Electronic unit for detecting the charging condition and/or the wear of a motor vehicle battery using the engine rotational speed and battery voltage

[0002] The invention relates to an electronic unit and to the method for detecting the charging condition and/or the wear of a motor vehicle battery according to the preamble of claim 1.

[0003] Many methods of determining the charging condition and/or the wear of a motor vehicle battery are known. For example, in German Patent Documents DE 37 12 629 A1 or DE 43 41 826 A1, it is suggested to derive the battery condition from the voltage break caused by the starter current. For this purpose, it is suggested to measure the battery current. From German Patent Documents DE 198 31 723 A1 and DE 44 06 193 A1, it is known to derive the battery condition from the voltage break caused by the starter current without measuring the battery current. In this case, particularly only the battery voltage and the battery temperature or internal-combustion engine temperature are taken into account. Furthermore, a charging condition determination by means of a quiescent voltage measurement is known.

[0004] All methods using a battery current measurement are relatively precise. However, since the battery current, particularly in large internal-combustion engines in motor vehicles, may amount to more than 1,000 A, very high-expenditure and expensive measuring systems are required which, in addition, have to permit a high time resolution. The precision of the methods without a battery current measurement is very low.

[0005] It is therefore an object of the invention to improve a system and a method respectively for detecting the charging condition and/or the wear of a motor vehicle battery of the initially mentioned type with respect to the costs and the precision.

[0006] This object is achieved by means of the characteristics of claims 1 and 4 respectively with respect to the device and the method. Advantageous further developments of the invention are the objects of the dependent claims.

[0007] The invention is based on the recognition that the battery voltage or the battery voltage break during a starting operation depends, particularly, on the drag moment of the internal-combustion engine. The drag moment, and thus the starter current or battery current, is subjected to influences by the temperature and manufacturing tolerances, but also changes by wear, equipment variants or insufficient maintenance. As determined empirically, all these quantities, similarly to affecting the battery current, also affect the rotational speed of the internal-combustion engine.

[0008] According to the invention, particularly an analysis of the battery voltage takes place as a function of the rotational speed of the internal-combustion engine during the starting operation with respect to the charging condition and/or the wear of the vehicle battery. The rotational speed of the internal-combustion engine can be digitally transmitted, for example, by way of a data bus (such as a CAN bus) which connects an internal-combustion engine control unit with the electronic unit. However, the electronic unit may also be part of a control unit of the internal combustion engine or of another control unit which itself detects and analyzes the rotational speed of the internal-combustion engine anyhow. In the electronic unit, a table, a characteristic curve or a characteristic diagram, for example, can be stored which assign a charging condition value and/or a wear value to the rotational-speed dependent battery voltage values. In this case, for example, the battery temperature can also be taken into account. However, in each case, no current measurement is carried out. The current as such therefore does not have to be measured in the case of this invention. Nevertheless, by taking into account the rotational speed of the internal-combustion engine, a high precision is achieved because, as determined empirically, the rotational speed of the internal-combustion engine acts, to a sufficiently precise degree, proportional to the current consumed by the starter.

[0009] As a further development of the invention, a preferably variable voltage threshold is defined as a function of the rotational speed of the internal-combustion engine. A poor charging condition and/or a high wear of the motor vehicle battery is/are detected when the battery voltage is below this voltage threshold within a defined time window, preferably at a defined point in time.

[0010] As a further development of the invention, the charging condition is also detected by a quiescent voltage measurement before the starting operation in order to be able to differentiate by means of the analysis of this quantity between a poor charging condition and a high wear when, during the starting operation, the battery voltage is below the defined voltage threshold. A poor charging condition is detected when previously there was a falling below a defined quiescent voltage threshold. A high wear is detected when previously the defined quiescent voltage threshold was exceeded.

[0011] As a result of the invention, a high-expenditure and expensive measuring of the battery current or of the current consumed by the starter during the starting operation can be eliminated. In comparison to all known methods without a battery current measurement, a clearly improved precision can be achieved. An automatic adaption of the method is achieved to different equipment variants of a vehicle (automatic transmission, air conditioner, type of internal-combustion engine) which have an effect on the drag moment of a vehicle. An adaption method shown, for example, in German Patent Document DE 197 50 309 can therefore be eliminated because these effects all enter into the course of the rotational speed of the internal-combustion engine.

[0012] The drawing illustrates an embodiment of the invention.

[0013] FIG. 1 is a definition according to the invention of a variable voltage threshold for the battery voltage which is dependent on the rotational speed of the internal-combustion engine; and

[0014] FIG. 2 is a view of the measurement of the battery voltage at a defined point in time.

[0015] In FIG. 1, rotational speed n of the internal-combustion engine is plotted on the abscissa and the battery voltage U_Battis plotted on the ordinate. FIG. 1 illustrates, by means of the solid line, the empirically determined relationship between the battery voltage U_Battmeasured at a certain point in time (for example, t_m) during many starting operations and the isochronously detected rotational speed n of the internal-combustion engine for an intact motor vehicle battery. Such a course is stored, for example, as an optimal characteristic curve in the electronic unit for implementing the method according to the invention. This relationship could also be stored as a characteristic diagram while additionally taking into account the battery temperature. The broken line illustrates the corresponding definition of a voltage threshold S=f(n) in the form of a tolerance band dependent on the rotational speed n of the internal-combustion engine. Above the threshold S, the motor vehicle battery can still be considered to be intact. A rotational-speed-dependent battery voltage value U_Battbelow the threshold S points to a poor charging condition or to a high wear.

[0016] In FIG. 2, the time t is plotted on the abscissa, and the battery voltage U_Batt, on the one hand, and the rotational speed value n* of the internal-combustion engine, on the other hand, are plotted on the ordinate. The rotational speed value n* of the internal-combustion engine corresponds to the digital and optionally smoothed value of the measured rotational speed n of the internal-combustion engine transmitted by the data bus. FIG. 2 shows the course of the battery voltage U_Battas well as the course of the rotational speed n of the internal-combustion engine in the form of the values n*, particularly during the starting operation. At the point in time t₁, the starter is started; at the point in time t₂, the rotational speed n or n* of the internal-combustion engine starts to rise in a detectable manner starting from zero [1/min]; and at the point in time t₃, the internal-combustion engine starts to run up under its own force. Within the time window of t₂to t₃, here at the point in time t_m, the battery voltage U_Batt, and isochronously the rotational speed n or n* of the internal-combustion engine is detected. The point in time t_mcan be defined by a fixed time period after the point in time t₂.

[0017] At the point in time t_m, the value U₁is obtained for the battery voltage U_Batt, the value nl is obtained for the rotational speed n* of the internal-combustion engine. By means of this value pair U₁, n1, a comparison with the threshold S in FIG. 1 takes place. Because the battery voltage U₁for the rotational speed value n1 of the internal-combustion engine is below the threshold value S=f(n), either a poor charging condition or a high wear is present. If the charging condition of the battery before the starting operation is known on the basis of a quiescent voltage measurement, a differentiation can be made as to whether the battery voltage during the starting operation was too low because of its wear or whether its charging condition was insufficient (not shown here). It is concluded, for example, that the charging condition is poor if previously there was a falling-below a defined quiescent voltage threshold. It is concluded that the wear is high if previously the defined quiescent voltage threshold was exceeded.