Feature ArticlesModular Design for Li-Ion and Li-Polymer Batteries for Undersea Environments
By David A. White
Senior Member, Technical Staff
SouthWest Electronic Energy Group
The battery industry is on the verge of a significant growth cycle in large format lithium-ion (Li-ion) battery systems due to expected demand for electric land vehicles. Important to this growth is what was once thought of as a detriment of the Li-ion chemistry—that it requires monitoring and control electronics for safety and reliability. Engineers at SouthWest Electronic Energy Group (SWE) are turning this supposed detriment into an advantage by using intelligent electronics to make battery systems that have capabilities that would not be practical, or even possible, without these electronic tools. While the land version of these battery systems is not necessarily suited for undersea environments, the same battery chemistry and electronics can be adapted for hadal zone regions deeper than six kilometers.
This article will show how a new concept of modularly designed Li-ion and Li-polymer batteries developed by SWE can be incorporated into low-production marine vehicles with the unique performance requirements of operating in a freezing and high-pressure environment. This new SWE battery system development methodology utilizes battery modules to construct complex battery systems.
Challenges of High Pressure
People familiar with undersea batteries that use conventional silver-zinc, lead-acid or nickel-cadmium chemistries know that although the name of the cell or battery may contain the term 'sealed,' these chemistries are not really sealed. They have to breathe, and when they are being fully charged, they have the undesirable characteristic of giving off highly flammable and explosive gases. Discharging can be done in a sealed environment, but full capacity charging is only safe in an unsealed and vented environment. Where these cells cannot be vented (as in operation in an oil bath), it is possible to undercharge them to prevent outgassing, but this is at the expense of reduced life. When these batteries are housed in a pressure vessel, it must be unsealed and vented during charge, then resealed for use, with the nagging knowledge that multiple unseal and reseal cycles can result in leaks.
Rechargeable Li-ion cells and batteries, introduced in the 1990s, have matured, and they promise to reduce or even remove the restrictions of conventional battery chemistries. The rechargeable Li-ion cell is two to four times more energy dense than other rechargeable chemistries, it is truly sealed, and it can be charged and discharged hundreds or thousands of times without outgassing.
Cylindrical Li-ion cells work fine within a pressure vessel, but they cannot work at hadal zone pressures in oil submersion because of air pockets within the metal-encased cells. However, a new packaged form of the same chemistry cell has been developed, called Li-polymer. The Li-polymer cell contains a Li-ion chemistry that is housed within a sealed foil pouch. The pouch is vacuum sealed, which removes almost all air pockets. When this cell is correctly constructed, it can be submerged in oil or flexible potting material. Charge and discharge cycling of cells has been tested at and above hadal zone pressures of 10,000 pounds per square inch.
The difficulty of these new Li-ion chemistries is that they require sophisticated electronics for monitoring, charge control, discharge control and balancing functions. Fortunately, these necessary electronics can also be made to survive hadal zone pressures.
It is also necessary to devise a means to uniformly distribute external pressure to the battery assembly. Oil encapsulation is an ideal way to uniformly distribute this pressure and to fill air spaces between components and cells. However, oil can allow submerged parts to move, and they may be damaged by differing orientation or shipboard shock and vibration. If the battery assembly is potted using a compliant material, it can then be made more resistant to uncontrolled orientation, shock and vibration. Finally, with oil or potting encapsulation, it is a necessity to seal the battery and the electronics away from salt water. This is typically achieved using housings with bladder seals that allow for the finite compressibility of oils and potting materials at the extreme hadal zone pressures.
Illustration of a Li-polymer battery pack containing four battery modules that SWE built for FMC technologies
Li-Ion Safety Issues and Testing
Safety is a high hurdle to overcome in a lithium chemistry battery system, where the battery energy density is many times higher than with previous chemistries. SWE has years of experience in the design of safe Li-ion battery systems. Safety is associated not only with battery use, but also with battery transportation. When the design effort is completed, its safety must be tested.
Transportation regulation and military organizations are often involved in regulating the safety of lithium chemistry batteries. For lithium chemistry batteries transported via air, land or sea, the transportation regulators for most countries, including those at the U.S. Department of Transportation (DOT), have settled on a common test requirement, section 38.3 of the United Nations Manual of Test and Criteria. For battery assemblies containing multiple battery cells, this test requires 24 completed battery assemblies. Testing typically irreversibly damages or destroys about half of these battery assemblies and stresses or uses a portion of the cycle life in the other half. If the battery is large and expensive, these tests can result in enormous capital expenditures both for labor and material. If the quantity of batteries being produced is not very high, the cost of these tests can kill lithium battery development projects.
The U.S. Navy regulates undersea battery systems and has developed a safety handbook, Naval Sea Systems Command S9310-AQ-SAF-010, which defines the assessment methods and destructive tests that must be performed on all lithium chemistry batteries used in, or transported on, Navy vessels. The required quantity of test batteries is not specified. The tests are designed to cause the destruction of the battery by high heat to determine the extent of potential damage that can result from the battery releasing its energy via either extended or violent battery disassembly.
Size Versus Safety and Reliability
Resolving the safety problem that can result from the high energy potential of a large battery system and the safety testing expense of such a system is a significant hurdle. However, this is not the only hurdle. A large battery system that must operate in an extreme undersea environment can be particularly unforgiving should there be a component failure. Personnel danger, property loss, down time, mission failure and significant maintenance expense are real risks in a large battery system. The system design must minimize these problems.
Smaller batteries provide a number of solutions. Personnel danger can be reduced. Battery safety test costs are lower if the battery is smaller. The risk of property loss is also reduced if the battery is smaller. Mission failure is less likely if the battery system has built-in redundancy. Maintenance expenses and down time are reduced if failed components are smaller, less expensive and easier to replace.
It is evident that the challenge is to build a large battery system using small, identical, easily replaceable component parts that work in a coordinated fashion, can be individually safety tested and can be installed in situ in an arrangement that is inherently redundant. This is a tall order. However, SWE has developed a design concept for a Li-ion battery system to meet all of these requirements: battery modularity.
Modularity Concept Development
SWE's battery modularity design methodology is to construct a complex rechargeable battery system using series and parallel combinations of identical, independent battery modules. Each battery module is a separable, self-contained, rechargeable battery of a convenient size for the on-site construction of a multiple-module battery system designed to meet DOT requirements for transport safety.
A predecessor to the modular battery construction concept was a large, 25.9 volt, 356 ampere-hour Li-ion battery constructed by SWE for Applied Research Laboratories at the University of Texas at Austin. It utilized 36 battery modules. The modules each had simple pack-protection circuits to prevent overcharge, overdischarge and overcurrent. The modules were not separable, did not contain cell balancing or module balancing and did not contain built-in charge control. This battery was constructed using cylindrical cells, so it had to be installed in a pressure vessel to maintain a nominal one atmosphere of pressure. Charging was done on the surface with the battery sealed within its pressure vessel.
Today, SWE's modular design methodology uses much more sophisticated module electronics. SWE's battery module has a means of internal charge control that allows it to be charged from multiple energy sources, such as power supplies, solar panels, fuel cells or a combination of these. SWE's modular battery system has the capability to use these multiple energy sources to charge while deployed.
SWE recently built a sophisticated battery pack using Li-polymer cells for FMC Technologies Inc. (Houston, Texas). The battery system contains eight four-battery module sections, each housed in a quarter-cylinder case. Each module section is mounted into a small pressure-equalization housing that contains pressure equalization fluid and a pressure equalization bladder. The system has a low-current power tether that is capable of slow charging the battery pack at depth for continuous mission utilization. This SWE battery system has been tested at 10,000 pounds per square inch while performing low-current charge and high-current discharge.
Another new battery system proposed by SWE for the Alvin manned submersible at Woods Hole Oceanographic Institute illustrates how modular design methodology can be used to construct a battery system as large as 115 kilowatt-hours.
The SWE battery module in this system is a 30 volt, 31 ampere-hour module. The battery uses 128 modules in a series and parallel arrangement to obtain a dual-skid battery system. Each skid contains eight sections of eight series-connected battery modules (64 modules). The eight sections are separated from one another by ideal OR'ing diodes, which prevent the failure of a battery section from affecting other battery sections.
Each of the two skids of batteries on the Alvin are charged at 32 kilowatts and discharged at 64 kilowatts. Full recharge time in this instance is as fast as two to four hours. Since the Alvin power requirements are 48 kilowatts, it can run at full power using only six of the 16 available sections. The Alvin battery system contains an RS-485 Modbus computer interface in each module. The computer interface monitors and controls each battery module and the whole battery system.
Balancing a Key Requirement
Modern Li-ion cell chemistries are remarkably robust in their ability to maintain balance. Nevertheless, field return data on high-series-count Li-ion batteries support the need for a robust balancing capability for complex battery systems.
For high-cell-count battery systems, battery pack imbalance is the dominant cause of pack failure. For this reason, SWE's robust balancing capability is designed into the entire battery system. In the instance where the module design concept is utilized, this means intramodule and intermodule balancing.
Electronic cell balancing is not new. Two common intramodule balancing methods are discharge balancing and charge transfer balancing. These two intramodule balancing methods are commonly only described for balancing across a complete and inflexible battery system using centralized control. For highly configurable battery systems constructed from independent rechargeable battery modules, there is a need for an intermodule balancing method.
SWE proprietary intermodule balancing algorithms and electronics have demonstrated the ability to meet this need in oceanographic applications during both charge and discharge at 0' C and 10,000 pounds per square inch.
Li-ion polymer cells have been successfully tested for both charge and discharge at hadal zone pressures. With an energy capacity two to four times greater than conventional chemistry cells, these cells significantly extend operation at depth. The Li-ion chemistry does not outgas during charge or discharge and can be safely housed within sealed containers. This feature allows faster, safer and more reliable deployment of Li-ion powered marine systems. It also provides continuous operation at depth.
Electronic balancing is a requirement for large Li-ion battery systems because the chemistry does not provide for overcharge balancing. With this restriction, engineers are discovering that the ability to automatically electronically balance all parts of a complex battery system leads to new paradigms in battery system design, use and maintenance. The new SWE battery system design provides battery modules that enhance safety and reliability, reduce costs, increase series connections, increase flexibility and provide more flexible charge control.
This article has been adapted from the paper 'Modular Design of Li-Ion and Li-Polymer Batteries for Undersea Environments,' published in the Marine Technology Society Journal.
David White is a senior member of the technical staff at SouthWest Electronic Energy Group. He has been designing large, state-of-the-art electronic systems for 39 years.