The Magazine for Underwater Professionals
Steatite, UK, reports on the development of a pressure tolerant lithium-sulfur battery pack for MAS (marine autonomous systems)
A two-year Steatite led project to develop a pressure tolerant battery pack has accomplished a new milestone. Due for completion in October 2017, the project has reached the halfway point and is now entering a crucial stage of battery development.
The consortium – consisting of Steatite, OXIS Energy, UK, MSubs, UK, and the National Oceanography Centre, UK – has made great headway in recent months, as the collaborative research and development project, supported by Innovate UK and the Defence Science and Technology Laboratory (Dstl), UK, continues at pace.
Focused primarily on marine vehicles, the project aims to exploit the inherent benefits of lithium-sulfur cell technology. These benefits include marine autonomous vehicles improved neutral buoyancy, considerably greater levels of safety and high energy densities, which will have huge benefits in terms of greater vehicle speed, endurance and payloads.
By enabling the vehicle to remain in the sea longer and achieve more on a per dive basis, launch and operational costs can be heavily reduced, resulting in much improved dive efficiency.
The project is progressing well with the cell development phase about to be completed (end of year one). We are now entering the battery development phase (first half of year two) involving continued development of Steatite’s pressure-tolerant multi-chemistry Battery Management System (BMS).
The system will be demonstrated in a deep-dive submarine in mid-2017.
The battery pack is designed to operate at depths of up to 6000 metres. This depth marks the lowest reaches of the Abyssal Zone, effectively the deepest regions of the continuous ocean floor. Almost 98% of ocean depth is above this level, with just 2% reaching into the Hadal Realm, a region consisting mainly of deep-ocean trenches situated along tectonic plate edge subduction zones.
With ocean pressure increasing by one atmosphere with every 10 metres of depth, the battery pack will need to withstand crushing pressures of 600 atmospheres at its 6000-metre limits, whilst still remaining fully functional.
This has formed the bulk of testing, and cell development work has successfully extended the capabilities of the OXIS lithium-sulfur (Li-S) cells.
To ensure reliability, extensive test and evaluation work at the National Oceanography Centre’s world renowned facilities have put the cells through their paces at over 600 atmospheres of pressure and low temperatures (0 to four degrees Celsius) representative of harsh, deep ocean conditions.
During previous iterations the composition of the cells were optimised to overcome the loss of performance at low temperatures. This means that the Li-S cells now deliver (almost) the same performance as at ambient conditions; a huge achievement, and testament to the unique skills and abilities of the consortium.
Operating at the very edge of their required limits, the latest iteration involves 12 ampere-hour cells, discharging at low temperature and high pressure. The effective neutral buoyancy energy density (NBED) is nearly double that of the lithium-ion reference cells (shown below). Our ability to exploit the planned improvements in the OXIS cell energy density over the next couple of years means that we remain on track to meet the project’s goals.
Several cells have been used to perform life tests, and we have now reached over 60 cycles for slow discharge, and 80 cycles of faster discharges.
Following this hugely successful test phase we are confidently progressing to the battery development phase, driving us ever closer to the reality of a functional battery pack able to be utilised on deep-dive marine autonomous systems to depths of 6000 metres.