Are ultra light batteries in electric boating’s future?

A UK battery company and US electric plane pioneer are teaming up to explore the potential of ultra light batteries that halve the weight needed to hold the same energy as the best lithium-ion cells.

The lithium-sulfur batteries developed by Oxis Energy in the UK have a power density (technically called a ‘high gravimetric energy density’) of 500 watt hours per kilogram. That’s double the density of the advanced Tesla 2170 lithium-ion cells used in the Model 3.

Unlike with many other battery ‘breakthroughs’, though, the Oxis batteries already exist. In fact lithium-sulfur batteries have been around since the 1960s, but Oxis has overcome some of the technical issues that have held them back.

Cycle life and safety issues have been overcome

lithium-sulphur battery pouch with a nail through itThe shortcomings of lithium-sulfur have been cycle life and safety. The chemical interaction between the cathode and the electrolyte during charging and discharging would eventually melt away the cathode, and the batteries were prone to overheating.

According to an article published by the the IEEE (Institute of Electrical and Electronics Engineers)  “Oxis says its design incorporates a ceramic lithium sulfide as a ‘passivation layer’, which blocks the flow of electricity—both to prevent sudden discharge and the more insidious leakage that can cause a lithium-ion battery to slowly lose capacity even while just sitting on a shelf. Oxis also uses a non-flammable electrolyte.” The company has been granted 186 patents with 87 pending, although not all are specifically for this product.

Ready to be tested in electric planes

The batteries come in a pouch format, and Oxis has put them through puncture and bullet tests that prove their safety, as well as 250 charge/discharge cycles. They think they can get it to 500 cycles within a couple of years.

They have now announced their collaboration with Bye Aerospace of Colorado. Bye developed and markets the eFlyer, a two seat electric airplane that is the first FAA-Certified all electric craft.

The companies have begun a 12-month collaborative program. With the reduced battery weight they are  exploring what they hope could be a doubling of air time from a single charge.

So what does this have to do with electric boats?

It wouldn’t be the first time learning from electric airplanes might be applied to electric boats. Australia’s WaveFlyer electric jetski came from the collaboration of the Electro.Aero company with a team at  University of Western Australia.

The uniting factor is energy density. The weight of batteries obviously plays an important role in keeping a plane in the air, but it’s also important in keeping a boat on top of the water, and for a planing boat less hull in the water means less drag, less energy consumption, longer range and/or higher speeds.

Electric motors are doing a terrific job for displacement hulls and smaller craft, but the physics of the weight/energy ratio continues to be a big consideration for the planing hulls.

An electric boat 'flies' above the water on its hydrofoils 'One of the approaches for reducing drag in electric boats is hydrofoiling, like the Candela electric speedboat and SeaBubbles electric water taxi which lift their hulls completely out of the water.

The team at Candela went to work on hydrofoiling because, they note, a 7.5m planing boat uses 12-18 times more fuel than a family car. While hydrofoils give Candela and others a 50% reduction in energy consumption, hydrofoiling isn’t a viable option for every type of boat use.

Hydrofoiling boats also have to be planned and constructed from the beginning. It’s difficult (to say the least) to convert an existing boat because the engineering is a lot more complicated than just attaching some foils to a hull.

For boat owners who may want to go electric with an existing planing boat, they inevitably have to compare against fossil fuel. Gasoline has an energy density of 12,000 watt hours per kilogram, so in the near future the weight of a battery is not going to provide the energy storage of a same weight gas tank.

Lighter batteries open up more possibilities

There are lots of advantages to going electric, to be sure, including more efficient engines, quiet operation and of course carbon emissions. But the physics still need to be addressed. Batteries with double the energy of existing technologies are a big step, though, especially for recreational boats that are taken out for relatively short high speed runs.

If the tests of Oxis and Bye Aerospace work out well , electric boating could be another area that can benefit from lighter, higher density batteries and increased range and speed.

Oxis is certainly no stranger to the marine world. Their batteries are already in use powering deep sea autonomous vehicles to depths of over 6,000 metres.

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