The invention relates to a method for operating a battery unit having at least two series-connected battery cells. The invention also relates to a use of the method for operating a battery unit in an electric vehicle (EV), in a hybrid vehicle (HEV) or in a plug-in hybrid vehicle (PHEV).
It is apparent that, in future, for static applications such as wind turbines, in motor vehicles which are designed as hybrid vehicles or electric motor vehicles and in electronic devices such as laptops or mobile telephones, new battery systems will be used which are subject to very high demands with regard to reliability, safety, performance and service life.
Here, use is made in particular of so-called lithium-ion battery cells. These are distinguished inter alia by high energy densities, thermal stability and extremely low levels of self-discharge. Lithium-ion battery cells have a positive and a negative electrode at which lithium ions can be reversibly accumulated and released again during a charging process and during a discharge process.
In the development of modern motor vehicles, a reduction of the fuel consumption of the motor vehicle is playing an ever more important role. Possible measures for reducing the consumption of a motor vehicle are to combine the internal combustion engine with an electric drive in a hybrid vehicle or to replace the internal combustion engine with a purely electric drive. The electric drive in hybrid vehicles normally comprises an electric motor which can also be operated as a generator, a battery unit as an energy store which provides a supply to the electric drive, and control electronics.
Plug-in hybrid vehicles furthermore offer the possibility of charging the battery unit via an external energy source. Here, the external energy source may for example be a charging station which is fed via the public electrical supply grid.
Purely electric vehicles, by contrast hybrid vehicles, do not have an internal combustion engine which can drive the motor vehicle, but rather have only an electric drive. Therefore, in the case of these motor vehicles, charging of said battery unit via an external energy source is necessary. Here, the battery unit is nowadays normally in the form of a lithium-ion accumulator, and is also referred to as traction battery.
Battery units in such electric vehicles are normally of modular construction. Here, numerous battery cells are interconnected in series, and form for example in each case one battery module. Further battery modules are for example combined to form in each case one subunit, and share for example a cooling device. A complete battery unit is constructed for example from multiple subunits. A battery control unit is provided for controlling and monitoring the battery unit.
The battery cells that belong to a battery unit generally have different capacities and different self-discharge rates owing to manufacturing tolerances or owing to aging. During the operation of a battery unit, it is necessary for the states of charge of the individual series-connected battery cells to be equalized with one another.
DE 10 2009 002 466 A1 has disclosed a method for equalizing load states of battery cells. Here, a battery cell which has a higher state of charge than other battery cells is partially discharged by means of a discharge resistance which is connected in parallel with respect to the battery cell.
A further method for equalizing load states of battery cells is disclosed in US 2007/0046260. Here, multiple resistances which can be connected in parallel are provided. A control unit calculates how many and which of the resistances provided are connected in parallel for the purposes of discharging the battery cell.
A further method for equalizing load states of battery cells emerges from US 2012/256592. In this case, too, a partial discharge of the battery cell is performed by means of a resistance connected in parallel with respect to the battery cell.