Many different applications, from consumer electronic devices to vehicles and power grids, use rechargeable batteries as energy sources. Typically, these applications require their batteries to be as safe and as efficient as possible. Overcharging a battery, for example, may compromise its reliability and reduce its useful lifetime. More alarmingly, over-discharging a battery can cause it to catch fire or even explode. Thus, any application that charges and/or discharges a battery typically includes circuitry to prevent overcharging and/or over-discharging.
Protecting a battery against these undesirable conditions, however, may be a complicated task. Many batteries, especially high-voltage or high-power batteries, are composed of a plurality of discrete cells, each of which is separately monitored. As more cells are included in the battery (e.g., to increase its voltage or power output), the chance of one cell performing at a level different from the other cells in the battery increases. For example, manufacturing defects or variations may cause each cell in a battery to have a different voltage output, charge capacity, maximum charging voltage, and/or minimum discharging voltage. Even perfectly matched cells may, over time, become mismatched with usage and age. The worst-performing cell may act as a “weak link” in the battery; an overcharge-protection circuit, for example, may halt charging of the battery when the worst-performing cell reaches its limit, despite the additional capacities of other cells in the battery.
Circuitry may be added to the battery to charge and/or discharge the cells to their individual capacities. This circuitry, however, may reduce a battery's efficiency and/or significantly increase its cost or complexity. A charge-shunting circuit, for example, dumps excess charge into resistors to protect battery cells. This charge dumping, however, generally wastes energy and produces heat. A switched-capacitor or flying-capacitor circuit is typically more efficient, but requires the use of a complicated, costly, and large capacitor and transformer network. Still other circuits allow a subset of electrically adjacent cells in a battery to be charged and/or discharged, but require a complicated, expensive switching network and, because only adjacent cells may be chosen, perform sub-optimally when, for example, cells in the middle of the battery fail or degrade.
A need therefore exists for a robust, efficient, and flexible battery-charging and -discharging system to optimize the charging and discharging of individual cells in a battery—for example, capable of fully charging and discharging any cell in a battery, regardless of variations therein, while protecting the cells from overcharging and over-discharging.