Portable underwater electrical power is needed for many commercial, recreational and military applications. A battery system is currently not available to meet these needs, which was the aim of this project. Lithium-ion battery cells (Panasonic (CGR18650E)) were chosen, based on their high energy density and availability. To increase their voltage, 8 battery cells were connected in series ("sticks") and protected by encapsulating them into a polycarbonate tube; and 6 sticks were housed inside a triangular aluminum case (module). Testing was performed to determine the consistency of individual cells, sticks and module and during discharge/charging cycles. The effect of ambient temperature (T-A) was determined by instrumenting them with thermocouples. In addition, voltage and current were measured and used to determine the heat generated within the battery cell and were compared to theory. From these data, a radial temperature profile was determined for two battery sticks in the battery module. Collapse pressure was determined and compared to theory. The Panasonic (CGR18650E) cells delivered 2291 mAh each over a wide range of T-A and discharge/charge rates. The theoretical and experimental data showed that the temperature within the battery sticks and modules did not rise above or below their operating temperature range (-20 and 60 degrees C), in agreement with the models. The tubes encapsulating the sticks withstood pressures down to 305 m of sea water (msw) which was predicted by modeling. The Panasonic (CGR18650E) cells, sticks and module demonstrated that they provided sufficient electrical power, reliably and safely to be used in the underwater environment (1800-2000 kPa, 305 msw) over a wide range TA, including high power requirement systems like an active thermal protection system that keeps a diver comfortable in extreme temperature conditions. The concept developed here can be modified to meet specific power requirements by varying the number of cell in series to achieve the desired voltage, and the number of sticks in parallel to provide the current capacity required. (C) 2010 Elsevier B.V. All rights reserved.