1. Field of the Invention
This Invention relates generally to a method for increasing the charge stored in a battery and, more particularly, to a system and method for determining the usable amount of charge to balance individual battery cells to optimize the usable charge of the battery.
2. Discussion of the Related Art
Electric-only and hybrid vehicles, such as battery electric vehicles, range extended electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and fuel cell hybrid electric vehicles, are becoming increasingly more prevalent. An electric-only vehicle will have the power supplied occasionally from an NC power grid that charges the battery, and the battery will discharge as the vehicle is driven. Hybrid electric vehicles generally combine a rechargeable battery system with power supplied by a power source, such as an internal combustion engine or a fuel cell system.
The battery system can be based on different chemistries including lithium-ion, nickel metal hydride, lead acid, etc. Depending on the battery characteristics, a control system should not allow a cell's charge to fall below a charge depletion floor or above an overcharging limit. The battery engineer takes performance, durability and safety into consideration when establishing the depletion floor and the overcharge limit. The depletion floor and overcharge limit are typically related to state of charge (SOC) and voltage.
Due to production variations, individual battery cell capacities, self-discharge resistance and other values vary resulting in the variation of the capacity of each battery cell to hold electric charge. In addition, how much of the cell's capacity is actually used also varies, because of the serial connection of the cells in the battery pack or module. For a serial configuration of cells, the electric current charges and discharges all of the cells simultaneously. This results in the cell with the least amount of charge determining the useable charge from the battery system, where usable charge remains locked-up in other cells of the battery system.
Some battery systems will attempt to get extra charge out of the other cells by redistributing the charge between cells based on cell voltage. An improvement over using voltage includes determining the actual electrical storage capacity and state of charge (SOC) of the cells in a multi-cell battery system.
A need exists for an algorithm that can accurately determine the charge to balance, that is extra charge that can be distributed (balanced) to the other cells so that the battery will supply the maximum usable charge and does not leave behind charge in cells when the first cell falls to the charge depletion floor.