Preliminary evaluation of rechargeable lithium-ion cells for an implantable battery pack

Abstract A preliminary evaluation of the performance characteristics of 1.08 Ah lithium-ion cells was undertaken utilizing operating conditions similar to that required for an implanted medical device, such as a ventricular assist device or total artificial heart, in order to determine their potential usefulness for this application. The major parameters studied at 22 or 37 °C were discharge-rate capability, specific energy and energy density, surface temperature, self-discharge and cycle life. The discharge loads used in the cycle-life study were either constant or pulsatile, with the constant discharge load being equivalent to the average of the pulsatile load. The lithium-ion cells showed high discharge-rate capability up to 1.5 A at 37 °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 discharges at 22 °C), before the first indication of cell polarization was noticed. The specific energy and energy density of cells discharged at 0.88 A to 2.5 V at 37 °C was 73 Wh/kg and 190 Wh/l, respectively (64 Wh/kg and 167 Wh/l at 22 °C). The internal resistance of the cells was calculated to be 198 mΩ at 37 °C (316 mΩ at 22 °C), which resulted in a relatively high, 8.0 °C, increase in surface temperature under a 0.88 A discharge load. The self-discharge of the cells at 37 °C was relatively low, with only a 1.3% loss in capacity being observed after 24 h. The lithium-ion cells yielded longer cycle lives at 37 °C (2 239 cycles) compared with 22 °C operation (1539 cycles) under similar 0.88 A discharge loads. The cells performed slightly better under constant discharge 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 indications are that these lithium-ion cells would be suitable for use in a rechargeable battery pack capable of powering implanted medical devices.