Method for determining the starting ability of a starter battery in a motor vehicle

A method for determining the starting ability of the starter battery in a motor vehicle. The current and voltage of the storage battery are measured before starting is begun, shortly after starting is begun and at short intervals of time during the starting procedure. The current/voltage value pairs for each instant are used to calculate a resistance value and the quantity of charge drawn from the storage battery, and the rate of rise in the resistance values as a function of the quantity of charge drawn is used to derive a measure of the availability of the storage battery during the starting procedure. In particular, the gradient .DELTA.RQ of the internal resistance as a function of the quantity of charge drawn and the turn-on internal resistance R.sub.E are stored in an arithmetical memory as a field as a function of the charge state or of the equivalent no-load voltage (U.sub.R) and the battery temperature and are compared with already recorded values or typical normal values.

FIELD OF THE INVENTION
 This invention relates to a method for determining the starting ability of
 the starter battery in a motor vehicle.
 BACKGROUND
 Of crucial importance for the ability of a starter battery to start a motor
 vehicle having an internal combustion engine are the charge state and the
 advancement of aging or the decline in capacitance, because these limit
 the current strength which can be drawn from the starter battery, or the
 power output thereof. Starting the engine is amongst the heaviest demands
 on the battery in the motor vehicle. The starting procedure is when it is
 most noticeable if the performance of a battery is reduced by aging.
 The reduction in the loading capacity of a battery over relatively long
 periods of time is caused, on the one hand, by the rise in the nonreactive
 internal resistance of the battery, and, on the other hand, by passivation
 phenomena on the active materials.
 In a modern motor vehicle, the properties of the generator, the battery and
 the electrical loads are matched to one another such that only an instance
 of several mishandling can result in battery charge states being so low
 that the starting ability is no longer assured. The request for a reliable
 indicator for battery replacement is, therefore, of great importance,
 particularly when safety-related electrical loads are installed, such as
 electric brakes or electrically assisted steering aids.
 DE-C 3901680 discloses a method for monitoring the cold starting ability of
 the starter battery for an internal combustion engine, in which the time
 profile of the voltage drop which occurs on starting is observed and
 evaluated. The evaluation is carried out on the basis of maximum ratings
 for a characteristic curve obtained from empirical values, and on the
 basis of the battery temperature.
 DE-C 3712629 describes a measuring device for the remaining useful life of
 a motor vehicle battery which records the battery voltage and the
 associated load current value before and after starting for the first time
 with the battery in the fully charged state, ascertains the
 temperature-compensated internal resistance, stores it in a memory and
 compares it with the internal resistance values ascertained for subsequent
 starting procedures in the internal combustion engine. An indication is
 then given on the basis of prescribed stored threshold values.
 DE-A 1 19750309 discloses a method for determining the starting ability of
 the starter battery in a motor vehicle, in which the voltage dip on
 starting is recorded and stored taking into account the temperature of the
 battery and the engine. The increase in the size of the voltage dip,
 associated with aging of the battery, as compared with the fresh values is
 used as a measure of battery aging.
 WO 99/17128 discloses the practice of comparing, during the starting
 procedure, the voltage values of adjacent voltage minima arising on
 account of the compression and decompression of the engine pistons. The
 difference between these values is used as a measure for indicating the
 battery state.
 Thus, it would be highly advantageous to provide a method for determining
 tie starting ability or the availability of a storage battery used for
 starting an internal combustion engine, wherein the method reliably
 ascertains the state of the battery.
 SUMMARY OF THE INVENTION
 Thus, the invention embodies a method for determining the starting ability
 of a starter battery in a motor vehicle including measuring current and
 voltage of the storage battery before starting is begun; measuring current
 and voltage of the storage battery shortly after starting is begun;
 measuring current and voltage of the storage battery at predetermined
 intervals of time during starting; calculating a resistance value and the
 quantity of charge drawn from the storage battery from current/voltage
 value pairs for each measurement; and determining a measure of
 availability of the storage battery during starting based on the rate of
 rise in the resistance values as a function of the quantity of charge
 drawn.

DETAILED DESCRIPTION OF THE INVENTION
 According to the invention, the current and voltage of the storage battery
 are measured before starting is begun, shortly after starting is begun and
 at short intervals of during the starting procedure. The current/voltage
 value pairs are used to calculate, for each instant, a resistance value
 and the quantity of charge drawn from the storage battery after starting
 begins. The rate of rise in the storage battery internal resistance as a
 function of the charge drawn is used to determine a measure of the
 availability of the storage battery for the starting procedure.
 In the case of the invention method for determining the starting ability or
 the availability of a storage battery used for starting an internal
 combustion engine, the two essential causes of battery aging are clearly
 distinguished: the purely nonreactive turn-on resistance and the
 resistances which additionally build up during starting over time. Since
 each resistance component can independently impede the starting procedure,
 different boundary conditions also apply for each one. The two resistances
 taken together provide a comprehensive description of the starting ability
 of a battery.
 In addition, the charge state and the battery temperature can be taken into
 account. By way of example, the charge state can be estimated from the
 no-load voltage measured before starting. A method for estimating the
 charge state from the no-load voltage is described in prior German Patent
 Application 198 47 648, the subject matter of which is incorporated by
 reference.
 As can be seen from FIG. 1, when the starting procedure is initiated, the
 current has a momentary peak value and the voltage has a maximum dip. As
 the starting procedure progresses further, the internal combustion engine
 is set in motion by the starter, with the result that the current falls
 again in waves, corresponding to the compression and decompression of the
 cylinders, and the voltage recovers accordingly. If a minimum speed of the
 internal combustion engine is reached, it continues to run automatically.
 If, for a battery which has aged a little and whose response is plotted in
 FIG. 1, the voltage is plotted against the current as in FIG. 2, then a
 relatively linear curve is obtained, corresponding to curve 2. In FIG. 2,
 point A corresponds to the value pair (U.sub.VS,I.sub.VS), i.e., voltage
 and current before starting, where there is generally still a low level of
 loading by customary loads. The instant of this measurement is not
 particularly critical. A measurement can be taken directly before starting
 or else several seconds beforehand, for example, two seconds beforehand,
 if no heavy loads are turned on otherwise.
 Point B in FIG. 2 shows the value pair (U.sub.E,I.sub.E), namely the
 voltage and current shortly after starting is begun. The goal is to select
 the instant of maximum current. It is usually sufficiently accurate to
 take a measurement no later than approximately 0.02 seconds after starting
 is begun, however. The gradient of the connecting line between points A
 and B, or the quotient of U.sub.VS -U.sub.E and I.sub.E -I.sub.VS, is the
 turn-on internal resistance R.sub.E.
 In the course of the starting procedure, values of U.sub.S and I.sub.S
 measured at short intervals of time are plotted as points in curve 2
 during the starting procedure. The measurement instants are 0.01 second
 apart, for example. In principle, the spacing, between the measurement
 instants needs to be selected such that the starting procedure has
 sufficient resolution, i.e., the temporal spacing may be between about
 0.001 sec. and about 0.1 sec. It is merely necessary to ensure that there
 is a sufficient number of measured values, for example, about 100 measured
 values for a normal average starting duration of about 1 sec.
 In FIG. 5, the internal resistance of the storage battery is plotted
 against the quantity of charge drawn from the storage battery for each
 measurement instant. According to the invention, the gradient of the
 resistance line as a function of the quantity of charge converted is used
 as a measure of aging. In the present example, in a range from 0 to
 approximately 1 second, the change in resistance per quantity of charge
 converted .DELTA.RQ, or the measure of aging, is largely independent of
 the determination instant.
 If the measured values have a relatively large degree of scatter in
 comparison with the compensating curves, it is advantageous to include
 further operands when ascertaining the measure of aging.
 Of particular interest in curve 2 of FIG. 2 is the range C, which is
 approximately parallel to line 1, that is, the range in which the gradient
 R.sub.S of curve 2 is approximately equivalent to the value R.sub.E.
 Thus, a voltage and current pair (U.sub.S,I.sub.S) is determined in the
 time range in which the gradients R.sub.E and R.sub.S are as near to the
 same size as possible, with R.sub.s being determined as the inclination of
 curve 2 using U I values at two respective instants, which are about 0.2 s
 apart, for example. The tangent to curve 2 in the range which is parallel
 to curve 1 intersects the voltage coordinate (I=O) at the equivalent
 no-load voltage U.sub.R. This tangent is virtually the resistance line of
 the battery in the range relevant to starting.
 If the same method is used with the current/voltage characteristic curve in
 FIG. 3, then the result in FIG. 4 is a representation of voltage as a
 function of current for a very severely weakened battery. It can be seen
 that the internal resistance, represented by the individual points in FIG.
 4, rises.
 If tangents are placed into the associated current/voltage values in this
 representation, then it is evident that the tangents fan out or that the
 gradient of the tangents, or the internal resistance, rises. In the case
 of the line denoted by 3 in FIG. 4, the internal resistance reaches a
 maximum. All the tangents have a common point of origin, which is
 approximately at U.sub.R. This knowledge makes it possible to correct the
 scatter of the measured values.
 The value .DELTA.RQ and the turn-on resistance R.sub.E are stored in an
 arithmetical memory as a field as a function of the charge state or of the
 equivalent no-load voltage and the battery temperature, and are compared
 with already recorded values or typical standard values. Relatively large
 differences, such as a gradient .DELTA.RQ&gt;0.001 m.OMEGA./As (ascertained
 for a customary starter battery with 60 Ah), indicate advanced battery
 aging or lack of charge. Whether this relates to battery aging or lack of
 charge is distinguished from the charge state. With a charge state of more
 than 40%, for example, there should be no starting problems at normal
 winter temperatures.
 In the case of fresh batteries with a sufficiently high charge state, or in
 the case of high battery temperatures, the starting procedure usually
 takes place so quickly that evaluation is not possible. In such cases,
 evaluation is not necessary, however, and the gradient of the resistance
 line .DELTA.RQ can be set equal to zero. As the battery starts to age,
 however, evaluation produces discrete values, which means that it is
 possible to react early enough by replacing the battery, for example. The
 deterioration in the battery's power. as determined in accordance with the
 invention, can be indicated to the driver if at prescribed limit value is
 exceeded, for example, or automatic consequences can be triggered, for
 example, in the charging technology, by aiming for a higher charge state
 to compensate for advancing aging. Such changes can, of course, be
 effected not only when a threshold value is reached, but also gradually,
 on the basis of ascertained mean values. Finally, an apparent sudden
 improvement in the battery can infer that it has been replaced, for
 example.