I. Introduction
Demand for electric vehicles with hybrid drive has soared worldwide due mainly to a recent sharp increase in fuel prices. According to a recent survey [7], in 2008 alone, 36.0% motorists worldwide want to buy a car with hybrid drive while 45.8% of them are interested in buying full-electric cars. Electric cars are powered entirely with electrical energy from tens of thousands of battery cells. These battery cells are grouped and assembled as a set of battery packs. Individual cells in a pack, which are exposed to, and must operate in a harsh environment, have different operating characteristics due to difference in their manufacturing tolerances, uneven temperature conditions across the pack, or non-uniform ageing patterns. These, in turn, have crucial effects on the charge/discharge of battery cells. In a series chain of battery cells, a weak battery cell with low capacity reaches its full charge state well before the rest of the battery cells in the chain, hence overcharging and overheating itself. On the other hand, when the weak cell cannot reach its full charge owing to a high self-discharge [2] and/or a short-circuited cell, good battery cells could become overcharged. In a series chain of battery cells, an open-circuited cell causes the others in the chain to become open-circuited as well. All of these phenomena eventually lead to a battery-cell failure, which is inevitable especially in large-scale battery packs.