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The Toyota Prius battery pack consists of 38 individual battery modules, each module contains 6 NiMH cells in series. This means that each pack contains 228 NiMH cells. Each cell has the potential to fail. This report investigates the mode of failure of Prius battery packs by first analysing a number of packs in the lab, and then road testing them in a Toyota Prius. The analysis of the battery packs show that some packs had aged “linearly”, that is in a balanced manner, such that the state of health of all modules remained similar. However, in other packs discrete modules had significantly different states of health. A pack that consists of cells that are matched in both state of health and state of charge delivers the best performance. The research also showed that the worst cell in the pack determines the overall pack performance. This was demonstrated by substituting reduced capacity or short-circuited modules into a functioning battery pack.