METHOD AND APPARATUS FOR DETERMINING THE STATE OF CHARGE OF A BATTERY

Abstract: A method of determining the state of charge of a motor vehicle battery (1) in which a stabilization load (2) is applied to the battery (1) after the engine is switched off to accelerate the stabilization of the charge distribution within the battery (1). The stabilization load (2) is applied until the terminal voltage of the battery (1) reaches a level corresponding to a predicted value of the state of charge of the battery (1). After a pre-determined delay of 8 hours or less, the terminal voltage of the battery (1) is measured and the state of charge derived therefrom.

Description: This invention relates to a method and apparatus for determining the state of charge of a battery and, in particular, the battery of a motor vehicle. A well known technique used in warehouses and garages for determining the state of charge of a battery involves the measurement of the open circuit terminal voltage of the battery after it has been stored for some time. The storage time is essential to allow any transient variation of charge distribution within the battery following charging or discharging thereof to stabilise and it is found that storage times of 30 hours or more are needed to obtain a reasonably accurate determination of the state of charge using this technique. Having measured the open circuit terminal voltage, a value for the state of charge is obtained from appropriate conversion tables. This technique is not able to provide an accurate determination of the state of charge soon after the battery has been subject to a charging or discharging current and is therefore unsuitable for use in determining the state of charge of a battery used in a motor vehicle which is rarely left idle for periods greater than 15 hours. US-A-4 390 841 describes an apparatus and a method for continuously monitoring and/or indicating energy that a battery power source has remaining at any time and/or can deliver for example to an electric vehicle, although the measurement may be carried out when the vehicle is at rest. A battery mathematical model forms the basis for monitoring the capacity prediction determined from continual measurement of the discharge current rate and stored battery parameters. The predicted capacity is used to provide a state-of-charge indication. Self-calibration over the life of the battery power supply is enacted through use of a feedback voltage based upon the difference between predicted and measured voltages to correct the battery mathematical model. Terminal voltage and current flow are determined every second and the state of charge is computed on the basis of the current rather than the voltage. The expected terminal voltage is also computed and is compared to the actual voltage to obtain a feedback voltage which is used to check on battery performance in the short term and for recalibration in the long term. FR-A-2 425 159 describes an apparatus and a method for controlling the state of charge of a battery in which, when the terminal voltage exceeds a predetermined level, charging is interrupted and the battery discharged for a predetermined time. After the discharging period, the terminal voltage is checked against a predetermined voltage to determine the state of charge of the battery. There is no mention of a stabilization period following the discharging period. GB-A-2 126 735 describes a method and apparatus for indicating storage battery capacity on the basis of a terminal voltage detected before a load is connected to the battery. The capacity is stored and then updated an unspecified predetermined time after the load has been disconnected from the battery. There is no consideration of the effect of a charging current applied to the battery. According to a first aspect of this invention there is provided a method of determining the state of charge of a battery, which is subjected to charging or discharging current, which method comprises the steps of applying a load to the battery and subsequently measuring the terminal voltage of the battery, wherein a specific predetermined load is applied to the battery, for a short time during a period when any charging or discharging current passing through the battery is due only to relatively small permanent loads, to produce a desired discharging current flow through the battery for accelerating stabilization of the charge distribution therein; stabilization of the charge distribution in the battery is allowed to proceed, during which time any charging or discharging current flowing through the battery is due only to said relatively small permanent loads; and the terminal voltage of the battery is measured a predetermined time of at least two hours following said short time and the state of charge of the battery is evaluated by comparing the measured voltage with stored data on the relationship between the terminal voltage and the state of charge of the battery. According to a second aspect of the present invention there is provided an apparatus for determining the state of charge of a battery which is subjected to charging or discharging current, wherein the apparatus comprises: means for applying, for a short time during a period when any charging or discharging current passing through the battery is due only to relatively small permanent loads, a specific predetermined load to the battery to produce a desired discharging current flow through the battery for accelerating stabilization of the charge distribution therein and subsequently removing said load; measuring means for providing a measurement of the terminal voltage of the battery after a predetermined time delay of at least two hours from when the specific predetermined load is removed, during which time delay any charging or discharging current flowing through the battery is due only to said relatively small permanent loads; and evaluating means for comparing the measurement of the terminal voltage of the battery from the measuring means with data stored in a memory device relating the terminal voltage of the battery to the state of charge of the battery and for indicating the state of charge of the battery. Preferred features and uses of this invention will be apparent from the subsidiary claims of the specification. This invention will now be described, merely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an arrangement for applying a stabilization load to a battery as used in a preferred embodiment of this invention; Figure 2 is a schematic diagram of an arrangement for evaluating the state of charge of the battery as used in a preferred embodiment of this invention; Figure 3 is a schematic diagram of an alternative arrangement for evaluating the state of charge of the battery; Figure 4 is a schematic diagram of an arrangement for providing a predicted value of the state of charge of the battery as used in a preferred embodiment of this invention. In the embodiment of the invention to be described, a stabilization load is applied to the battery being tested after any charging or discharging current flowing therethrough has fallen below a given level, this load acting to accelerate stabilization of the charge distribution in the battery and thus enabling an accurate determination of the state of charge of the battery to be effected sooner than would otherwise have been possible. The stabilization load may, typically, be applied for a period of 2 to 5 minutes and the state of charge determined after a further settling time of 8 hours or less. The embodiments described relate to a battery of a motor vehicle and in this situation it is convenient if the stabilization load is applied to the battery when the engine of the motor vehicle is switched off and any relatively large loads on the battery, eg those due to lights and heating devices, are removed. This may be achieved by sensing when the engine is switched off and checking that any remaining discharge current is less than a value such as 50mA. The relatively small, permanent loads such as those due to electronic clocks and electronic management systems, which cannot be switched off unless the battery is disconnected have little effect upon the method. Figure 1 illustrates a method of applying a stabilization load to a battery 1. The battery 1 to be tested may be connected to a stabilization load 2 by means of a switch S1. The switch S1 is controlled by a comparator 3 which receives as a first input a measurement of the terminal voltage of the battery 1 and a second imput from a ROM 5 connected to receive a predicted, digitised value of the state of charge of the battery 1 and to convert this into a corresponding voltage in accordance with stored data on the relationship between the terminal voltage and the state of charge of the battery. When the engine is switched off, the switch S1 is arranged to be closed by means not shown and is kept closed for a period determined by the comparator 3. In the illustrated arrangement, the load 2 is connected to the battery 1 until the terminal voltage thereof reaches a level corresponding to the predicted state of charge of the battery whereupon the output of the comparator 3 inverts and opens switch S1. The terminal voltage of the battery 1 is thus rapidly brought from a transient level imposed by earlier charging or discharging to a level corresponding to its predicted state of charge as the applied load accelerates the stabilization of charge distribution in the battery although further settling of the charge distribution will continue after application of the stabilization load. The prediction of a value for the state of charge of the battery will be described in detail below with reference to Figure 4. Once the charge distribution in the battery has stabilized to a sufficient degree by means of application of the stabilization load, a further pre-determined settling period is allowed before the state of charge of the battery is determined. This period, which may be as short as 2 hours, may be measured by any suitable timing device. Figure 2 illustrates a first method of determining the state of charge of the battery after this settling period. In this method, the terminal voltage of the battery is measured when it is on open circuit or is subject only to relatively small, permanent loads. A ROM 6 is connected to receive a measurement of the terminal voltage of the battery from an analog to digital convertor 7 and is arranged to provide an uncompensated signal for the state of charge of the battery in accordance with stored data on the direct relationship between the terminal voltage and the state of charge of the battery. As this relationship is temperature dependent, this signal is subject to a correction factor provided by a ROM 8 which is connected to receive a measurement of the battery temperature from an analog to digital convertor 9, the ROM 8 converting the temperature signal into a correction factor in accordance with stored data on the relationships between terminal voltage, temperature and state of charge of the battery. The outputs of the ROMs 6 and 8 are combined to produce a measurement of the state of charge of the battery. The battery temperature is measured by any suitable device such as a thermistor (not shown). Figure 3 illustrates an alternative method of determining the state of charge of the battery after the settling period. In this method, the terminal voltage of the battery is measured whilst a known current is drawn therefrom. A ROM 10 is connected to receive a measurement of the terminal voltage and of the current drawn from the battery from analog to digitial convertors 11 and 12 and is arranged to provide an uncompensated measurement of the state of charge of the battery in accordance with stored data on the relationships between terminal voltage, drawn current and state of charge of the battery. These relationships are also temperature dependent and are therefore subject to a correction factor provided by a ROM 13 which is connected to receive a measurement of the battery temperature via an analog to digital convertor 14, the ROM 13 converting the temperature measurement into a correction factor in accordance with stored data on the relationships between terminal voltage, drawn current, temperature and state of charge of the battery. The outputs of the two ROMs 10 and 13 are combined to produce a measurement of the state of charge of the battery. The current drawn from the battery is measured by the voltage across a resistor (not shown) in series with the battery making allowance, if necessary, for any small, permanent loads on the battery. A method of providing a predicted value for the state of charge of the battery is illustrated in Figure 4. In this method, the current flowing through the battery whilst it is being charged and discharged is measured with respect to time to provide an estimate of the variation of the state of charge of the battery since the previous determination thereof and thus to provide a predicted value for the state of charge of the battery. The current flowing through the battery is measured by the voltage across a resistor (not shown) in series with the battery and this measurement is provided via an analog to digital convertor 15 to a digital integrator 16 which integrates the current with respect to time to provide an approximate amp-hour signal A. As the charge efficiency of a battery is temperature dependent, the accuracy of the prediction can be improved by applying a correction factor B determined by a ROM 17 which is connected to receive a measurement of battery temperature from an analog to digital convertor 18 and to receive a signal based on the previous measurement of the state of charge of the battery. The ROM 17 is arranged to convert the measurements of temperature and previous state of charge into a correction factor in accordance with stored data on the relationships between charge efficiency, temperature and state of charge of the battery. The signals A and B are combined to provide an estimate of the variation of the state of charge of the battery since the last determination thereof from which a predicted value of the state of charge can be derived. This predicted value is used to control the duration of the stabilization load applied to the battery as described in relation to Figure 1. The described apparatus may conveniently be in the form of a microprocessor connected to sensors measuring the various parameters required. The methods and apparatus described above enable a reasonably accurate determination of the state of charge of the battery to be provided relatively soon after any charging or discharging current passing through the battery has fallen below the given level and it is therefore feasible for a determination of the state of charge of a battery of a motor vehicle to be accurately updated when the vehicle is left idle over night. The vehicle can therefore be provided with an indicator for providing the driver with an indication of the last determination of the state of charge of the battery and thus provide a warning if the battery is not sufficiently charged to start the engine. When the car is being used, and the battery thus subject to charging and discharging currents, the described apparatus monitors the current to provide an on-going prediction of the state of charge of the battery. This on-going prediction may, if desired, also be indicated to the driver although it is to be understood that it is merely an estimation which is used, as described above, to determine the duration of the application of the stabilization load when the state of charge is next determined after the engine is switched off and major loads on the battery removed. The stabilization load applied to the battery may be approximately 10 amps and, as mentioned above, may have a duration of between 2 and 5 minutes. In order to accomodate a situation in which a fault occurs which prevents the terminal voltage of the battery from reaching a level corresponding to the predicted state of charge, an upper limit of about 10 minutes is preferably applied to the duration of the stabilization load. The various relationships between terminal voltage, temperature, state of charge, etc. described above may differ for different types of battery so the apparatus described is preferably provided with a plurality of sets of data appropriate to the more common types of battery. The selection of the appropriate data set for the battery being tested may be determined by a manual adjustment of the apparatus, or alternatively, may be sensed automatically by determining the characteristics of the battery being tested by measurements of its terminal voltage and of current drawn therefrom. It may also be advantageous for a motor vehicle to be provided with apparatus for determining the state of charge of the battery using both of the methods described in relation to Figures 2 and 3 since circumstances may arise in which one or other of these methods provides a more accurate or speedier measurement of the state of charge. The selection of the method to be used in any particular circumstance can be governed by a microprocessor in accordance with knowledge of the battery's immediate history. The determination of the state of charge of the battery may also be used to control the charging of the battery by substantially reducing or switching off the charging current when the state of charge reaches a pre-determined level. Thermal runaway of the battery can thereby be prevented.

Claims: A method of determining the state of charge of a battery, which is subjected to charging or discharging current, which method comprises the steps of applying a load (2) to the battery (1) and subsequently measuring the terminal voltage of the battery, characterised in that a specific predetermined load (2) is applied to the battery (1), for a short time during a period when any charging or discharging current passing through the battery is due only to relatively small permanent loads, to produce a desired discharging current flow through the battery for accelerating stabilization of the charge distribution therein; stabilization of the charge distribution in the battery (1) is allowed to proceed, during which time any charging or discharging current flowing through the battery is due only to said relatively small permanent loads; and the terminal voltage of the battery (1) is measured a predetermined time of at least two hours following said short time and the state of charge of the battery is evaluated by comparing the measured voltage with stored data on the relationship between the terminal voltage and the state of charge of the battery. A method according to claim 1, characterised in that the method includes the steps of measuring the current passing through the battery (1) whilst it is being charged and discharged, estimating the variation in the state of charge since the previous determination thereof, and providing a predicted value of the state of charge of the battery. A method according to claim 2, characterised in that the specific predetermined load (2) is applied to the battery (1) for a period of time such that the terminal voltage thereof is brought to a level substantially corresponding to the predicted value of the state of charge of the battery. A method according to claim 2 or 3, characterised in that the method includes the steps of measuring the temperature of the battery (1), and modifying the predicted value of the state of charge of the battery in accordance with stored data on the relationship between temperature and charge efficiency of the battery. A method according to any preceding claim, characterised in that the predetermined time is 8 hours or less. A method according to any preceding claim, characterised in that the method includes the steps of measuring the temperature of the battery (1) and evaluating the state of charge of the battery from stored data on the relationships between terminal voltage, the temperature and the state of charge of the battery. A method according to any preceding claim, characterised in that the method includes the step of selecting the stored data used to evaluate the state of charge of the battery (1) in accordance with the characteristics of the battery being tested. A method according to claim 7, characterised in that the method includes the step of determining the characteristics of the battery being tested by measurements of the terminal voltage and of current drawn from the battery. A method according to any preceding claim, characterised in that the load applied to the battery (1) is approximately 10 amps and is applied for between 2 and 5 minutes. A method of controlling the state of charge of a battery in which the state of charge thereof is determined in accordance with any preceding claim, characterised in that the charging current is substantially reduced or switched off when the evaluated state of charge reaches a predetermined level to thereby prevent thermal run away of the battery (1). Apparatus for determining the state of charge of a battery which is subjected to charging or discharging current, characterised in that the apparatus comprises: means for applying, for a short time during a period when any charging or discharging current passing through the battery (1) is due only to relatively small permanent loads, a specific predetermined load (2) to the battery to produce a desired discharging current flow through the battery for accelerating stabilization of the charge distribution therein and subsequently removing said load; measuring means (7, 11) for providing a measurement of the terminal voltage of the battery (1) after a predetermined time delay of at least two hours from when the specific predetermined load (2) is removed, during which time delay any charging or discharging current flowing through the battery is due only to said relatively small permanent loads; and evaluating means for comparing the measurement of the terminal voltage of the battery from the measuring means with data stored in a memory device (6, 10) relating the terminal voltage of the battery to the state of charge of the battery and for indicating the state of charge of the battery. Apparatus as claimed in claim 11, characterised in that the apparatus includes monitoring means (15) for measuring the current passing through the battery (1) whilst it is being charged or discharged and estimating the variation in the state of charge since the previous determination thereof and providing a predicted value of the state of charge of the battery. Apparatus as claimed in claim 12, characterised in that the apparatus includes control means (3) for removing the load on the battery when the measured terminal voltage reaches a level corresponding to the predicted value of the state of charge of the battery. Apparatus as claimed in claim 11, 12 or 13, characterised in that the apparatus includes sensing means for sensing the temperature of the battery and in that the evaluating means is arranged to evaluate the state of charge from stored data on the relationships between the terminal voltage, the temperature and the state of charge of the battery. A motor car having apparatus as claimed in any one of claims 11 to 14, characterised in that the stabilization means is arranged to apply the said load to the battery when the engine of the motor vehicle is switched off and any remaining discharging current is due only to permanent loads. A motor vehicle as claimed in claim 15, characterised in that the vehicle includes indicating means for providing the driver with an indication of the most recent evaluation of the state of charge of the battery.

Inventors: BARBOUR, PETER WATSON, MACKIE, CARMICHAEL, WILLIAMS, MALCOLM
Applicants: JAGUAR CARS LIMITED
Application year: 1986
Main classification: H02J7
IPC codes: H02J7, G01R31