The invention relates to an equalizing system for electrochemical accumulator batteries that can be used notably in the field of electrical and hybrid transport and the embedded systems. The invention relates in particular to the batteries of lithium-ion (Li-ion) type which are well suited to this kind of application, because of their ability to store high energy with a low mass. The invention is also applicable to supercapacitors.
An electrochemical accumulator has a nominal voltage of the order of a few volts, and more specifically 33 V for the Li-ion batteries based on iron phosphate and 4.2 V for an Li-ion technology based on cobalt oxide. If this voltage is too low compared to the requirements of the system to be powered, a number of accumulators are placed in series. It is also possible to arrange in parallel with each associated accumulator in series one or more accumulators in parallel in order to increase the available capacity and to supply higher current and power. The associated accumulators in parallel thus form a stage. A stage consists of at least one accumulator. The stages are connected in series to achieve the desired voltage level. The association of the accumulators is called an accumulator battery.
The charge or discharge of an accumulator is reflected respectively in an increase or decrease in the voltage at its terminals. An accumulator is considered to be charged or discharged when the latter has reached a voltage level defined by the electrochemical process. In a circuit using a number of accumulator stages, the current flowing through the stages is the same. The level of charge or of discharge of the stages therefore depends on the intrinsic characteristics of the accumulators, namely the intrinsic capacity and the parasitic series and parallel internal resistances, of the electrolyte or of contact between the electrodes and the electrolyte. Voltage differences between the stages are then possible because of the manufacturing and aging disparities.
For an Li-ion technology accumulator, excessively high or low voltage, called threshold voltage, can damage or destroy the latter. For example, the overload of an Li-ion accumulator based on cobalt oxide can cause thermal runaway thereof and start a fire. For an Li-ion accumulator based on iron phosphate, an overload is reflected in a breakdown of the electrolyte which reduces its life and can damage the accumulator. An excessive discharge which leads to a voltage less than 2 V, for example, mainly causes oxidation of the current collector of the negative electrode when the latter is made of copper and therefore deterioration of the accumulator. Consequently, the monitoring of the voltages at the terminals of each accumulator stage is mandatory when charging and discharging for both safety and reliability reasons. A so-called monitoring device in parallel with each stage provides this function.
The function of the monitoring device is to track the state of charge and discharge of each accumulator stage and to transmit the information to the control circuit in order to stop the charging or the discharging of the battery when a stage has reached its threshold voltage. However, on a battery with a number of accumulator stages arranged in series, if the charging is stopped when the most charged stage reaches its threshold voltage, the other stages may not be totally charged. Conversely, if the discharging is stopped when the most discharged stage reaches its threshold voltage, the other stages may not be totally discharged. The charge of each accumulator stage is then not exploited optimally, which represents a major problem in transport and embedded type applications that have high autonomy constraints. To overcome this problem, the monitoring device is generally associated with an equalizing device.
The function of the equalizing device is to optimize the charge of the battery and therefore its autonomy by bringing the accumulator stages connected in series to an identical state of charge and/or discharge. There are two categories of equalizing devices, the so-called energy dissipation equalizing devices and the so-called energy transfer equalizing devices.
With the energy dissipation equalizing devices, the voltage at the terminals of the stages is made uniform by diverting the charge current from one or more stages that have reached the threshold voltage. As a variant, the voltage at the terminals of the stages is made uniform by discharging one or more stages that have reached the threshold voltage. However, such energy dissipation equalizing devices present the major drawback of consuming more energy than necessary to charge the battery. In fact, it is necessary to discharge a number of accumulators or divert the charge current of a number of accumulators for the last accumulator or accumulators that are a little less charged to finish their charging. The energy dissipated can therefore be very much greater than the energy of the charge or charges that have to he terminated. Furthermore, they dissipate the excess energy as heat, which is not compatible with the integration constraints in transport and embedded type applications, and the fact that the life of the accumulators becomes much shorter when the temperature rises.
The energy transfer equalizing devices exchange energy between the accumulator battery or an auxiliary energy network and the accumulator stages.
The patent U.S. Pat. No. 5,659,237 for example discloses a device that makes it possible to transfer energy from an auxiliary network to stages via a “flyback” structure with a number of outputs and using a coupled inductance as storage element. The latter is a specific component in that it is dedicated to this application. Consequently, the cost of such a component is prohibitive in relation to the function to be fulfilled.
Also, the patent CN1905259 discloses a device that makes it possible to transfer energy from the stages to the battery and that uses an inductance for each accumulator as storage element. However, this device does not opt for an optimized energy transfer for the equalizing of the batteries in the transport and embedded type applications. In practice, the end of charge of a battery is determined by the last stage to reach the threshold voltage. To terminate the charging of a battery, the energy is taken from one or more stages and it is restored to all the stages. When one or more accumulator stages is/are a little less charged, the energy is not then transferred as a priority to the latter which needs/need it but also to the stage or stages from which the energy is taken. The equalizing therefore requires energy to be taken from all the stages at the end of charging in order to avoid charging them to too high a voltage. The equalizing is therefore done with high losses because of the large number of converters in operation. Furthermore, the accumulators already at the end of charge have useless alternating or direct current components passing through them.
The aim of the invention is therefore to propose an enhanced charge equalizing system that does not have these drawbacks of the prior art.
To this end, the subject of the invention is a charge equalizing system for batteries comprising at least two accumulator stages connected in series, each stage comprising an accumulator or at least two accumulators connected in parallel, characterized in that said system comprises:
Said equalizing system may also comprise one or more of the following characteristics, taken separately or in combination:
The invention also relates to a device for charging a charge equalizing system as defined above.
Other features and advantages of the invention will become more clearly apparent on reading the following description, given as an illustrative and nonlimiting example, and the appended drawings in which:
a is a flow diagram schematically illustrating an exemplary embodiment of the control of charging devices of the equalizing system of
b is a diagram associated with
In these figures, the elements that are substantially identical are given the same references.
Charge Equalizing System
The subject of the invention is an equalizing system 2 for such an accumulator battery 1, comprising at least two accumulator stages Etj connected in series.
This equalizing system 2 comprises a control device 3, and a plurality of identical charging devices 5 for each accumulator stage Eti.
Each charging device 5 is connected to the negative pole, denoted Ni, and to the positive pole, denoted Pi, of each accumulator stage Eti, and also to the positive pole, denoted P, and to the negative pole, denoted N, of the accumulator battery 1. The charging devices 5 are controlled by the control device 3.
In the example illustrated in
According to an alternative, two controlled switches are used in place of the diodes D1i, D11 and D2i, D21. A rectification said to be of synchronous type is then possible. The efficiency of the circuit can be increased by reducing the voltage drop in the passing state of the component.
This charging device 5 is differentiated from the prior art inasmuch as it does not have any common reference between the input and the output, as is the case for a “buck-boost” type configuration, and inasmuch as it does not use any transformer, as is the case for a “flyback” type configuration.
A variant embodiment consists in adding a capacitor connected between the positive Pi and negative Ni poles of each accumulator stage. The capacitor is configured to filter the current ripple from the charging device 5. A smooth direct current is thus supplied to each accumulator stage.
It is possible to also add a capacitor (not represented) between the terminals N and P of the battery. It is configured to filter the ripple due to the charging device 5. Thus, the current supplied by the battery is smoothed.
The charging device 5 (
Operation in discontinuous conduction mode is preferred because it presents the advantage of being easier to implement and to carry out at lower cost. This is because, in discontinuous conduction mode, the current from the inductance L1i is canceled by definition before each period of the control signal for the switches SW1i and SW2i. The value of the current flowing through the inductance L1i, when the two switches SW1i and SW2i are closed, can be deduced from the voltage applied to the terminals of the inductance L1i, from the energy storage time in the inductance L1i and from the value thereof.
Thus, and contrary to the operation in continuous conduction mode (FIG. 3′), it is no longer necessary to implement a current sensor 12 associated with a regulation loop 13 and with a current reference variable 15, as well as with a current control device 14, for example a switching in pulse width modulation mode by the transistors SW1i and SW2i operating as switches, for each of the accumulator stages Eti in series.
Moreover, in discontinuous conduction mode, the control of the switches SW1i and SW2i in pulse width modulation mode can be replaced by a fixed conduction time control.
According to an exemplary embodiment of the control of the charging devices 5 by the control device 3, use is made of a single clock 6, a shift register 7 and controlled switches or “AND” logic functions 8 (
The shift register 7 avoids having the switches SW1i and SW2i of the different charging devices 5 of the different stages Eti closed simultaneously, which would result in an excessive discharge current. The input signal E of the shift register 7 is supplied by the control device 3. The latter also controls one of the two inputs of each “AND” logic function 8. The second input of each “AND” logic function is connected to an output of the shift register 7. The control of a charging device 5 is effective when the two inputs of the “AND” logic function 8 are in the high state.
This control makes it possible to minimize the instantaneous current consumed by the control circuit unlike a control for which all the charging devices 5 are controlled at the same time. Furthermore, this control makes it possible to reduce the effective current supplied by the battery 1 compared to a synchronized control of the charging devices 5, and therefore to minimize its overheating.
Moreover, with reference to
Thus, the connection of the switches SW1i and SW2i of the charging devices 5 is made at the terminals of ten to twelve stages Eti. The voltage withstand strength of the diodes and controlled switches is limited, according to the technology of the Li-ion battery, to approximately 45 V-60 V, which is a standardized voltage withstand strength value in the field of semiconductors. The maintenance of a large number of individual modules 9, as is the case for electric vehicles, is made easier.
According to a variant embodiment, use is made, in addition to the charging devices 5 for each accumulator stage Eti, of identical charging devices 5 by the series connection of N stages Eti forming an individual module 9.
It is also possible to use one or more of the charging devices 5 implemented at the terminals of a series connection of N stages to supply energy to an auxiliary network 10, such as, for example, the 12 V network for the vehicles (
Moreover, the equalizing system 2 may comprise a voltage measuring device (not represented) to measure the voltage of each accumulator stage Eti and to transfer voltage information to the control device 3 which can use this voltage information to determine whether an accumulator stage Eti has to be charged and accordingly control the associated charging device 5 when such is the case.
Operation of the Equalizing System in Discontinuous Conduction Mode
The operation of the equalizing system 2 is described below with reference to
When the control device 3 controls a transfer of energy to a stage Eti the stage Et1 in the example illustrated, the switches SW11 and SW21 of the charging device 5 in parallel with the corresponding stage Et1 are closed simultaneously and during a conduction time t1. The circulation of the current during this conduction time t1 is schematically represented by dotted lines in
The inductance L11 henceforth stores energy. The current iL11 through the inductance L11 increases proportionally to the voltage applied to its terminals, equal to the voltage of the N stages (
At the end of the time t1, the switches SW11 and SW21 open simultaneously. The current iL11 in the inductance L11 at this instant reaches a peak value Ipeak, equal to the voltage applied to the terminals of the inductance when the switches SW11 and SW21 are closed, multiplied by t1 and divided by the value of the inductance.
At the end of the time t1 and until the end of the period of operation T of the charging device 5, the switches SW11 and SW21 are in the open state; the diodes D11 and D21 are passing until the cancelation of the current in the inductance L11. The circulation of the current during this phase is schematically represented by the alternation of two dots and a dash in
The operation of the charging device 5 is identical regardless of the accumulator stage Eti to which it is connected and therefore makes it possible to continue charging certain stages.
Dimensioning
Representation in Equation Form
The dimensioning of the charging device 5 of
When the switches SW1i and SW2i of one and the same charging device 5 are closed during a conduction time t1, the current increases in the inductance L1i (iL1i). By disregarding the voltage drop in the passing state of the switches, the current iL1i(t) in the inductance L1i is expressed:
At the end of the time t1, the switches SW1i and SW2i open and the current in the inductance iL1i reaches the peak value Ipeak:
At the end of the time t1 until the current iL1i is canceled, the diodes D1i and D2i of one and the same charging device 5 conduct. The current iL11 in the inductance L1i decreases according to the following law, with Vd being the voltage drop in the passing state of the diode.
The operating phase corresponding to a zero current, when the diodes are blocked, until the end of the period T, defines the discontinuous conduction mode.
From the equations 2 and 3, the conduction time t1 that is not to be exceeded (t1(max)) for the charging device 5 to operate in discontinuous conduction mode can be defined. This time is determined by considering that the current in the inductance is canceled at T. To consider the worst case, the time t1(max) should be evaluated for the maximum input voltage Ve and the minimum output voltage Vs. Furthermore, the voltage drops of the diodes can be disregarded to consider the worst case.
The output current of the charging device 5 is equal to the current conducted by the diodes D1i and D2i. The average output current of a charging device 5 is calculated from the equation 5. The average output current (Is(avg)) is proportional to the square of the input voltage Ve2 and inversely proportional to the output voltage Vs and to the voltage drop of the diodes D1i and D2i. To supply the desired average current regardless of the voltage of the accumulator stage Eti, the maximum output voltage and the minimum input voltage must be taken into account,
The current in the charged stage or stages is not equal to the output current of the charging device 5. In fact, the energy stored by the inductance L1i of a charging device 5 is supplied by the accumulator battery 1. This current is therefore supplied by the stage or stages that is/are charged. The current supplied to the charged accumulator stage or stages is therefore equal to the algebraic sum between minus the current through the switches SW1i and SW2i plus the current conducted by the diodes D1i and D2i. By considering N, the number of charging devices 5 in operation, the average value of the current of the charged stage or stages (IEt(avg)) is obtained using the equation 6. For the equation 6 below, it is considered that, over the same operating period T, the current is supplied by the battery 1 to the charging devices 5 and also from the charging devices 5 to the stages Eti. If the number of charging devices 5 in operation is equal to the number of stages Eti connected to the input of the charging devices 5, the average current of the stages is equal to 0.
To illustrate the equations introduced previously, the dimensioning of two charging devices 5 is considered.
The first relates to a charging device 5 which can be used to continue the charging of a stage Eti and which is connected to the terminals of ten stages.
The second relates to a charging device 5 which can be used to continue the charging of a series association of ten stages and which is connected to the terminals of a hundred stages, that is to say, to the terminals of ten series associations, each therefore consisting of ten stages in series.
The dimensioning of the charging device 5 is divided into 2 steps, namely, first of all, the calculation of the conduction time t1 of the switches SW1i and SW2i for an operation of the charging device 5 in discontinuous conduction mode (equation 4), then, the calculation of the value L1i to supply, at the output of the charging device the desired average current (equation 5).
The assumptions for the dimensioning of the two charging devices 5 are as follow
For the two charging devices 5, the time t1(max) is calculated by using the minimum voltage drop of the diodes D1i and D2i, the maximum input and minimum output voltage of the charging device. Then, the maximum inductance L1i is calculated by using the maximum voltage drop of the diodes and the minimum input and maximum output voltage of the charging device 5.
For a charging device 5 that can be used to charge a stage Eti, the time t1 and the inductance L1i are given below (result 1). Fast Schottky-type diodes are taken into account.
For a charging device 5 that can be used to charge a series association of ten stages, the time t1 and the inductance L1i are given below. Bipolar diodes are taken into account.
In these examples, L1 is a maximum value. However, for reasons of robustness of he system, inductances of lower values can be used.
Simulations
As an example, two simulation results are illustrated for a charging device in operation that can be used to charge a stage (
The accumulator battery 1 consists in this example of a series association of ten accumulator stages each comprising an accumulator. An accumulator is represented by a voltage source Vi and an internal resistance RI in series, equal to 0.010 ohms for each accumulator. For reasons of legibility of the diagram, the accumulators above and below the accumulator that is on charge are associated to each comprise a single voltage source and a series resistance.
The operating frequency of the charging device 5 is set arbitrarily at 50 kHz.
The conduction time of the switches SW1i and SW2i is set at 1.631 μs. The value of the inductance L1i is set at 9.1 μH (cf. result 1).
First Simulation
For the first simulation, most of the accumulators are charged to the threshold voltage 2.5 V and one accumulator is charged to the voltage V7 of 3.6 V. The charging device 5 is connected in parallel to the accumulator which has the highest charge voltage, or 3.6 V (here, the seventh). The stages below the seventh accumulator are associated with a voltage source V1-6 of 15 V and an internal resistance R1-6 of 0.060 ohms, and similarly the stages above the seventh accumulator are associated with a voltage source V8-10 of 7.5 V and an internal resistance R8-10 of 0.030 ohms.
This example illustrates the extreme case of operation for which the average output current has to be 1 A (minimum average current).
As described previously, the current iL17 increases in the inductance L17 during a conduction time t1, a time during which the switches SW17 and SW27 are closed. It is interesting to note that, during this phase, the current is supplied by the accumulator battery 1, via the current iV7 supplied by the accumulator during this phase. At the end of the time t1, the value of the current reaches a peak value Ipeak, of the order of 4.6 A in our example. From the time t1, the current in the inductance decreases and is supplied to the accumulator. The circuit operates in discontinuous conduction mode because the current is canceled before each operating period of the charging device 5.
The average output current Is7(avg) is equal to 1.0 A, as desired. A minimum average current of 1 A is well respected regardless of the voltage value of the charged accumulator and the voltage value of the accumulator battery.
Second Simulation
For the second simulation, the accumulators are mostly charged to the threshold voltage of 3.6 V and one accumulator is charged to the voltage of 2.5 V. The charging device 5 is connected in parallel to the accumulator which has the lowest charge voltage, or 2.5 V. This example illustrates the extreme case of operation for which the charging device 5 has to operate in discontinuous conduction mode.
As described previously, the current iL17 increases in the inductance L17 during a conduction time t1, a time during which the switches SW17 and SW27 are closed. At the end of the time t1, the value of the current reaches a peak value Ipeak, of the order of 6.1 A in our example. From the time t1, the current in the inductance decreases and is supplied to the accumulator. The circuit operates in discontinuous conduction mode because the current is canceled before each operating period of the charging device 5. The operation in discontinuous conduction mode is well observed regardless of the voltage value of the charged accumulator and the voltage value of the accumulator battery.
The average output current Is7(avg) is equal to 2.3 A. It is well above the minimum value of 1 A.
Other simulations have been implemented. The charging device 5 has been validated for the entire voltage variation range of the accumulator (2.5 V-3.6 V) and of the battery 1 (25 V-36 V). The charging device 5 has also been validated regardless of the position thereof, namely at the terminals of the stage 1, of the stage 6 or of the stage N. The operation of the charging device 5 with a number of charging devices 5 operating in parallel has also been validated. The charging device 5 that can be used to charge ten stages Eti in series and connected to the terminals of a hundred stages Eti has also been validated by this approach.
Number | Date | Country | Kind |
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1000481 | Feb 2010 | FR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP2011/051684 | 2/4/2011 | WO | 00 | 10/12/2012 |