Charge circuit for battery cells

A device for charging a battery system having a number of individual voltage sources situated in a series circuit is provided, which device uses a voltage source and a respective bypass associated with each of the individual voltage sources. A charge current IL is supplied from the voltage source via the bypasses to the individual voltage sources as a function of their charge condition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and a device for charging a battery system having a plurality of voltage sources.

2. Description of Related Art

Published German patent document DE 101 50 376 describes a device for equalizing the charge condition of accumulators connected in series. According to this approach, a capacitor is provided, as well as a plurality of switches situated between the capacitor and the accumulators. Furthermore, the device has a logic which is used for measuring the voltages applied to the accumulators and, when a predefined differential threshold value between the voltages applied to the accumulators is exceeded, for activating a driver controlling the switches. The capacitor is connectable for the purpose of a charge exchange alternatingly with the individual accumulators via the switches. In addition, the device known from published German patent document DE 101 50 376 has two accumulators connected in series and four switches. A first of the switches is situated between a first terminal of the capacitor and the terminal of the accumulators connected in series, farther away from the ground, a second of the switches is situated between the second terminal of the capacitor and ground, a third of the switches is situated between the second terminal of the capacitor and the point of connection between the two accumulators, and a fourth of the switches is situated between the first terminal of the capacitor and the point of connection between the two accumulators.

If a plurality of battery cells is connected in series in order to increase the output voltage of the battery system, the voltage across the individual cells is established, among other things, as a function of their “health condition” or of the prevailing internal resistance, in addition to other influencing factors when the battery system is charged. If battery cells which are sensitive to overvoltages are used, the charging operation of the battery system is terminated when the maximum allowable cell voltage is attained at one of the individual cells connected in series.

The more homogeneously the individual cells are able to be charged, the higher the service life of the battery system. Since, during the charging operation, there is inhomogeneity within the individual cells of the battery system, in which the maximum allowable cell voltage value is attained before other individual cells attain this value, the charging operation is switched off, although some of the cells are not yet fully charged. On discharging, the not fully charged individual cells initially attain a predefined lower voltage value and the discharging operation is terminated, although individual cells in the battery system which are capable of outputting their charge are still available. This effect of inhomogeneity may become reinforced over the operating time and as a function of the number of charging and discharging cycles to which the battery system is subjected. The greater the inhomogeneity, the smaller is the difference between available and chargeable energy. Due to the above-described effect, the battery system is no longer able to be used effectively and must therefore be replaced. In general, this effect of inhomogeneity is counteracted by a careful selection of the individual cells having identical characteristics for serial circuits in a battery system, and by using a charge transfer in which a charge is transferred from individual cells which are in a better condition to individual cells which are in a worse condition via transformer circuits. In contrast, the selection of the individual cells represents a relatively high degree of complexity. Transformers, which were previously used for this purpose, require a large installation space and are expensive. Furthermore, removal of heat during the homogenization operation requires expending a considerable effort.

BRIEF SUMMARY OF THE INVENTION

An object of the present invention is to increase the service life of a battery system by keeping the individual cells in a homogeneous charge range among them.

An energy source for obtaining a more homogeneous charge distribution which is available on the system side and is connected to the battery system from the outside is used for charging the battery system. No transformers are needed for transporting the charge to the individual cells of the battery system; the selection of the individual cells of the battery system may be determined in a larger bandwidth.

The more homogeneous charge distribution within the battery system is achieved by adding a bypass for each individual cell of the battery system. Either at least one resistor or at least one inductor may be connected in series into the bypass for current limitation.

Each bypass around each individual cell of the battery system is associated with a bypass switch for bypassing the charge current past each individual cell. The intensity of the bypass current may be regulated via a timed activation of the bypass switch in such a way that the mean current from the individual cells is approximately equal to 0, so the individual cell is not discharged when the bypass is switched on. The following equation applies to the current regulation:
IBz=IBy−IL≈0
where:IBy=current through the individual cell bypassIBz=current from an individual cellIL=charge current of the battery system

To detect the value 0 of the current from an individual cell IBz, either the voltage of the individual cells may be monitored so that the maximum allowable limiting value is not exceeded, or a sensor may be used for each individual cell which measures or represents the current through the individual cell.

The bypasses associated with the individual cells of the battery system are activated during the charging operation of the battery system when the voltage across the particular individual cell reaches the maximum allowable voltage value. These individual cells are fully charged in this case and need no further charging. The individual cells which, in contrast, have lower charge conditions, have not yet reached their maximum allowable voltage value and may therefore receive an additional charge. The charging operation is completed when either all individual cells or a previously defined number of individual cells have reached their maximum allowable voltage value.

Charge current ILthrough the voltage source or current source may be limited with the aid of a smart control by the battery charge controller. The charge current limitation is activated when one or more bypasses associated with the individual cells of the battery system are activated.

DETAILED DESCRIPTION OF THE INVENTION

The sole FIGURE,FIG. 1, embodiment of the device proposed invention for charging a battery system using a number of individual voltage sources connected in series.

It is apparent from the drawing that the charge circuit proposed according to the present invention has a voltage source10via which a battery system12is charged. Battery system12includes individual voltage sources14.1,14.2, . . .14.n, which are situated in a series circuit16within battery system12. Any number of individual voltage sources14.1,14.2, . . .14.n may be situated in battery system12. It is important that individual voltage sources14.1,14.2, . . .14.n are situated in series circuit16. Battery system12according to the illustration in the drawing is associated with a battery charging controller24.

Each of individual voltage sources14.1,14.2, . . .14.n of battery system12has a bypass18.1,18.2, . . .18.n. The particular individual voltage source14.1,14.2, . . .14.n may be connected to bypasses18.1,18.2, . . .18.n associated with each individual voltage source14.1,14.2, . . .14.n. Either at least one resistor or at least one inductor may be serially situated in each of bypasses18.1,18.2, . . .18.n for current limitation. In addition, a bypass switch20.1,20.2, . . .20.n is associated with each of bypasses18.1,18.2, . . .18.n. Furthermore, in the embodiment variant of the charge circuit illustrated in the drawing, a voltmeter26is contained in each bypass18.1,18.2, . . .18.n. Instead of voltmeter26monitoring the particular voltage in individual voltage sources14.1,14.2, . . .14.n, a sensor may also be associated with the particular individual voltage sources14.1,14.2, . . .14.n which either measures the current flowing through the particular individual voltage source14.1,14.2, . . .14.n or represents the current flowing through the particular individual voltage source14.1,14.2, . . .14.n.

Due to the configuration illustrated in the drawing of the charge circuit for charging battery system12, voltage source10provided on the system side and connected to battery system12from the outside is used for obtaining a homogeneous charge distribution within battery system12. No transformers are needed for transporting the charge to the particular individual voltage sources14.1,14.2, . . .14.n; furthermore, the selection of individual voltage sources14.1,14.2, . . .14.n for battery system12may be determined in a greater bandwidth, i.e., using a greater tolerance.

The homogeneous charge distribution due to the approach proposed according to the present invention is obtained by the fact that, when a bypass18.1,18.2, . . .18.n is connected to the particular individual voltage source14.1,14.2, . . .14.n, discharge of the particular individual voltage source14.1,14.2, . . .14.n is avoided by regulating the intensity of bypass current IBy,1 . . . nusing timed control24of the particular bypass switch20.1,20.2, . . .20.n. The regulation takes place in such a way that a mean currentIBz,1 . . . napproximately equal to 0 results from individual voltage sources14.1,14.2, . . .14.n.

To detect a value approximately equal to 0 of current IBz,1 . . . n, on the one hand the voltage across individual voltage sources14.1,14.2, . . .14.n may be detected with the aid of voltmeters26. During the charging operation, the voltage across the particular individual voltage sources14.1,14.2, . . .14.n may not exceed the maximum allowable limiting value. On the other hand, the current through the particular individual voltage sources14.1,14.2, . . .14.n, denoted by IBz,1 . . . n, may also be detected using a sensor associated with one of the particular individual voltage sources14.1,14.2, . . .14.n, which either measures or represents the current.

Bypasses18.1,18.2, . . .18.n are activated at individual voltage sources14.1,14.2, . . .14.n during the charging operation of battery system12by voltage source10situated on the outside when the voltage across the particular individual voltage sources14.1,14.2, . . .14.n reaches the maximum allowable voltage value. In this case, those individual voltage sources14.1,14.2, . . .14.n are fully charged and need no further charging by charge current ILsupplied by voltage source10. In contrast, those individual voltage sources14.1,14.2, . . .14.n having lower charge conditions have not yet reached their maximum allowable voltage value and may be further charged by externally situated voltage source10.

The charging operation is completed when either all individual voltage sources14.1,14.2, . . .14.n or a previously defined number of individual voltage sources14.1,14.2, . . .14.n has reached their maximum allowable voltage value.

To complete the charging operation, i.e., the supply of charge current ILby voltage source10into battery system12, either the criterion “all cells” or the criterion “defined number of cells charged to the maximum allowable voltage value” may be used. This is a function of the application of the charge circuit proposed according to the present invention for battery system12or of the allowable power loss which may be removed by the bypass circuits.

To protect the bypass circuits, in particular bypass switches20.1,20.2, . . .20.n, charge current ILthrough the voltage source or current source10may be limited via control by battery charging controller24. The charge current limitation for charge current ILmay be activated in particular when one or more of bypasses18.118.2, . . .18.n are activated. “A” denotes sensors for detecting the current in individual voltage sources14.1through14.n, which may be used as an alternative to voltmeters26. Resistors or inductors may be connected in series in respective bypasses18.1through18.n; furthermore, bypass switches20.1through20.n may be designed as semiconductor switches rather than as discrete switches.

An inhomogeneous charge distribution within individual voltage sources14.114.2, . . .14.n within battery system12may be avoided via the device for charging battery system12proposed according to the present invention.