The Toyota Prius battery pack consists of 38 individual battery blades, each blade contains 6 NiMH cells in series. This means that each pack contains 228 NiMH cells. Given this number of individually manufactured cells combined into a large battery pack, any individual cell or a blade getting weaker than all other linearly degrading cells, will become the performance limiting element in the pack. For example in a situation where one blade weakens down to 1200mAh compared to all other 37 blades maintaining approximately 2400mAh capacity will make the car run only 1.3 km in EV mode, compared to 2.6 km if all 38 blades are of 2400mAh capacity. In order to identify such individually weak cells, a supercapacitor based monitoring system is designed and the paper indicates the approach in developing this system, together with some details explaining the general performance of the overall system and how the new monitoring system can help in managing the battery pack life issues in the Toyota Prius.