Method and device for charging a battery

A method for charging a battery, in particular a lithium ion battery, be performing the following: charging the battery using a constant charging current in a first phase, charging the battery using a constant charging voltage in a subsequent second phase, ending the charging as a function of a specifiable boundary value of the charging current in the second phase. In this context, the following operations are provided: comparing a guide voltage specified for setting the constant voltage to at least one stored switch-off value determined as a function of the boundary value, and ending the charging when the guide voltage reaches the switch-off value. Also described is a device for charging the battery.

FIELD OF THE INVENTION

The present invention relates to a method for charging a battery, in particular a lithium ion battery, having the following steps: The battery is first charged using a constant charging current in a first phase, then, in a subsequent second phase, using a constant charging voltage, the charging being terminated as a function of a specifiable boundary value of the charging current in the second phase. The present invention also relates to a device for charging a battery, in particular for carrying out the method described above, having at least one current regulator for charging the battery using a constant charging current in a first phase and at least one voltage regulator for charging the battery using a constant voltage in a subsequent second phase, as well as using a device for ending the charging as a function of a specifiable boundary value of the charging current during the second phase.

BACKGROUND INFORMATION

Methods and devices for charging a battery are believed to be discussed in the related art. By the term battery one should also understand, in this instance, individual or several cells of a battery or of an accumulator. For lithium ion batteries it is particularly known that one may use the so-called IU charging method, which is also designated as CCCV charging method (constant current constant voltage). In this method, the battery is charged in a first phase (I charging) at constant current, while the charging voltage increases. The current is limited, in this instance, by a current regulator of the device, or rather, of the charging unit. Upon reaching a selected end-of-charge voltage at the battery, one switches over from constant current control to voltage control, at which the battery is loaded further at constant voltage. At an increasing charging level of the battery, the charging current drops automatically. It is known that, as a criterion for ending the charging, one may use the falling below of a specifiable boundary value by a falling charging current.

In order to carry out or map the abovementioned charging method, the recording of the charging current and its preparation in signal technology are required.

SUMMARY OF THE INVENTION

It is provided, in accordance with the present invention, that a guide voltage, specified for setting the constant voltage, be compared to at least one stored switch-off value that is determined as a function of the boundary value, and the charging is ended when the guide voltage reaches the switch-off value. Thus, it is provided, in this instance, that for ending the charging process, not the charging current itself, but a switch-off value, that takes into account the boundary value of the charging current, is used as the criterion. The charging current, that drops off during the second phase, is consequently taken into account indirectly. The switch-off value is selected in such a way that it corresponds to the guide voltage at the time of the reaching of the specified boundary value by the charging current. All in all, a charging process may thereby be ended at a favorable time in a simple way without the detecting and evaluating of the charging current having to take place during the second phase.

The switch-off value may be calculated and/or ascertained by empirical investigation. The investigations may be carried out so that the charging current as well as the guide voltage are recorded and compared to each other in the second phase. At the time at which the charging current reaches the specifiable boundary value, the current guide voltage is ascertained and stored.

The guide voltage may be specified as a function of a source voltage, a battery voltage and/or as a function of component parts used for the voltage control. The whole controlled system of the voltage regulator may be taken into account in determining the guide voltage, so that an optimal degree of charging is attained.

An individual switch-off value may be filed for each battery that is to be charged. Thus it is possible, when using the advantageous method, to charge a plurality of batteries in optimal fashion, or to adapt the method automatically to different batteries.

Especially, for the comparison, one of the individual switch-off values may be selected as a function of the battery voltage of the respective battery. The abovementioned variation of the guide voltage at the same current and at different battery voltage, has the result that the switch-off time over various battery voltages is subject to a certain fuzziness. By taking into account, or rather, by filing characteristics values individual to each battery, as was described above, one is able to compensate for this fuzziness by selecting the appropriate switch-off value for the battery to be charged. During the second phase, the battery voltage is constant, and is determined by the development of the battery itself. The battery voltage is easy to record using measuring techniques.

The device according to the present invention stands out in that the device includes at least one control unit, which compares a guide voltage present at the output of the voltage regulator to a switch-off value, stored in a memory of the device and determined as a function of the boundary value, and that it ends the charging when the guide voltage attains the switch-off value. Thus, the device includes a memory in which at least one switch-off value is stored, or rather, as described above, becomes stored. The control unit accesses the stored switch-off value and compares it to the guide voltage present at the output of the voltage regulator, which, as stated above, is used for attaining the degree of charging. When the guide voltage reaches the switch-off value, which corresponds to the time at which the charging current reaches the specifiable boundary value, the charging process is ended. To end the charging process, a switching element may be provided which is able to be actuated by the control unit and which cuts the electrical connection to the battery.

Additional advantages proceed from the method described above. The voltage regulator and/or the current regulator may include at least one operational amplifier, so that the voltage regulation and the current regulation take place in an analog manner and are implemented cost-effectively.

The control unit may include at least one microcontroller, to which the guide voltage is supplied, and which compares this to the switch-off value(s) stored in the memory. Particularly, at least one analog/digital converter may be assigned to the microcontroller, with the aid of which the guide voltage is recorded and supplied to the microcontroller for comparison.

According to one advantageous further development of the device, a measuring arrangement is provided which recognizes the battery that is to be charged. This arrangement may be, for example, a voltage measuring device which records the voltage of the battery to be charged and concludes thereby that it is a certain battery. Thus, during the comparison of the guide voltage to the switch-off value, the individual switch-off value assigned to the battery may be selected in a simple manner.

In the following text, the present invention is to be elucidated in greater detail with the aid of the drawings.

DETAILED DESCRIPTION

In a schematic representation,FIG. 1shows a device1for charging a battery2. Device1has an alternating voltage source3or is able to be connected to it. Source voltage UACsupplied by alternating voltage source3is supplied to a rectifier4, which converts alternating voltage UACto a direct voltage U4, which is supplied to a switch5. Switch5may be one or more semiconductor elements, and particularly may be MOSFET's, bipolar transistors or IGBT's. Alternating voltage U5, that is to be output by switch5, is set by specifying the switching frequency. The former is supplied to a transformer6, which transforms alternating voltage U5to an alternating voltage U6, in a known manner. A second rectifier7is postconnected to transformer6, and it rectifies alternating voltage U6, and provides it at terminals8,9to battery2.

In addition, device1includes an analog current regulator10as well as an analog voltage regulator11, each of which includes an operational amplifier12and13, respectively. Current regulator10is developed to charge the battery using a constant charging current, while voltage regulator11is developed to charge the battery using a constant voltage. At the output of voltage regulator11, a guide voltage UFis set, which is recorded using a voltage divider14and supplied to a control unit15, which includes a microcontroller. A memory, in particular a non-volatile memory17is assigned to microcontroller16, in which comparison values are stored, which will be discussed in greater detail below. For potential isolation, an opto-coupler18follows current regulator10and voltage regulator11, whose output is connected to an input of switch5.

The method for charging battery2will be explained with the aid of the diagram shown inFIG. 2. First, using current regulator10, battery2is charged at a constant charging current IL. In this first phase I, charging voltage ULof battery2increases. As soon as charging voltage ULhas reached a specifiable maximum voltage value, as characterized by time t1, the current-regulated method changes to a voltage-regulated method, in that in phase II, following first phase I, battery2is charged by voltage regulator11at a constant voltage. In second phase II, current ILof battery2drops off. In order to hold charging voltage ULconstant, guide voltage UFis varied. A microcontroller16records guide voltage UF, in this context, and compares it to the values stored in memory17.

The stored values are switch-off values which correspond to guide voltage UFat a time t2, at which the loading current ILreaches a boundary value, as of which the charging process is to be broken off. Individual switch-off values stored in memory17refer to the different batteries that are to be charged by device1, so that, for each battery to be charged, an individual switch-off value is stored. The switch-off values have been ascertained ahead of time by empirical trials. For the purpose of recording and comparing guide variable UFto the respective switch-off value, an analog/digital converter19is assigned to microcontroller16, which is connected to voltage divider14. The microcontroller now compares the guide voltage to the corresponding switch-off value. At a given battery voltage UBatt, microcontroller16is able to branch to the corresponding storage location in memory17, and draw upon the individual switch-off value, stored there, to form the switch-off criterion. When guide voltage UFreaches the switch-off value, the charging process is ended by device1, for instance, by opening switch5. Thus, control unit15, having microcontroller16and analog/digital converter19, as well as memory17, together form a device20for ending the charging.

The time to end the charging process individually for each battery or cell is able to be determined by providing individual switch-off values. Thus, for example, one may take into account that the charging current ILof another battery reaches the specifiable boundary value only at a later time t3, as indicated inFIG. 2.

By device1described above, as well as by the method described, the time for ending the charging process is consequently not directly determined as a function of charging current IL, as usual, but as a function of guide voltage UF. Because of that, in particular, a recording arrangement for recording charging current ILmay be omitted, and device1as well as the method for charging the battery may be configured to be cost-effective and simpler.