PRELIMINARY EVALUATION OF RECHARGEABLE LITHIUM-ION CELLS FOR AN IMPLANTABLE BATTERY PACK

A preliminary evaluation of the performance characteristics of 1.08 Ah lithium-ion cells was undertaken utilizing operating conditions simil ar to that required for an implanted medical device, such as a ventric ular assist device or total artificial heart, in order to determine th eir potential usefulness for this application. The major parameters st udied at 22 or 37 degrees were discharge-rate capability, specific ene rgy and energy density, surface temperature, self-discharge and cycle Life. The discharge loads used in the cycle-life study were either con stant or pulsatile, with the constant discharge load being equivalent to the average of the pulsatile load. The lithium-ion cells showed hig h discharge-rate capability up to 1.5 A at 37 degrees C, with over 74% of their rated capacity being obtained and a midpoint voltage of over 3.3 V ( > 72% of rated capacity and > 3.3 V for up to 1.0 A discharge s at 22 degrees C),before the first indication of cell polarization wa s noticed. The specific energy and energy density of cells discharged at 0.88 A to 2.5 V at 37 degrees C was 73 Wh/kg and 190 Wh/l, respecti vely (64 Wh/kg and 157 Wh/l at 22 degrees C). The internal resistance of the cells was calculated to be 198 m Omega at 37 degrees C (316 m O mega at 22 degrees C), which resulted in a relatively high, 8.0 degree s C, increase in surface temperature under a 0.88 A discharge load. Th e self-discharge of the cells at 37 degrees C was relatively low, with only a 1.3% loss in capacity being observed after 24 h. The lithium-i on cells yielded longer cycle lives at 37 degrees C (2 239 cycles) com pared with 22 degrees operation (1539 cycles) under similar 0.88 A dis charge loads. The cells performed slightly better under constant disch arge loads than under pulsatile loads of equivalent average current (0 .83 A average) with cycles lives of 2279 cycles versus 1941 cycles and operating times were 1.6 +/- 1.1 min (mean) longer. Preliminary indic ations are that these lithium-ion cells would be suitable for use in a rechargeable battery pack capable of powering implanted medical devic es.