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Lithium-Ion Subsea Battery System

By Leon Adams • David White

SeaSafe in WHOI's under-ice ROV.

The subsea industrial equipment, vehicle and oceanography markets need battery solutions that deliver more electrical capacity at less weight than old technology sealed lead acid (SLA). Lithium-ion technology offers four times the energy density of SLA, but the batteries need to be safe, reliable and easy to use in subsea deployment. Also important are achieving configurable size and capacity flexibility, long-lasting subsea operating life, subsea pressure tolerance, and precertified testing for shipping standards. Project time and cost savings by avoiding custom battery development are also preferred.

Meeting these needs challenged SouthWest Electronic Energy's (Missouri City, Texas), or SWE, research and development team to create SeaSafe, a safe, modular, flexible and reliable battery system. SeaSafe lithium-ion batteries deliver longer life and longer missions than lead acid, with four times more energy, six times more available energy at colder temperatures typical of the seabed and eight times longer cycle life.

SeaSafe's advantages over sealed lead-acid batteries are that lithium ion does not outgas during charge. SWE incorporates smart, automatic battery management in each battery module that monitors constantly, preventing charging and discharging errors. SeaSafe also provides health and status reporting on demand, unlike most SLA batteries.

The benefits for subsea applications include longer operating life for deep-sea oil and gas infrastructure electronics, lights, and backup; safer, longer-lasting missions for manned underwater vehicles; lighter weight and local instant power for ROVs; and deeper dives and longer missions for AUVs.

SeaSafe battery solutions are implemented in two commercial off-the-shelf battery product configurations: SWE SeaSafe Smart Battery Modules, which are lithium-ion battery module building blocks, and SWE SeaSafe Battery System, which is pressure-tolerant and constructed using a pressure equalization case designed to hold four SeaSafe Modules with parallel integrator isolators (PII). Support software includes SeaSafe Observer, which is PC-based and monitors the health and status of battery modules and/or systems.

SeaSafe Battery Modules
The SeaSafe Smart Battery Modules are available in a 29-volt, 28-ampere-hour or a 24-volt, 28-ampere-hour size. The modules are based on industrial-grade, long-lasting lithium-ion polymer cells. Safety, reliability, flexibility and ease of use are achieved because each module has a built-in autonomous battery management system (BMS) with safety protection. No external connections or controls are required to manage critical safety features.

Multiple modules can be connected in series for total battery system voltage and/or parallel for total battery system amp-hour capacity. The BMS and battery modules have been developed and tested for more than three years, with extensive internal validation testing on short-circuit behavior, safety functional behavior, communications and capacity gauging. In addition, the modules completed pressure verification tests of multiple cycles to 6,000-meter depth capability (10,000 pounds per square inch) at an independent mechanical engineering firm in Texas. Maximum state-of-charge is configurable and is managed by the BMS to optimize extended operating life up to 15 years. Charge is provided by off-the-shelf constant-voltage/constant-current power supplies.

The module enclosure is fiberglass filled with thermally conductive, flame-retardant polyurethane potting material. In addition to its thermal benefits, the potting material also dampens shock and vibration. The modules are UN DOT 38.3 certified for transportation. They can be configured in customer-designed cases or within an SWE-supplied SeaSafe battery case.

SeaSafe Battery Systems
SWE SeaSafe Battery Systems are built with SeaSafe Battery Modules installed in an SWE-designed pressure-equalized case. Up to four SeaSafe Modules can be configured within one case in a series/parallel combination of modules to meet voltage and amp-hour capacity needs, delivering up to 132 volts or 112 amp-hours maximum. Multiple cases can be stacked and connected to increase voltage and/or amp-hour capacity. The system is pressure tolerant down to 6,000 meters. The pressure-equalized case is built rugged from 316 stainless steel and incorporates SEA CON (El Cajon, California) WET-CON connectors.

The case has a translucent urethane bladder that equalizes ocean-depth pressure with the battery modules immersed within the case in pressure-compensation oil. The dimensions of the case are 14.8 by 15.6 by 17.8 inches.

When SeaSafe Battery Modules are connected in a series string, many autonomous safety features are redundant, thus increasing reliability. For instance, if one string of two series-connected modules is accidently shorted, both modules monitor the shorting current, with two opportunities for the short to be autonomously interrupted. To continue this article please click here.

Leon Adams is the chief sales and marketing officer at SouthWest Electronic Energy, where he has more than four years of experience in lithium and lithium-ion battery applications, product definition, sales and marketing management, and customer support experience. Previously, he spent 28 years at Texas Instruments, leading embedded processing and digital signal processing product lines for industrial and consumer applications, including battery-powered solutions. He is a member of the Marine Technology Society and has a master's from the University of Texas at Austin and a bachelor's in engineering physics from Murray State University.

David White is a senior member of technical staff at SouthWest Electronic Energy in Missouri City, Texas. He graduated from Texas A&M University in 1970 with a bachelor's in electrical engineering. He has been designing state-of-the-art computer, seismic and battery systems for Texas Instruments, Halliburton, Input/Output, SouthWest Electronic Energy and others for more than 40 years. He is a Marine Technology Society member, an emeritus member of the Society of Exploration Geophysicists and a life senior member of the Institute of Electrical and Electronics Engineers.

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