6.25 MWh in a 20 foot container. A standard 20 foot container has an internal volume of about 33 cubic meters. Dividing 6,250 kWh by 33 cubic meters yields approximately 190 kWh per cubic meter, which is 190 Wh per liter at the container level. At the pack level, sodium ion announcements suggest 175 Wh per kilogram is plausible. Even if ship hardening reduces that to 140 Wh per kilogram, that corresponds to 7.14 kg per kWh at 140 Wh per kilogram and 5.71 kg per kWh at 175 Wh per kilogram. The physics did not change. The inputs did.

To assess what this means, a consistent modern reference envelope is required. For volumetric density, 150 to 220 Wh per liter at the container DC block level is a defensible band anchored by current containerized BESS products. For gravimetric density, 140 to 175 Wh per kilogram is a reasonable planning range for ship suitable packs in the near term. For cost, $65 per kWh is the floor observed in competitive Chinese BESS tenders. Adding integration, class compliance, and shipboard DC collection and monitoring pushes that to perhaps $80 to $150 per kWh for containerized battery modules delivered to port, and $100 to $180 per kWh for fully installed and classed systems onboard. These are the numbers against which earlier maritime battery studies must now be measured.

This article comes after I tested the theses that underlie my long-term projection of maritime energy demand through 2100, shown in the energy demand graphic, against studies over the past few years. The projection assumes that roughly 40% of today’s fossil fuel bulk trade declines materially as coal and oil shipments fall away, and that about 15% of global iron ore trade enters structural decline as steel recycling increases and electric arc furnaces displace blast furnaces. Those shifts do more than reduce total tonnage. They change the geometry of global shipping. Fewer long-haul fossil cargoes mean fewer ultra-long legs with multi-GWh energy requirements. At the same time, more regional trade, reshoring of manufacturing, and electrification of industrial supply chains increase the relative share of short and medium-distance routes.

I have been iterating my assumptions of 100% electrification of inland shipping, majority electrification of short sea shipping and hybridization of deep sea shipping for several years. That’s been based on early studies and examples, but now more studies and data are in, and more complete assessment can be done. Key questions are whether the studies were aggressive or conservative based on real world metrics, and what that means for their findings.

Inland shipping provides the clearest demonstration of the shift. The Yangtze River 700 TEU electric container ships operating on routes of roughly 1,000 km use swappable containerized batteries. Trade reporting describes roughly 36 battery containers distributed across the route for two vessels operating in a pool model The vessels carry only the containers required for a given leg, while the majority of the battery inventory remains ashore charging at terminals.

If a given leg requires, for example, 50 to 60 MWh of usable energy, the onboard battery mass at 140 Wh per kilogram would be roughly 360 to 430 tons, and at 175 Wh per kilogram roughly 285 to 345 tons. On a 700 TEU vessel displacing many thousands of tons, that is very manageable. The swap model decouples charging time from berth time. Containers are winched off, charged ashore over many hours at modest power, and swapped back on. The constraint shifts from onboard storage density to yard throughput and grid connection capacity. Because only a fraction of the battery pool is onboard at any time, vessel deadweight and TEU penalties are driven by leg energy, not by total route inventory.

The Moon et al Nature Energy 2025 paper on the United States domestic fleet assumed battery system weights of roughly 20 to 21 kg per kWh, equivalent to about 48 Wh per kilogram. That is three to four times heavier than modern pack level densities. Their modeling still found that a large share of domestic vessels under 1,000 gross tons could electrify cost effectively under reasonable carbon pricing and grid decarbonization assumptions. Replacing 21 kg per kWh with 6 to 7 kg per kWh reduces system mass by about 70%. That materially lowers draft penalties and vessel retrofit constraints. The LBNL and MARAD 2023 report made similar conservative assumptions on mass and cost. Under 2025 battery densities and prices, inland and harbor craft electrification becomes easier than those papers projected. This doesn’t eliminate the real headwinds facing maritime vessel decarbonization in the United States given the Jones Act and deindustrialization impacts that effectively killed the commercial ship building industry in the country, and the challenges related to higher battery prices and lower battery availability.

Short sea ferry and RoPax studies reinforce the pattern. Katumwesigye et al in the Journal of Cleaner Production modeled a 110 MWh fully electric RoPax ferry on the Helsinki to Tallinn route. They used conservative state of charge windows and replacement assumptions. Under modern pack densities and containerized battery logistics, the same 110 MWh requires 785,714 kg at 140 Wh per kilogram or 628,571 kg at 175 Wh per kilogram. That is 629 to 786 tons of battery mass. For a large RoPax vessel, that is significant but not prohibitive. If containerized battery swapping is integrated into port operations, charging power constraints diminish and usable state of charge windows can widen. Battery only operation on fixed short sea ferry routes is not speculative. It is a practical engineering problem with a capital solution.

Recent developments in the ferry sector illustrate how quickly electric propulsion is moving from experimentation toward very large commercial application. In May 2025 an Australian shipbuilder launched the China Zorilla, a 130 metre fully electric fast ferry designed to carry up to 2,100 passengers and 225 vehicles across the River Plate between Buenos Aires and Uruguay. Its battery system exceeds 40 MWh and, at roughly 250 to 275 tons of batteries, is by far the largest battery installation yet put into a single vessel of any kind, driving eight electric waterjets for propulsion and marking a milestone in battery-electric shipbuilding. The Viking Line Helios project has been announced to carry about 2,000 passengers and 650 cars entirely on battery power between Helsinki and Tallinn in the early 2030s. Roughly 70% of new ferry orders globally now feature electric drivetrains, many of them scaled up for high passenger and vehicle counts on fixed short sea routes. Those examples show that ferries have leapfrogged smaller electrification niches to become a proving ground for large battery systems, and they offer context for the article’s arguments about where battery-dominant propulsion is now credible and where hybrid remains the structural choice.

Nivolianiti et al. 2025 in Energy Conversion and Management modeled hybrid short-sea passenger vessels combining photovoltaic generation, batteries, and diesel generation, and evaluated multiple storage chemistries including lead-acid, lithium-ion, and nickel-iron. Their optimization favored lead-acid at 80% depth of discharge under their assumed cost and degradation inputs. That result reflects the battery cost and density landscape embedded in the model rather than a structural limitation of electrification. Under a modern envelope of $80 to $150 per kWh containerized battery systems and 140 to 175 Wh per kilogram pack-level densities, the relative advantage of high-cycle-life lithium-ion or sodium-ion systems strengthens considerably, especially when swappable containerized batteries and electrified ports are assumed. The hybrid architecture in their work remains rational for operational resilience and peak smoothing, but the need for substantial diesel contribution is less compelling when battery cost declines by a factor of three and system mass falls by a factor of three relative to older kg per kWh assumptions. Their conclusion that hybridization is optimal for short-sea passenger vessels should be read as conditional on conservative battery economics, not as evidence that battery-only operation is structurally infeasible.

As a key note, one of the most persistent concerns in maritime battery discussions is thermal runaway, and it has had real design consequences. High energy chemistries such as NMC have higher specific energy at the cell level, but they also carry greater risk of exothermic failure propagation, which drives requirements for fire suppression systems, blast protection, gas detection, ventilation, physical separation between modules, and redundant battery rooms. All of that adds weight and volume beyond the cells themselves. In early marine battery installations, the installed system weight per kWh was often two to four times the cell weight equivalent once structural steel, cooling, suppression systems, and access clearances were included. That is why many studies ended up modeling 20 kg per kWh system mass or volumetric densities in the tens of kWh per cubic meter. Lithium iron phosphate changes that balance. LFP has lower energy density at the cell level than NMC, but it has a much higher thermal stability threshold and far lower risk of runaway propagation. Sodium ion pushes that further, with chemistry that is intrinsically less prone to combustion and does not rely on scarce metals. In a maritime context, that matters more than squeezing another 20% out of cell-level Wh per kilogram. If the chemistry reduces suppression, segregation, and blast containment requirements, the installed pack-level mass and volume penalties shrink. The result is that safer chemistries can deliver higher effective system-level energy density once marine safety overhead is accounted for, even if their cell-level numbers are lower than NMC.

The boundary emerges in merchant short sea shipping where sea leg energy demand rises above 200 MWh. The MMMCZCS 2024 pre feasibility study modeled a 1,100 TEU feeder in the Western Mediterranean with a worst case sea leg energy demand of about 320 MWh. At 80% usable state of charge, gross capacity required is 320 divided by 0.8, which equals 400 MWh. Using 6.25 MWh per 20 foot container, that is 64 containers. On a 1,100 TEU vessel, 64 TEU represents 5.8% of nominal slot capacity. The mass at 140 Wh per kilogram is 2,857,143 kg, or 2,857 tons. At 175 Wh per kilogram it is 2,285,714 kg, or 2,286 tons. A 2,300 to 2,900 ton battery mass penalty is meaningful but not fatal. If freight rates are sufficient and if ports are electrified with buffering batteries, 100% battery propulsion is technically viable even at this energy level. The economic sensitivity is real. Hybridization in this regime becomes a design optimization to reduce battery mass and inventory, not a structural necessity.

The MMMCZCS study assumed battery system costs around $300 per kWh and battery room volumetric densities of 29 to 47 kWh per cubic meter. Against a modern containerized DC block density of about 190 kWh per cubic meter, those density assumptions are conservative by a factor of two to four. Against $80 to $150 per kWh for containerized battery modules, $300 per kWh is conservative by a factor of two to four. Their conclusion that an 80% battery and 20% fueled generator hybrid at sea is the most reasonable pathway reflects those inputs. Updating the inputs shifts the balance toward a larger battery share for many short sea routes. Hybrid remains attractive for schedule resilience and adverse weather margins, but it is no longer forced by battery density and cost alone.

CIMAC paper 158 in 2025 reaches a similar hybrid conclusion using comparable density and cost assumptions. Updating those inputs moves the feasibility boundary outward. Kistner et al in Energy Conversion and Management X assumed LFP system level densities of about 154 Wh per kilogram and 152 Wh per liter and battery system costs of €460 to €500 per kWh. The density assumption is not far from modern pack level density. The cost assumption is far above 2025 BESS auction levels. Their conclusion that only relatively short container routes are economically viable shifts substantially under $80 to $150 per kWh. A route that required €460 per kWh to pencil may look very different at $100 per kWh.

Kersey et al in Nature Energy 2022 used a baseline battery cost of $100 per kWh and a volumetric density of 470 Wh per liter, with a near future scenario of $50 per kWh and 1,200 Wh per liter. The cost assumptions align with modern BESS prices. The volumetric density assumptions are aggressive for containerized ship battery systems. A reduction from 470 Wh per liter to 190 Wh per liter increases required battery volume by a factor of about 2.5. That raises cargo forfeiture and draft impacts relative to their baseline. Even so, their identification of draft as a hard constraint for long routes remains valid. On a 5,000 km route, a small neo Panamax ship might require 5 GWh of storage. At 175 Wh per kilogram, 5,000,000 kWh divided by 0.175 equals 28,571,429 kg, or 28,571 tons of batteries. That mass increase materially alters draft and hull resistance. Cost declines do not eliminate mass scaling.

Deep sea hybridization remains structurally necessary for multi day, multi GWh legs. Ocean crossings of 10,000 km at typical container ship power levels can require 8 to 12 GWh of propulsion energy. Even at 175 Wh per kilogram, 10 GWh of storage weighs 57,142,857 kg, or 57,143 tons. That exceeds the deadweight of many mid size container ships and drives draft beyond safe limits. Containerized swapping mid ocean is capital intensive and logistically complex. Ports can electrify coastal corridors, but they cannot dissolve ocean distance. Hybrid architectures for deep sea ships will likely combine batteries for port entry, maneuvering, and substantial portions of journeys with more expensive biomethanol or ethanol for mid-ocean segments. My assumption is that Kersey et al’s draft limitations, adjusted for actual energy density, will be the factor that determines the ratio between battery electric and liquid fuel as a propulsion energy source.

Ports become the central constraint for inland and short sea electrification. The real bottleneck is not battery chemistry but megawatts at the quay. Buffering batteries at ports smooth ship charging loads into steady grid draws. Containerized batteries charging 24/7/365 while port operations go up and down. High capacity interconnects and offshore wind HVDC links can deliver low carbon power at scale to coastal hubs. Solar installations across warehouses and terminals shave daytime load. In my port electrification roadmap, I modeled phased port electrification including buffering batteries, HVDC from offshore wind, maximized local and nearby solar, and high capacity grid connections. That analysis showed that port side storage reduces peak demand charges and grid upgrade requirements by shifting ship charging into predictable load profiles. With ports designed as energy hubs, swap container models become viable at scale.

The final synthesis is straightforward. Inland shipping is firmly in the battery only column under 2025 density and cost metrics. Short sea shipping up to about 200 MWh per sea leg is strongly battery only with swap and electrified ports. Between 200 and 400 MWh per leg, battery only is technically feasible but economically sensitive, and hybridization appears as a pragmatic choice rather than a physical requirement. Deep sea shipping remains hybrid because mass and draft scale linearly with energy demand and ocean distances remain long. The maritime battery studies of the past five years were not wrong. They were written for a battery market that no longer exists. Updating the inputs shifts the electric and hybrid boundary outward, but it does not eliminate the ocean.