The present invention relates to a method for determining a maximum charge current and a maximum discharge current of a rechargeable battery for a hybrid electric vehicle. More particularly, the present invention relates to a method for determining a maximum charge current and a maximum discharge current of the battery in order to increase battery efficiency, to prevent damage to the battery, and to increase the lifespan of the battery.
Generally, the dischargeable power of a battery varies in accordance with the state of charge of the battery. In an electric vehicle, in order to prevent damage to the battery, the motor controller receives a varying chargeable (i.e., charging and discharging) current, depending on the stage of the state of charge for the battery. For example, if the state of charge of the battery is above 30%, the maximum discharge current can be utilized. However, as the state of charge decreases, the maximum discharge current also decreases, to protect the battery.
The allowable discharge current according to the state of charge may be determined through experimentation, as shown by the graph in FIG. 1 depicting one example of a maximum discharge current. Because the battery of an electric vehicle is mainly discharged during the operation of the vehicle, it is useful to determine the maximum discharge current at various stages of the state of charge of the battery.
In addition to being discharged, the battery of a hybrid electric vehicle is also periodically charged. The discharging of electricity from the battery and the charging of the battery occur alternately. In order to increase the efficiency of the hybrid electric vehicle and to improve fuel economy, the battery of the hybrid electric vehicle preferably outputs a maximum discharge current and preferably receives a maximum charge current.
Different techniques have been proposed for determining the maximum discharge current, but a method for determining the maximum charge current has not previously been introduced. The maximum discharge current cannot simply be the same as the maximum charge current, or the battery will not operate at its maximum efficiency.
FIGS. 2A and 2B demonstrate how some prior-art techniques underestimate maximum charge and discharge currents, resulting in available power that is not utilized, represented by the shaded portions of the graph.
The information disclosed in this Background section is only for enhancement of understanding of the background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art that is already known to a person skilled in the art.
In a preferred embodiment of the present invention, a method for determining a maximum charge current of a battery comprises determining a steady-state charge resistance based on a current state of charge of the battery, and determining a no-load charge voltage based on the current state of charge of the battery. The method further comprises calculating a maximum temporary charge current, and calculating a maximum charge current based on the maximum temporary charge current.
Preferably, the maximum temporary charge current is calculated by the following equation: [(the maximum charge voltagexe2x88x92the no-load charge voltage)/(the steady-state charge resistance)].
Also, preferably, the maximum charge current is calculated by the following equation: [(the maximum temporary charge current)*(predetermined charge time period gain)].
In another preferred embodiment of the present invention, a method for determining a maximum discharge current of a battery comprises determining a steady-state discharge resistance based on a current state of charge of the battery, and determining a no-load discharge voltage based on the current state of charge of the battery. The method further comprises calculating a maximum temporary discharge current, and calculating a maximum discharge current based on the maximum temporary discharge current.
Preferably, the maximum temporary discharge current is calculated by the following equation: [(the no-load discharge voltagexe2x88x92the minimum discharge voltage)/(the steady-state discharge resistance)].
It is also preferable that the maximum discharge current is calculated by the following equation: [(the maximum temporary discharge current)*(predetermined discharge time period gain)].
In other preferred embodiments, the methods described above can be implemented as logic in a battery management system.
In yet another preferred embodiment of the present invention, a method of more accurately determining maximum charging and discharging currents for a battery of a hybrid electric vehicle comprises determining a plurality of maximum voltage differentials corresponding to a plurality of possible state of charges for the battery. Each maximum voltage differential is based on a difference between a predetermined voltage limit for the battery and a no-load voltage value for each of the plurality of possible state of charges.
The method also comprises determining a plurality of steady-state resistances corresponding to the plurality of possible state of charges, and measuring a first state of charge of the battery. Finally, the method comprises determining one of a maximum charge current and a maximum discharge current by dividing the maximum voltage differential corresponding to the first state of charge by the steady-state resistance corresponding to the first state of charge.