Design and testing of a rechargeable, pressure-compensated, lithium-ion battery module for underwater use
DeMauro, Edward P.
MetadataShow full item record
Active thermal protection is important for underwater divers when performing missions in extreme temperature conditions. In order to allow for the divers to have the greatest amount of maneuverability, an active thermal protection system must be able to carry its own power source to provide for the cooling/heating needed to keep the diver comfortable. This active thermal protection system is a closed system, composed of a tubesuit perfused with water by pumps, an electrical control system, an electrical buss, and a power source. Lithium-ion battery cells were chosen to provide energy, based on their high energy density. Eight battery cells are connected in series to form sticks, and either six or nine sticks are housed within triangular aluminum casings, forming battery modules. The battery cells must be protected from exposure to high external pressure due to electrochemical reactions and the evolution of gases that can cause explosions. A system is needed, also, to supply a positive pressure differential to the active thermal protection components, as the components have flat designs which are not pressure resistant. Each battery stick is enclosed within a plastic tube before being inserted into a battery module, in order to protect the battery stick from high external pressure. Experimentation and analysis was performed in order to determine the type of plastic material used, and to ensure that the tube could withstand high hydrostatic pressure. A tube was placed inside of a water tank, and the internal pressure of the water tank was increased until failure was observed in the tubes. The collapse pressure was recorded and compared to theory in order to determine the practicality of the theory. A fatigue test was also performed on the tubes to study how they performed under repeated loading and measure the amount of deformation that occurred in a practical application. Testing was also performed, using Type T, 24 gauge copper-constantan thermocouples, PVC-T-24-180-SE, to measure the temperature of the battery cells as a function of time. In addition, voltage and current information, obtained from testing, was used to determine the heat generated with in the battery cell as a function of time. From these data, a radial temperature profile was plotted, at a given time, for two battery sticks in the battery module. These graphs showed that the heat transfer within the battery sticks corresponds to low-Biot number heat transfer, and that the temperatures do not rise above the operating temperature range given by the manufacturer, which is between -20 and 60°C during discharge of the cells. Heat flux data was also calculated in order to understand how the battery sticks responded to the ambient water temperature, using a transient model. The completed active thermal protection system, with battery modules, was tested in order to determine the total dead space volume of the system. An experiment was devised, using a manometer to calculate the dead space volume. This data was compared to data obtained from measuring the amount of water each battery module held, along with estimating the dead space volume from the measurements of the modules and other components. The dead space volumes of the active thermal protection system components are 8 L for the flatpack, 1.6 L for the electrical buss, and 3.15 L for the 9-stick battery module, and 2.5 L for the 6-stick battery module. Knowledge of the dead space volume was used for the design of a pressure compensation system for the active thermal protection system, preventing the battery modules from collapsing under external hydrostatic pressure by using a pressure-reducing regulator and relief valve. Results show that the completed active thermal protection system is capable of providing the power requirements needed for keeping an underwater diver comfortable in extreme temperature conditions. Polycarbonate tubes were selected for protection of the battery sticks from external pressurization. Temperature measurements and analyses show that the battery cells remain within the manufacturer's acceptable thermal operating range. Pressure compensation experiments, likewise, show that the active thermal protection system is capable of maintaining a positive pressure differential at depth, keeping the active thermal protection system components from collapsing.