Abstract:
The invention relates to a device for measuring battery voltage including an accumulator (Ai), characterized in that said measuring device comprises: an alternating current generator (II), at least one first diode (D 1   i ) and at least one second diode (D 2   i ) arranged in series such that said first diode (D 1   i ) is connected to the positive pole (+) of said accumulator (Ai) by means of the cathode thereof and to the cathode of said second diode (D 2   i ) by means of the anode thereof and such that said second diode (D 2   i ) is connected to the negative pole (−) of said accumulator (Ai) by means of the anode thereof and to the anode of said first diode (D 1   i ) by means of the cathode thereof, a reading capacitor (C 1 ) connected to the central point (N) between said first (D 1   i ) and second (D 2   i ) diodes and to a means ( 3 ) for determining the voltage across the terminals of said accumulator (Ai) from the voltage variation at the central point (N).

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is the National Stage of International Application No. PCT/EP2010/065660, filed on Oct. 18, 2010, which claims the benefit of the priority date of French Application No. 0905005, filed on Oct. 19, 2009. The content of these applications is hereby incorporated by reference in its entirety. 
     FIELD OF DISCLOSURE 
     The invention relates to a measuring device for batteries of electrochemical accumulators, which can be used in particular in the field of electrical and hybrid transport and the embedded systems. 
     The invention relates in particular to the batteries of lithium-ion type (Li-ion) which are well suited to this kind of application, because of their capacity to store a strong energy with a low weight. 
     BACKGROUND 
     The expression “battery of accumulators” is used to mean the series connection of a number of accumulators or of accumulators in parallel. 
     The charging of an accumulator is reflected in an increase in the voltage at its terminals. An accumulator is considered to be charged when it has reached a voltage level defined by the electrochemical process. 
     Furthermore, the lithium-ion technology batteries have a minimum threshold voltage below which the accumulators of the battery may be damaged or destroyed, which can result in malfunctioning of the battery, for example the current is blocked and the battery no longer delivers any energy. 
     Furthermore, in a circuit using a number of accumulators connected in series, it is highly probable that the level of charge of each accumulator is not the same. This is because it is possible for one accumulator to be discharged before the other accumulators of the battery. 
     Consequently, it is essential to stop the discharging of the battery when the voltage of the least charged accumulator falls below the minimum threshold voltage, even if there is still energy in the other accumulators to avoid a malfunctioning of the battery and damage to the least charged accumulator. 
     It is then essential to monitor in detail the voltage of each accumulator during discharge to check that no accumulator reaches the minimum threshold voltage. 
     SUMMARY 
     The objective of the invention is therefore to enable the voltage of each accumulator to be monitored in a simple manner so as to prevent an accumulator from falling below the minimum threshold voltage in order to prevent damage to the accumulator and therefore malfunctioning of the battery. 
     To this end, the subject of the invention is a voltage measuring device for a battery comprising an accumulator, characterized in that said measuring device comprises:
         an alternating current generator,   at least one first and at least one second diodes connected in series, such that said first diode is linked to the positive pole of said accumulator by its cathode and to the cathode of said second diode by its anode, and said second diode is linked to the negative pole of said accumulator by its anode and to the anode of said first diode by its cathode,   a reading capacitor linked to the alternating current generator and to the mid-point between said first and second diodes, and to a means for determining the voltage at the terminals of said accumulator based on the voltage variation at the mid-point, said determination means comprising:
           a third and a fourth rectifying diodes in series such that the anode of the third rectifying diode is linked to the cathode of the fourth rectifying diode and to said reading capacitor,   a filtering capacitor linked on the one hand to the cathode of the third rectifying diode and to the anode of the fourth rectifying diode,   a resistor mounted in parallel with the filtering capacitor.   
               

     Such a voltage measuring device comprises, for each accumulator, two diodes and a capacitor, which are components that are simple to implement and therefore allow for the production of circuits that are not particularly complex at low cost, without having an excessive weight or volume, to check that no accumulator of the battery reaches a voltage below a predefined minimum threshold voltage. 
     Said measuring device may also comprise one or more of the following features, taken separately or in combination:
         said first and second diodes have a threshold voltage substantially equal to 0.6 V,   said measuring device also comprises an injection capacitor linked to the current generator and to the mid-point,   said measuring device is a voltage measuring device for a battery comprising at least two accumulators in series, and said device comprises a measuring device for one of the accumulators, a first and a second diodes in parallel with each accumulator, a plurality of link capacitors respectively linking two consecutive mid-points, said plurality of link capacitors being mounted in series, the determination means determining the lowest voltage at the terminals of the accumulators,   the measuring device also comprises an injection capacitor linked to the alternating current generator on the one hand and to any mid-point on the other hand,   the third and fourth rectifying diodes are configured to subtract the threshold voltages of said first and second diodes from the voltage signal passing through said third and fourth rectifying diodes, so as to retain only the voltage information relating to the accumulator,   said measuring device comprises at least one booster capacitor mounted in parallel with a predetermined number of the link capacitors,   the alternating current generator is implemented by a voltage generator and a resistor which are mounted in series, said voltage generator generating a square signal with a frequency of 100 kHz and an amplitude of 0 to 10 V.       

     The invention also relates to a monitoring device comprising a measuring device according to the invention, characterized in that it comprises a means for comparing the measured voltage of said accumulator to a predefined threshold voltage and a control device for stopping the discharging of the battery when said measured voltage is below the predefined threshold voltage. 
     The invention also relates to a battery of accumulators incorporating such a monitoring device. 
     This battery is, for example, a lithium-ion battery. 
     Other features and advantages of the invention will emerge from the following description, given as a nonlimiting example, in light of the appended drawings in which: 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  schematically represents a device for measuring the voltage of an accumulator of a battery, 
         FIG. 2  schematically illustrates a device for measuring the voltage of two accumulators of a battery, and 
         FIG. 3  schematically represents a device for measuring the voltage of a number of accumulators in series of a battery comprising booster capacitors. 
     
    
    
     In these figures, identical elements are given the same references. 
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an embodiment of a voltage measuring device  1  for an accumulator battery that makes it possible to measure the voltage of an accumulator Ai, in this case A 1 , of the battery. 
     The measuring device  1  comprises:
         an alternating current generator I 1 , or in a variant, a voltage generator generating, for example, a square signal with a frequency of 100 kHz and amplitude of 0 to 10 V and a resistor R′ in series ( FIG. 3 ),   a first D 1  and a second D 2  diodes connected in series ( FIG. 1 ),   a reading capacitor C 1  linked to the mid-point N between the first D 1  and second D 2  diodes, and to a determination means  3  for determining the voltage at the terminals of the accumulator A 1  based on the voltage variation at the mid-point N.       

     As can be seen in  FIG. 1 , the first diode D 1  is linked to the positive pole + of the accumulator A 1  by its cathode and to the cathode of the second diode D 2  by its anode, and the second diode D 2  is linked to the negative pole − of the accumulator A 1  by its anode and to the anode of the first diode D 1  by its cathode. 
     In the positive alternation of the current I 1 , the first diode D 1  is passing and the second diode D 2  is blocked. The current I 1  then passes through the first diode D 1 . The voltage at the mid-point N is then equal to the sum of the voltage of the accumulator A 1  on the positive pole + with the threshold voltage of the first diode D 1 . 
     On the other hand, in the negative alternation, it is the second diode D 2  which is passing and the first diode D 1  which is blocked. The current then passes through the second diode D 2 . The voltage at the mid-point N is then equal to the sum of the voltage on the negative pole − of the accumulator A 1  with the threshold voltage of the second diode D 2 . 
     Consequently, the voltage variation at the mid-point N is equal to the sum of the voltage V 1  of the accumulator A 1  with the threshold voltages of the first D 1  and second D 2  diodes. 
     As an example, the first D 1  and second D 2  diodes have a threshold voltage of 0.6 V. For a voltage V 1  for example of 2.6 V of the accumulator A 1 , the voltage variation at the mid-point N is therefore equal to 3.8 V. 
     The voltage signal which arrives at the reading capacitor C 1  is a signal exhibiting a signal variation amplitude which corresponds to the voltage variation at the mid-point N and a DC (Direct Current) component. 
     The reading capacitor C 1  eliminates the DC component so as to have only information concerning the signal variation amplitude. 
     Moreover, in the example illustrated, the determination means  3  comprises:
         a third D 3  and a fourth D 4  rectifying diodes in series such that the anode of the third rectifying diode D 3  is linked to the cathode of the fourth rectifying diode D 4  and to the reading capacitor C 1 ,   a filtering capacitor C 2  linked on the one hand to the cathode of the third rectifying diode D 3  and to the anode of the fourth rectifying diode D 4 , and   a resistor R mounted in parallel with the filtering capacitor C 2 .       

     The passage of the voltage signal in the two rectifying diodes D 3  and D 4  rectifies this signal to obtain a signal of constant sign and also makes it possible to subtract the threshold voltages of the first D 1  and second D 2  diodes. 
     The signal is then filtered with the filtering capacitor C 2  which makes it possible to obtain a substantially DC voltage. 
     The voltage Vs measured at the terminals of the resistor R is then substantially equal to the voltage V 1  at the terminals of the accumulator A 1 . 
     Such a measuring device forms part of a voltage monitoring device for a battery. 
     The voltage Vs can then be transmitted to a comparator of the monitoring device (not represented) which compares the measured voltage Vs to a predefined threshold voltage, and when the measured voltage Vs is below the threshold voltage, a control device of the monitoring device stops the discharging of the battery for example by disconnecting the battery from the discharge system in which it is installed. 
     Furthermore, for greater accuracy, it is possible to provide an injection capacitor Ci linked to the mid-point N, and to the current generator I 1 . 
       FIGS. 2 and 3  illustrate a second embodiment in which the measuring device makes it possible to monitor the voltage of each accumulator Ak of a battery comprising a number of accumulators Ak connected in series. 
     The example of  FIG. 2  shows two accumulators A 1  and A 2  connected in series. 
     In parallel with each accumulator A 1 , A 2 , a first and a second diodes are provided D 1   1 , D 2   1  respectively D 1   2 , D 2   2 . A link capacitor C 1  links the two mid-points N 1  and N 2 , respectively between the two diodes D 1   1 , D 2   1 , and the two diodes D 1   2 , D 2   2 , and a reading capacitor C 1  is linked to the determination means  3  and to the mid-point N 1 , or equally to the mid-point N 2  between the diodes D 1   2  and D 2   2  instead of the mid-point N 1 . The example illustrated in  FIG. 2  for the accumulator A 1  therefore shows the measuring device of  FIG. 1  for one Aj of the accumulators. 
     As an example, the accumulator A 1  has a voltage V 1  less than the voltage V 2  of the accumulator A 2 . In this case, in the positive alternation of the current I 1 , it is the first diode D 1   1  of the accumulator A 1  which is passing and the second diode D 2   1  of the accumulator A 1  which is blocked, and the current I 1  passes through the first diode D 1   1 . And, in the negative alternation, it is the second diode D 2   1  of the accumulator A 1  which is passing and the first diode D 1   1  of the accumulator A 1  which is blocked, and the current I 1  passes through the second diode D 2   1 . 
     The amplitude of the voltage present at the mid-points is identical on all the mid-points N 1 , N 2 , only the DC component of the signal is different. This voltage amplitude is equal to the sum of the voltage of the accumulator A 1  that has the lowest voltage with the threshold voltages of the first D 1   1  and second D 2   1  diodes. 
     The measurement of the voltage of the accumulator A 1  that has the lowest voltage is therefore done on any of the mid-points N 1 , N 2  using the determination means  3 . 
     Furthermore, by absorbing current, the accumulator A 1  that has the lowest voltage is charged. Because of this, its voltage increases until it reaches the voltage of the accumulator A 2  which is the most charged. The accumulators of the battery are thus balanced. 
     The measuring device  1  may also comprise an injection capacitor Ci linked to the alternating current generator I 1  and to any mid-point N k , in this case the mid-point N 1  between the diodes D 1   1  and D 2   1  associated with the accumulator A 1 . 
     In the example of  FIG. 3 , the measuring device  1  is used to monitor the voltage of 10 accumulators Ak (1≦k≦10) in series of a battery. 
     Nine link capacitors C 1   m , in which m varies from 1 to 9 in this example, respectively link two consecutive mid-points and are mounted in series. An injection capacitor Ci can also be provided, linked on the one hand to the alternating current generator and on the other hand to any mid-point. 
     As for the reading capacitor C 1 , it is linked to any mid-point, in this case the first mid-point, and to the determination means  3 . 
     Take the case where the accumulator A 8  has the lowest voltage, for example 2.6 V whereas the other accumulators A 1  to A 7 , A 9  and A 10  have a voltage greater than 2.6 V, for example of the order of 3 V for the accumulator A 4  or 3.3 V for the other accumulators. 
     As previously, the amplitude of the voltage present on the mid-points is identical on all the mid-points and is equal to the sum of the voltage of the accumulator A 8  that has the lowest voltage with the threshold voltages of the first D 1   8  and second D 2   8  diodes. 
     The determination means  3  can then determine the voltage at the terminals of the accumulator A 8  from this voltage variation, for example using third D 3  and fourth D 4  rectifying diodes and the filtering capacitor C 2  as described previously. 
     Furthermore, by absorbing current, the accumulator A 8  that has the lowest voltage is charged, and its voltage increases until it reaches the voltage of the second most discharged accumulator, that is to say the one that has the second lowest voltage, in the example illustrated in  FIG. 3  it is the accumulator A 4  that has a voltage of the order of 3 V which is the second most discharged accumulator. Thereafter, a number of accumulators among the most discharged with similar voltages will be charged simultaneously until they reach the voltage of the third most discharged element, until all the accumulators have an identical voltage, for example in this case 3.3 V. 
     Furthermore, in order to guarantee that all the mid-points have the same voltage variation so as to be able to measure the voltage of the least charged accumulator, that is to say the one that has the lowest voltage, regardless of its position, booster capacitors C 3   1  to C 3   3  can be added in parallel with the link capacitors C 1   1  to C 1   9 . In the example illustrated in  FIG. 3 , the booster capacitor C 3   1  is mounted in parallel with the capacitors C 1   1  to C 1   4 , the booster capacitor C 3   2  is mounted in parallel with the capacitors C 1   5  to C 1   9 , and the booster capacitor C 3   3  is mounted in parallel with the capacitors C 1   1  to C 1   9 . 
     The components of such a measuring device are therefore simple to implement and inexpensive, in order to know, in a simple manner, when an accumulator crosses the minimum threshold voltage risking causing damage to the accumulator and malfunctioning of the battery. 
     Furthermore, because of their light weight and their small bulk, these components can be incorporated without difficulty in a battery of accumulators. 
     Furthermore, since all the components are incorporated in one and the same battery, they are all subjected to the same temperature conditions, so that the drifts of the characteristics of the diodes change together, which ensures a good balance and an accurate measurement regardless of the temperature.