Evaluation of nickel-cadmium battery packs for mechanical circulatory support devices

The mismatching of battery cell capacities within a rechargeable battery pack can result in reduced cycle life and operating time, as well as an increased safety risk due to gas release, which is a concern when used in implantable medical devices, such as a ventricular assist device (VAD). Two groups of four nickel/cadmium (Ni/Cd) battery packs containing 10 cells each (i.e., 12 volt packs) were evaluated for their cycle life, safe operating time, and the impact of mismatched cell capacities within the pack. Four control battery packs containing capacity matched cells (within +/- 0.4% of mean) and four battery packs each containing one low capacity cell (4.7-10.7% below mean) together with nine high capacity matched cells (within +/- 0.6% of mean) were charge/discharge cycled at 37 degrees C using a 9.7 W average pulsatile discharge load and a 30 minute discharge time on each cycle. Periodically, the battery packs were fully discharged down to 10 volts and each cell was monitored for negative voltages. The capacity matched and mismatched packs yielded similar performance curves, with an initial decline in mean total operating time until a plateau was reached of 41.1 +/- 1.5 and 41.8 +/- 1.8 minutes of discharge time, respectively. The mean total operating times of the matched and mismatched packs began to decline again after approximately 670 and 635 cycles, respectively. Cell voltage monitoring showed no negative voltages, indicating that no gases were released from the cells.(ABSTRACT TRUNCATED AT 250 WORDS)