Abstract:
A method for the continuous measurement of the efficiency of a battery, especially a battery installed in motor vehicles, comprising the following phases: —determination of the state of charge SOC % of the battery installed in the vehicle, both during key-off phase and key-on phase, without the direct measurement of current through progressive readings of battery voltage, both in the absence and in the presence of electrical load and considering the time elapsed since the beginning of each discharge phase; —determination of the state of health SOH of the same battery through the application of a controlled current load and the comparison of the resulting voltage drop compared to predetermined parameters; —determination of the state of function SOF of said battery through an estimation of its residual autonomy in function of the discharge rate or current rate and of the evolution of SOC %; —correlation and normalization of the above described parameters (SOC, SOH, and SOF) depending on the battery operating temperature;—memorization of at least the values of initial SOC % and SOH detected during the key—off phase.

Description:
FIELD 
     The present invention relates to a method for the continuous measurement of the efficiency of a battery, especially a battery installed in motor vehicles, in addition to the respective device based on the use of this method. 
     More specifically, the present invention relates to a method which provides information concerning the state of charge (SOC), state of health (SOH) and state of function (SOF) of batteries during use in a motor vehicle; the invention, according to this method, also concerns a device connected directly to the battery installed in the motor vehicle, capable of continuously monitoring the above-mentioned battery states and of detecting the approach of critical conditions which do not allow engine restart. 
     BACKGROUND 
     As is known, the starter batteries used in motor vehicles for engine start-up and for powering electrical equipment are prone to gradual discharge and, with the engine running, the alternator compensates for their loss of energy. However, over time, battery efficiency is reduced, even in the presence of constant recharging from the alternator. This drop in efficiency is not usually detected, nor signalled to the user, and usually results in said users needing to address the problem when it is too late, i.e. when the battery has insufficient charge and/or functionality to start the engine. This situation causes great inconvenience, and is often made worse by the fact that users find themselves in places lacking a readily available battery replacement service. 
     It should also be considered that increasing numbers of vehicles are equipped with ‘Stop and Start’ technology. This includes devices which automatically switch off the engine during stops—in order to reduce atmospheric emissions of harmful gases, as well as to reduce fuel consumption. In these conditions a battery, in poor condition, may have difficulty in withstanding continuous engine restarts and the danger of a sudden enforced stop is therefore real. In an attempt to solve this problem a method and a device have been designed, by the same applicant, which enables the detection of battery efficiency, as described in Italian patent No. 1.357.179/2003. Such patent right, however, refers to the determination of the state of charge of batteries in open circuit conditions. The state of health of the battery, indicated by SOH (State of Health), is taken as an identification parameter of the battery condition and its value determines whether said battery is charged, uncharged or to be replaced. The SOH is a function of the state of charge SOC of the battery, directly determinable from the open circuit voltage of the battery itself and its degree of ageing, which results in quantitative terms, in an increase in the battery&#39;s internal resistance, which the method described in the above named patent measures through the application of a controlled current load under predefined conditions. This method, although valid, is more suitable for the realization of a bench device, given that it identifies the state of charge in open-circuit or key-off conditions. 
     Other methods and devices are also known for detecting the state of charge of batteries in key-on conditions, but these solutions are based on the measurement of currents, and hence result in complex and expensive equipment. Solutions of this type are known, for example, from U.S. Pats. No. 6,453,129 and 6,369,578 in addition to DE 19952693 and EP 0908737. 
     SUMMARY 
     The object of the present invention is to overcome the above mentioned drawbacks. 
     More specifically, the object of the present invention is to provide a method and a device which enables the continuous monitoring of the SOC, SOF and SOH states of a battery and the respective variations, both in conditions of open circuit or key-off, as well as closed circuit or key-on conditions. 
     A further object of the invention is to provide a method and a device for determining the residual autonomy of the battery. 
     A further object of the invention is to provide a method and a device capable of detecting the current rate at which the battery is discharging. 
     These and other objects are still achieved by the method for the continuous measurement of the efficiency of a battery and by the device utilizing such method according to the invention in accordance with the enclosed claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The characteristics of the method for the continuous measurement of the efficiency of a battery of the present invention can be better understood from the detailed description that follows, which refers to the enclosed figures which should be considered as having an illustrative and non-limiting aim, respectively: 
         FIG. 1  is a block diagram relating to the calculation of the state of health SOH of the battery, according to the method of the present invention; 
         FIGS. 2 ,  3  and  4  serve as reference diagrams of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the enclosed figures, the method for the continuous measurement of the efficiency of a battery, according to the present invention, includes at least one first phase which determines its State of Charge (SOC) and State of Health (SOH) levels, with the vehicle engine switched off, i.e. in key-off condition. The SOC % is determined by the open circuit voltage (OCV) of the battery, which is reached only after a period in which the vehicle has been left with its engine switched off. This OCV value is obtained from the relation V ocv =f (t), which extrapolates the value of stabilized OCV in function of the key-off time. 
     The open circuit voltage OCV enables the calculation of the initial SOC % which, when quantified, permits the determination of SOH; this is achieved by applying a controlled current load (e.g. between 15 and 50A) and recording, at the same time, the voltage drop ΔV (i.e. the difference between the two voltage values measured) which occurs in a given time period. The latter is preferably between 0.1 and 30 seconds. 
     The SOH value obtained in this manner is a function of SOC % and ΔV, i.e. SOH=f (SOC %, ΔV). The result indicates the state of health of the battery, allowing the determination of whether it is in a condition to start the engine with particular, but not exclusive, reference to the presence of a Stop and Start device in the vehicle. 
     The two parameters SOC % and ΔV, also consequently like the parameter SOH, are a function of the temperature. The values which are determined through such measurements must therefore be fixed in relation to the temperature noted at the time of the measurements. 
     Using the SOC % and ΔV data, the initial value of SOH is defined through the cartesian plane of  FIG. 2  which illustrates the subdivision of the areas that differentiate the conditions of a battery in a dynamic situation. First of all, two quantities are fixed that define the limits within which engine start-up is guaranteed; in particular, a SOC % min  is established, below which engine start-up is not guaranteed and a ΔV max  above which engine start-up is not guaranteed. At this point, the value of ΔV that is recorded following the application of the load, is normalized within the area delimited by the curves [A] and [B] as shown in  FIG. 2 , so that the parameter SOH is converted into a numerical value between 0 (curve [B]) and 1 (curve [A]), through the relation:
 
( b−ΔV )/( b−a ),
 
where “b” denotes the voltage drop that occurs, by application of the controlled current load, at a certain SOC % value, corresponding to the points lying on curve [B] in  FIG. 2  and above which the resulting SOH indicates that the battery is not in good condition. Instead “a” indicates the minimum voltage drop that occurs at a certain SOC % value by application of the controlled current load and corresponding to the points lying on curve [A]. In order to correlate the value of said relation:
 
SOH 25° C.(where 25° indicates the reference temperature of the battery) =( b−ΔV )/( b−a )
 
at the state of charge of the battery and thus evaluate the degree of ageing in relation to that specific state of charge, it is sufficient to check to what extent this point deviates from the corresponding value on curve [A] through the additional relation: 1−[(b−ΔV)/(b−a)], which gives the value E3 corresponding to a number between zero and 1, indicative of the degree of ageing of the battery in relation to a given SOC % value. The E3 values approaching zero refer to new batteries with a certain state of charge, whereas values close to 1 concern batteries with a certain degree of ageing, to which greater attention should be paid, which is expressed, according to the method of the present invention, in a more frequent monitoring of the states: SOC %, SOF and SOH.
 
     After all, in fact, if E3≦1−[(b−ΔV)/(b−a)], the battery can be considered efficient, otherwise an alarm (in sound and/or visual form) is activated on a particular device connected to the battery, or alternatively the functions connected with this parameter will be inhibited or the calculations to determine the states SOC %, SOH and SOF and their possible variations will be repeated with greater frequency. 
     The above described key-off phase involves the measurement, calculation and storage of the initial SOC % and SOH values. 
     The voltage at the battery terminals is monitored during the key-on phase, so as to distinguish when the battery is being charged by the alternator, or when a discharge phase starts. 
     The voltage is monitored according to the following model: 
     ti=0 and Vi=Vo, where Vi is the battery voltage measured at time ti=0 in which the device starts the voltage measurement, 
     tn=ti+Δt, where Δt is the time interval when the battery voltage is detected, and V n =V tn . 
     When the following situations are encountered:
 
 V   tn   −V   (tn−Δt) &gt;−(0.05−0.5) V;  
 
 V   tn   −V   (tn−2Δt) &gt;−(0.05−0.5) V;  
 
 V   tn   −V   (tn−3Δt) &gt;−(0.05−0.5) V;  
 
     the battery begins to discharge, therefore it is necessary to set the function to determine the discharge rate. 
     If the battery has been diagnosed in discharge condition, then the discharge rate or the current rate is determined (described in detail below) by means of the three parameters: initial SOC %, V n  and t n  which represent respectively: 
     SOC %=state of charge, V n =battery voltage at the time t n  and t n =time in seconds counted from the beginning of a discharge phase. 
     Furthermore, based on the discharge rate or the calculated current rate and the initial SOC % value of the battery, it is possible to determine the residual autonomy of the same battery according to that current rate and according to the evolution of SOC % during the discharge. 
     Considering the fact that, in normal conditions of use, the current varies continuously, this phase of the procedure is repeated in order to continuously update the battery&#39;s actual SOC % value, using the method of the present invention. In particular, it is possible to detect the change in discharge rate of the battery by observing the voltage and then continuously updating the calculation of the discharge rate and, hence, the SOC % and of the remaining battery power to the n-th time in which the calculation is started. To determine the discharge rate or the current rate, in the event in which the battery has been diagnosed in a discharge condition (and not being charged by the alternator) the following quantities are established:
 
 V   n   ,t   n   ,t   n −3 Δt= 0,SOC % init =SOC % tn−3Δt  
 
     Thus a “y o ” parameter and an additional A1 parameter are identified, both being functions of the battery voltage, V n , as well as the initial state of charge of the battery. 
     The parameter y o  is a numerical coefficient, like that of A1, whose numerical values can be determined from the initial SOC % and the voltage V n  detected by the device at time t n    
     In particular, y o  is a function of the voltage V n  through the relation:
 
 y   o   =B 1*(− V   n )+ B 2*(− V   o ) 2   +B 3
 
     The dependence of y o  on the initial SOC % is derived from the parameters B1, B2 and B3 which vary linearly in function of the initial SOC % of the battery. 
     In the event of A1, the dependence on the voltage Vn is expressed by the following relation:
 
 A 1= C *exp(−exp(− k 1*((− V   n )− k 2)))
 
     In which k2 and k3 are constants, while through parameter C the value of A1 is corrected in function of the initial SOC %. In fact, C is a function of the initial SOC %. 
     Through the two parameters A1 and y0 described above, one can determine the current rate i(A) at which the battery is discharging through the following relation:
 
 i ( A )=− k   3 ·ln[( t   n   −y   o )/ A 1],
 
where k3 is a constant and i(A) corresponds to the current in amperes.
 
     At this point, given the value i(A) that indicates at which current rate the battery is discharging, it is possible to calculate the SOC % value of the battery at the time t n . The value of i(A) is used to determine the parameter m, which indicates the gradient, or angular coefficient, of the linear correlation between the SOC %, which evolves during the discharge, and the time t n , (described later) through the relation:
 
 m=k   4   ·[i ( A ) 2]+k   5   ·[i ( A )]+ k   6  
 
in which k 4 , k 5  and k 6  are numerical coefficients that allow the mathematical correlation between the current i(A) and the angular coefficient m, subsequently used to determine the variation of the SOC %. These numerical coefficients, k 4 , k 5  and k 6  vary depending on the capacity of the battery on which the device is installed.
 
     The trend that indicates the variation of the angular coefficient “m” in function of the calculated current discharge rate is shown in  FIG. 3 . 
     Through the value “m” it is therefore possible to calculate the SOC % at time t n  with the following relation:
 
SOC %= m ·( t   n )+SOC % init  
 
     The relative trend is illustrated in  FIG. 4  specifically, and by way of example, for 100 Ah batteries with an initial SOC % of 90%. This trend was also verified in the other charge states and in lower capacity batteries. 
     The voltage is systematically monitored and when it detects a variation that indicates a drop, for example, of between 3% and 7% from the initial SOC % value, the SOH value of the battery is calculated at time t n , by the application of a controlled current load according to the previously described methods and is verified according to the block diagram of  FIG. 1 . In relation to this diagram, it is established whether the new SOH value calculated at time t n  is indicative of a battery which is still capable of withstanding a discharge rate or not, and the respective values are transmitted in output. 
     The above calculation, which allows the estimation of the current rate under which the battery is discharging and the gradual evolution of SOC %, allows the determination of residual battery autonomy, which is defined by the SOF. In fact, during a discharge phase, the continuous variation of the current load that draws on the battery involves a continuous change in the rate of evolution of SOC %. Thus, having set a SOC % min , the SOF will be a parameter, a function of i(A) (current rate at which the battery is discharging), of the SOC % value fixed as SOC % min , of SOC % calculated at the time t n  and, hence, also of the temperature.
 
SOF= f ( i ( A ),SOC % min ,SOC %,T° C.).
 
     The continuous variation of i(A) and consequently of SOC % will therefore result in a continuous variation of the residual battery autonomy, which will be expressed by the parameter SOF. 
     Said device, installed in the vehicle and in electrical contact with the battery, by which it is powered, includes at least one temperature sensor and one voltage sensor, with connecting wires to the battery terminals, a predetermined and controlled resistive load, an electronic monitoring, recording, processing and data storage card, must be able to transmit in output the processed information (encoded or not), with the possible transmission of an audible and/or visible signal to the driver of the vehicle. 
     Although the invention has been described above with particular reference to one embodiment, which should be considered as having an illustrative and non-limiting aim, numerous modifications and variations will appear obvious to a person skilled in the art, in light of the above description. Therefore the present invention intends to cover all modifications and variations that fall within the spirit and scope of protection of the following claims.