High-Performance Battery Engineering for Undersea Applications
Increasing demands are being placed on battery power for undersea applications such as vehicle propulsion, operating portable equipment such as cameras or measurement devices, and operating installed equipment such as telecom infrastructure. Factors such as longer missions and higher peak-energy demands from more sophisticated and intelligent systems call for greater energy density, improved reliability, greater safety and high resilience to the elevated pressures experienced in undersea environments.
Overall, the design of the battery system must ensure high levels of reliability and safety, so as to minimize danger to personnel as well as disadvantages such as property loss, down time, mission failure, and high maintenance costs resulting from battery failures in the field.
Lithium battery technology has several advantages over other types, particularly its higher energy density. However, creating a lithium-based battery system capable of delivering the optimum performance and meeting reliability and safety-acceptance criteria, at the right price, demands careful attention to aspects such as cell technology, cell balancing, charge control and production quality.
This white paper describes these issues and discusses potential solutions that can be built into a lithium battery pack for undersea applications.
Preferred Battery Chemistry
Modern undersea missions require battery chemistry offering significantly higher energy density than existing chemistries such as Lead-Acid, Alkaline, Ni-Mh, or Ni-Cd. This is necessary to supply all the energy requirements of modern equipment, and to support longer manned or unmanned missions.
More modern lithium-metal and lithium-ion (Li-ion) battery technologies have matured and now offer up to four times greater energy density than the older technologies. They also overcome many of the associated limitations, particularly those related to operating or charging the battery in sealed conditions during use.
Applications and Environment
Because water has its densest phase a few degrees above freezing, temperatures near the sea bed are generally in the region of 4-5AC. This is comfortably within the usual operating range of a lithium battery.
The pressure experienced by the battery pack can be very much greater than normal atmospheric pressure, depending on the depth at which the equipment is required to operate. The pressure exerted on equipment operated near or on the sea bed can be as high as 10,000 psi.
High outside pressures are capable of deforming the battery casing and bursting seals, leading to effects such as contamination of the electrolyte and failure of the battery. To combat this, the battery pack and other subsystems may be mounted in a pressurized container, depending on the application, to minimize the pressure exerted on the battery module including any internal control circuitry. Lithium batteries are known for their suitability for use under high pressures in oil-filled or potted enclosures. For example, within other critical markets such as the down-hole oil and gas industry, lithium packs are operating in harsh applications where extreme pressure, high shock and vibration are commonplace during drilling and measurement operations.
A suitable battery system for sub-sea applications must be able to operate below the surface in a sealed environment. Lithium-metal, Li-ion and Li- polymer batteries provide an ideal solution as they can be recharged with no need for venting, since (unlike lead-acid or Ni-Cd batteries) the battery generates no gases during recharging. Since there is no need to disturb the sealing mechanism, the risk of early seal failure is greatly reduced and batteries can be recharged more easily on the surface or in situ, if required.
As with all cell chemistries, lithium-type batteries are not immune to failures in the field. There is a risk of fire or explosion if lithium batteries are overcharged or allowed to overheat. Some high-profile failures have been seen in the computer industry, which have resulted in the recall of large numbers of notebook PCs. The main causes of lithium battery failures are overheating, overcharging, and imbalances between cells. Proper control of charging, including temperature monitoring, is therefore essential to ensure robust and reliable performance in mission-critical applications.
The rate of charging, for lithium battery technologies, is relatively inflexible, and is typically around 1C or less. The battery is initially charged at maximum charge current until the rated voltage is reached. The current then falls as the maximum voltage is reached, and charging terminates when the current falls to below 3% of the rated value. The maximum voltage for a lithium battery is typically around 4.1V-4.3V per cell. Overcharging to a higher voltage can cause instability, gassing and temperature increase giving rise to risk of fire. For this reason, protection circuits are implemented to prevent excessive charge voltage from being applied and to halt charging if the temperature increases to critical levels.
It is also important to provide circuitry that will protect the battery against becoming over discharged, by shutting down the system when the battery voltage reaches a minimum level. This is typically in the region of 2.7V-3.0V per cell (in the design phase, it should be considered that, although the charged voltage is a nominal 4.2V, the typical on-load voltage may be reduced to 3.2V to 3.3V).
Circuitry to control charging and prevent over discharge can be implemented externally, in a battery-specific charger, or internally within the battery itself. Either approach may have advantages: external charge control may permit smaller, lower-cost batteries; on the other hand, integrating the circuitry allows a variety of energy sources such as a DC power supply or a fuel cell (or a combination of sources) to be used more easily.