FR2891094A1 - Battery charging method, involves tracking maximum power point of source by indirectly measuring consumed power and modulating duty factor of input stage based on measurement of consumed power - Google Patents

Abstract

The method involves progressive charging of a storage capacitor (201) from voltage pulses generated from an input stage upstream of a photovoltaic source (200) according to a duty factor. A break over threshold of a predetermined voltage at terminals of the capacitor is determined. The discharge of the capacitor in the battery is controlled. A maximum power point of the source is tracked by indirectly measuring consumed power and modulating the duty factor of the input stage based on the measurement. An independent claim is also included for a device for charging a battery from a complex direct current source.

Description

A method and device for charging a battery from a source of

complex direct current

  The present invention is directed to a method of charging a battery from a complex DC source. It also relates to a battery charger device from a complex DC source, implementing this method.

  Some DC sources have a complex behavior, where neither the voltage nor the current is sufficient independently to define the power available.

  It is for example known that a battery is not a pure source of voltage, and that said voltage will fluctuate depending on the current consumed.

  It is particularly known that a source of photovoltaic electric power (PV) has this property of providing at a given load and constant luminous flux, the maximum of electrical power at an optimum operating point (voltage x current) determined by its manufacture. .

  This operating point is designated by the acronym MPP (Maximum Power Point). When the luminous flux varies, the MPP of a photovoltaic source also varies, in a non-linear manner in both voltage and current.

  Methods are known in the art that tend to optimize at any time the power captured from a complex source such as a photovoltaic panel by monitoring (continuation) of the MPP.

  These methods are referred to by the acronym MPPT (Maximum Power Point Tracking). The application of these methods generally requires the sampled measurement of the power by acquisition of voltage and current.

  A particular case of application is the charging of a battery from a photovoltaic source, which is within the scope of the invention.

  According to the most usual methods, efforts are made to supply the battery under load with a voltage that is substantially constant and greater than that of the battery.

  In the case of a photovoltaic source, the stabilization of this voltage adds to the complexity of the MPPT process.

  The patent application 04 03012 of March 23, 2004, published under the number FR2868218, describes a pulse charging method which consists of charging an intermediate storage capacitor, to detect the crossing of a predetermined voltage threshold - greater than the voltage of the battery - at the terminals of this capacitor and to discharge said capacitor to the battery at this precise moment.

  According to the description provided in this patent application, an input stage of the charger converts the electric current supplied by a DC source in the form of high voltage pulses ensure the progressive charging of said storage capacitor.

  Such a charger thus already ensures by its principle operation insensitive to voltage fluctuations of the source. As already described by way of example in the document FR2868218, this already largely offsets the fluctuations specific to a complex source.

  The object of the present invention is to provide an improvement of the charging method disclosed in the aforementioned document FR2868218, specific to a complex source, comprising at least one automatic adjustment of the MPPT type.

  This object is achieved with a method for charging a battery from a complex DC source, having cyclic sequences each comprising: - a progressive charge of a storage capacitor from voltage pulses generated from a stage input downstream of said source, in a duty cycle, -a detection of a threshold crossing a predetermined voltage across said storage capacitor, controlling: - a discharge of said storage capacitor in the battery.

  According to the invention, the method further comprises a tracking (tracking) of a maximum power operating point (MPP) of said source, comprising: an indirect measurement of the power consumed from a measurement of the time of charging of the storage capacitor, and - a modulation of the duty cycle of said input stage, as a function of the indirect measurement of power consumed.

  When the method according to the invention is implemented in a charging device in which the input stage comprises power switching means, the modulation of the duty cycle can advantageously implement a so-called perturbation technique in which an increment of ATon constant time is added or subtracted from the last known best value of the active control time of said switching transistor.

  The method according to the invention may also include an exploitation of the indirect measure of power consumed to manage a safety stop value for said power consumed.

  It can also be provided that the method according to the invention further comprises a test of external conditions, in particular of thermal limit or of full charge of the battery, leading to a modulation control of the duty cycle so as to reduce or limit the power consumed. .

  According to another aspect of the invention, there is provided a device for charging a battery from a complex DC source, implementing the method according to the invention, comprising: - means for progressively charging a capacitor of storing from voltage pulses generated from an input stage in val of said source, in a duty cycle, means for detecting a threshold for crossing a predetermined voltage across said storage capacitor, and means for discharging said storage capacitor into the battery, controlled by said detecting means.

  According to the invention, the charging device further comprises means for tracking a maximum power function point (MPP) of said source, comprising - means for providing an indirect measurement of the power consumed from a measurement of the charge capacitor storage time, and - means for modulating the duty cycle of said input stage as a function of the indirect measurement of power consumed.

  It will be demonstrated in the following that the method according to the invention can be implemented at very little cost by keeping the diagram of the base of the charger where: - the measurement of its precharging time is used as an indirect measure of the power consumed, - modulates the duty cycle of the converter input stage to continually adjust to the MPP of the source.

  Pulse Width Modulation (PWM) modulation is not new in itself and is already used in regulation in all sorts of devices such as the so-called switching power supplies.

  The novelty comes from the combination of the battery charging method disclosed in the document FR2868218, already tolerant in itself to the fluctuations of the source, with: an indirect measurement of the power consumed by the cycle time, an algorithm of followed by the optimum power by modulation (PWM) of the input converter.

  This requires no additional hardware means from the moment the charger has a microcontroller as control unit / control.

  The result is a universal charger of low cost, functional in a wider operating range than most known chargers, and comprising at least one mode of operation: - specifically adapted to a complex source such as photovoltaic, 5 - largely insensitive to fluctuations of this source, and - sensing at all times a maximum of its power (MPPT function), - including an optional function of power limitation. The description of a preferred embodiment of the invention will be based on the case of a photovoltaic source (PV acronym) which is a good example of a complex source.

  - Figure 1 is a reminder in the form of typical curves of the known characteristics of a photovoltaic source.

  - Figure 20 is a block diagram of a charger according to the invention, arranged under control of a microcontroller.

  FIG. 3 illustrates a characteristic storage capacitor charge and discharge cycle disclosed in FR2868218 and in the present invention.

  - Figure 4 is a block diagram of the software integrating into the control of the charger MPPT function.

  With reference to FIG. 1, the behavior of a PV source with luminous flux is usually represented by a so-called IV (intensity x voltage) curve whose typical shape is given by the diagram 1a.

  At extreme operating points: when the PV circuit is open the intensity is zero and the voltage is maximum: the power P = I x V is therefore zero; - when closing the circuit of the short-circuited PV the intensity is maximum but under a zero voltage: the power is zero also Between these extreme values the PV behaves roughly like a constant current source, up to an inflection. At a particular point after this inflection in the direction of the increasing currents the power P = I x V passes through a maximum, which is by definition the MPP.

  A given PV is characterized by a family of such curves (diagram lb) with the luminous flux as parameter. A family of MPP points is obtained which describes precisely under which conditions a maximum power will be obtained according to the fluctuations of the luminous flux (in practice, according to the sunlight.) The optimization process called MMPT consists in presenting to the source PV an impedance variable load, dynamically controlled to stay closer to the MPP; this is represented by the diagram lc.

  As a servo in the sense of automation, MMPT methods must take into account that the MPP setpoint is not a constant and is not under control.

  Various known strategies are used in existing charge controllers, the best-performing of which is known as perturbation is to make small variations of the impedance, and: - to stay on the current impedance if the power remains essentially stable, - if not, modify the impedance in the sense that makes the power grow. To do this, it is necessary to have: means for measuring the variations of the instantaneous power; means for modulating the impedance of the load.

  Referring to Figure 2, this diagram faithfully reproduces the general architecture of a battery charger as described in FR2868218 (Figure 1).

  The source is a PV 200. The capacitor 201 of high capacity performs a filtering, that is to say a smoothing of the voltage provided by the PV 20 to overcome fluctuations of very short duration, which as one know would be detrimental to servo stability.

  The induction coil 202, fed from the filtering capacitor 201, is in series with a switching transistor 203 driven by a pulse width modulated (PWM) type control signal 204. This all-or-nothing signal is at a predetermined time Tp, and active (transistor passing) for a time Ton <Tp.

  The report (Ton / Tp) is called the duty cycle. When averaged over time, the application of a variable duty cycle constitutes, in a known manner, the equivalent of a modulation of the load impedance of the PV 200 combined with the filtering capacitor 201.

  The modulation of Ton is thus provided with means for controlling said load impedance.

  The high-voltage pulses, obtained at the junction of the coil 202 and the transistor 203 in response after it has been cut off by the control signal 204, are injected to a storage capacitor 205 via a diode 206. The energy charge of this capacitor increases. progressively with each cycle of the signal 203; this results in an increasing voltage across said capacitor.

  A second transistor 207 is arranged to switch the capacitor 205 to the battery under control of a control signal 208.

  The control signals 204 and 208 are controlled by a control unit 209, which will preferably be implemented in the form of a programmable microcontroller.

  To lighten the description, it has not shown the power supply of the control unit 209, which will be realized: - either from the PV 200 via a regulator, - or from the battery in charge, via a power supply galvanically isolated from any known type, either from an auxiliary source such as an electric battery.

  The choice of one or the other mode of supply does not affect the invention. This choice is purely engineering design.

  The control unit 209 comprises inter alia: a PWM generator 210 which via an amplifier 211 supplies the control signal 204, an output port 212 which via an amplifier 213 supplies the control signal 207, a comparator 214 which is used to detect the crossing of a predetermined voltage threshold across the storage capacitor 205.

  The reference input (-) of the comparator 214 is connected to a voltage reference Vref 215, which can, as in the document FR2868218, be constituted by a resistor bridge, or by a Zener diode in series with a high resistance.

  The variable input (+) of the comparator 211 is connected to a resistor bridge (216, 217) which has a voltage proportional to that of the storage capacitor 205.

  For the practical embodiment of a charger according to the invention, with a typical capacity of 10 watts from a conventional solar panel to any battery with a nominal voltage of 12 volts, here is an indication of a list of electronic components that can be used to assemble this charger: function number type Value (s) or (Figure 2) reference Solar panel Nominal 12 V 10 Watt 201 Polarized capacitor 330 pF 25 V filtering 202 Coil On ferrite core 125 pH, 5 A 203 PWM switching BUZ100 MOSFET transistor 205 Polarization capacitor 100 pF 60 V storage 206 Transistor MOSFET switching BUZ100 discharge 207 Schottky diode 8QT100 209 Microcontroller control unit PIC16F684 (Microchip) 211 TC4420 MOSFET driver amplifier (Microchip) 213 TC4420 MOSFET driver amplifier (Microchip) 215 Voltage reference Zener diode 2.4 V 216 Voltage divider Resistance 10 kΩ 217 Voltage divider Resistance 100 kQ Referring to Figure 3, the charger operates in accordance with the teachings of FR2868218.

  The PWM generator controls the switching (switching) of the input current and the step charging of the storage capacitor.

  As soon as the control unit detects, thanks to the comparator (214), the crossing of the predetermined threshold voltage Vth (th as threshold in English) thanks to the comparator, it delivers a calibrated signal (port 212) for the discharge of the storage capacitor in drums.

  It is this mechanism that constitutes the actual battery charging function. It results in a curve of the voltage at the terminals of the storage capacitor as a function of time in the form of sawtooth, having a gradual growth slope 301 and a short discharge 302.

  the duration T of the progressive charge (301), from the battery voltage Vbat to the crossing of the threshold Vth will be called in the following preload time.

  It is observed that the energy transferred to each precharge is substantially constant: E: = (1 / 2xCxVth2) - (1 / 2xCxVbat2) where C is the capacitance of the storage capacitor. In the short term, the battery voltage varies very little.

  It follows that the measurement of the precharge time T constitutes an indirect sampled measurement of the power P passing through the charger at a given moment, since by definition the power varies inversely: P = E / T Sampling of time The precharging device T constitutes, according to the invention, the only means for measuring the variations of the instantaneous power required for MPPT control.

  It is an essential feature of the invention to use only this unique means of indirect power measurement. This has the advantage of being free as long as any current microcontroller integrates at least one function of measurement of the time intervals (timer function, in English).

  In combination with the already known advantages of pulse charger flexibility, this new feature cancels and replaces the current and voltage measurement devices required for most current MPPT processes.

  In summary, the optimal power servo MPPT process is realized by using: - the sampling of the precharge time as a means of indirect measurement of the power through at a given time, - the variation of the duty cycle (PWM) of the signal hash as a means of controlling the average input impedance of the charger.

  With reference to FIG. 4, the program diagram (algorithm) described here by way of example is not limiting. It is inspired by known methods known as perturbation.

  Insofar as it uses the same characteristic means described above, any other program scheme having equivalent functionality can be used without departing from the scope of the invention.

  Advantageously, in this simple version, a constant time increment tTon is used which will be added to or subtracted from the last known best value of the active control time of the switching transistor (203).

  The program performs, after initialization, a loop with three production steps - as many large cycles - followed by a test step and a conditional correction step.

  At each step the PWM generator is set and the power passed through the charger during the next preload is read indirectly as a time count.

  As described above with reference to Figure 3, it is recalled that the power is inversely proportional to the precharge time.

  In step 401 the duty cycle PWM is set with Ton-ATon, then the precharging time is set: Tl.

  At step 402 the duty cycle PWM is set with Ton, then the precharge is read: T2.

  In step 403, the duty cycle PWM is adjusted with Ton + ATon, then the precharging delay is measured: T3.

  At the end of this sequence (step 404), the differences (T2-T1) and (T2-T3) are tested in absolute value with respect to a predetermined numerical value H (H as: hysteresis.) If these two values are less than H, in other words if the power varies very little, the loop is repeated without modifying Ton. This is a known precaution against servo instabilities.

  In the opposite case (step 405), one and only one of the two differences is positive - see curves in FIG. 1 - and will decide in which direction one will gain power by modifying by an increment the central value Ton.

  At this step, therefore, the correction is made: if (T2-T1)> 0 then Ton F-Ton-ATon; it is necessary to reduce the duty cycle - if (T3-T1)> 0 then Ton F- Ton + ATon; it is necessary to increase the duty cycle and then return to the loop in step 401.

  It will be noted that if one is in the vicinity of the MPP the loop produces only small complementary and alternating variations of the duty cycle around a central value Ton which remains stable. The average input impedance therefore remains substantially constant, especially since the filter capacitor 201 plays its smoothing role.

  Thus the tracking function of the maximum power is realized.

  It can be seen that, advantageously, the servocontrol described uses only very basic arithmetic, and therefore lends itself well to simplicity of programming and very fast execution on a cheap microcontroller of the RISC type.

  According to another characteristic of the invention, it is also possible, as an option, to use the indirect power information to manage a safety stop value, which will limit the power passing through the charger to a predetermined higher value.

  In order to make clear the main function of enslavement, it has not shown the precautions and operating comfort devices that any person skilled in the art will identify the potential utility, and which will become part of the program.

  In particular, tests of external conditions such as a thermal alarm or the detection of full charge state of the battery can be taken into account in order to reduce the power (short duration or PWM pulse blocking) in order to prevent a rise in power. temperature or overload that would damage this battery.

  The control unit can also be provided to control lights, audible alarms, display etc. for the comfort of the user (operating state, anomalies, etc.) The addition of such usual protection or operating comfort functions does not make it outside the scope of the invention.

  Of course, the invention is not limited to the examples that have just been described and many adjustments can be made to these examples without departing from the scope of the invention.

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Claims (10)

  A method for charging a battery from a complex DC power source, having cyclic sequences each comprising: a progressive charge of a storage capacitor from voltage pulses generated from a downstream input stage of said source, according to a duty cycle, a detection of a threshold crossing a predetermined voltage across said storage capacitor, controlling - a discharge of said storage capacitor in the battery, characterized in that it further comprises a tracking of a maximum power operating point (MPP) of said source, comprising: an indirect measurement of the power consumed from a measurement of the storage capacitor charging time, and a modulation the duty cycle of said input stage, as a function of the indirect measure of power consumed.   2. Method according to claim 1, implemented in a charging device in which the input stage comprises power switching means, characterized in that the modulation of the duty cycle implements a technique called disturbance in which a constant time increment ATon is added or subtracted from the last known best value of the active control time of said switching transistor.   3. Method according to one of claims 1 or 2, characterized in that it further comprises an exploitation of the indirect measure of power consumed to manage a security stop value for said power consumed.   4. Method according to one of the preceding claims, characterized in that it further comprises a test of external conditions, including thermal limit or full charge of the battery, leading to a modulation control of the duty cycle so as to reduce or limit the power consumed.   5. A device for charging a battery from a complex DC source, implementing the method according to one of the preceding claims, comprising - means for progressively charging a storage capacitor from pulses voltage generated from an input stage in val of said source, in a duty cycle, means for detecting a threshold for crossing a predetermined voltage across said storage capacitor, and means for discharging said capacitor from storage in the battery, controlled by said detection means, characterized in that it further comprises means for tracking a maximum power function point (MPP) of said source, comprising: - means for providing an indirect measurement of the power consumed from a measurement of the charging time of the storage capacitor, and - means for modulating the duty cycle of said input stage e n function of the indirect measurement of power consumed.   6. Device according to claim 5, characterized in that the indirect power consumption measurement means and / or the duty cycle modulation means are implemented within a microcontroller.   7. Device according to one of claims 5 or 6, wherein the input stage comprises power switching means, characterized in that the modulation means of the duty cycle implement a so-called disturbance technique in which an increment constant time ATon is added or subtracted from the last known best value of the active control time of said power switching means.   8. Device according to one of claims 5 to 7, characterized in that the modulation means are further arranged to manage a safety stop value for said power consumed, from the indirect measurement of power consumed.   9. Device according to one of claims 5 to 8, characterized in that the modulation means are further arranged to reduce or limit the power consumed according to a test of external conditions, including thermal limit or full load drums.   10. Application of the method or device according to any one of the preceding claims, in charge of a battery from a photovoltaic source.