FR3106447A1 - ELECTRICAL ENERGY STORAGE ELEMENT AND ASSOCIATED SAFEGUARDED POWER SUPPLY - Google Patents

Abstract

The invention relates to an electrical energy storage element (23) comprising:– a first terminal (20) intended to be connected to a DC voltage (Vbatt);– a second terminal (21) intended to be connected to a voltage reference (Vref); and– at least one supercapacitor (24) connected between said first terminal (20) and said second terminal (21);said at least one supercapacitor (24) being connected in series with at least one resistor (25) connected in parallel with short-circuit means (26) configured to short-circuit said at least one resistor (25) when the voltage across the terminals of said at least one resistor (25) is lower than a threshold value. Figure for abstract: Fig 2

Description

ELECTRICAL ENERGY STORAGE ELEMENT AND ASSOCIATED SAFEGUARDED POWER SUPPLY

The present invention relates to an electrical energy storage element as well as a backed-up power supply incorporating such an electrical energy storage element.

The invention relates more particularly to an energy storage element intended to guarantee the power supply of a load. Thus, the invention can be applied in multiple fields for which it is desired to guard against power outages. For example, the invention can be implemented to power devices in industrial systems. This type of electrical device can be an assembly line control automaton, a router, etc.

The invention finds a particularly advantageous application to guard against short power cuts, that is to say cuts of a few milliseconds to a few seconds.

PRIOR ART

To protect a load from a power outage, it is known to use a backed-up power supply, that is to say a power supply incorporating at least one energy storage element making it possible to supply the load when the voltage on the network is insufficient.

Conventionally, as illustrated in FIG. 1 of the state of the art, a backed-up power supply 100 comprises an AC voltage input 15 , connected to an electrical network of a building or an agglomeration, and an output 16 of DC voltage Vs connected to an electrical device or load 12 to be protected.

Input 15 is connected to a first voltage converter 18 configured to transform the AC voltage from input 15 into a DC voltage Vbatt whose voltage level is adapted to the charging voltage of an energy storage element 230 . The DC voltage Vbatt is injected at the input of a second voltage converter 19 configured to transform the DC voltage Vbatt into a DC voltage Vs adapted to the needs of the load 12 . The energy storage element 230 is connected between this DC voltage Vbatt and a reference voltage Vref , for example ground.

As long as the mains power supply is above a threshold value, the energy storage element 230 is not used to supply the load 12 , and the power supply for the load 12 is supplied by the network via the two converters 18 - 19 . During this phase, if the level of charge of the energy storage element 230 is lower than the DC voltage Vbatt , the energy storage element 230 is recharged.

When a network cut occurs, the energy storage element 230 is configured to supply the energy necessary for the load 12 . Thus, during this phase, the energy storage element 230 gradually discharges according to the consumption of the load 12 .

There are several technologies for forming an energy storage element 230 . The most used technology implements one or more lithium-ion batteries 240 connected between two terminals 20 - 21 . These lithium-ion batteries 240 conventionally have a voltage level of between 13 and 16 V: 13 V corresponds to the discharged state of the batteries, while 16 V corresponds to the fully charged state of the batteries. Thus, for a saved power supply 100 integrating lithium-ion batteries 240 , as illustrated in FIG. 1, the DC voltage Vbatt is conventionally 16 V.

To guarantee the operation of the energy storage element 230 , the backed up power supply 100 also comprises a supervision unit 17 configured to regularly test the level of charge of the energy storage element 230 .

To do this, the supervision unit 17 controls, by the signal Com , the voltage level delivered by the first converter 18 so as to reduce the voltage delivered by the first converter 18 slightly below the voltage Vbatt , for example to 15.5 V. With this voltage of 15.5 V, even if the energy storage element 230 is completely discharged, the second converter 19 is capable of supplying the voltage level Vs required by the load 12 . This decrease in voltage is used to measure the voltage in front of the second converter 19 , by the signal Mes , to ascertain whether the energy storage element 230 is capable of discharging to compensate for the drop in voltage. The energy storage element 230 is considered functional if, during a voltage decrease to 15.5 V, the signal Mes is 16 V.

Certain regulations require this test of the energy storage element 230 to be carried out every minute. However, this test causes a cycle of aging of the lithium-ion batteries 240 and the repetition of this test limits the life of the lithium-ion batteries 240 , even when these are not actually used to protect a load 12 during a power outage.

In addition, lithium-ion batteries 240 impose significant transportation constraints. For example, it is complex to transport 240 lithium-ion batteries with a capacity of more than 100watt per hour, because specific regulations must be followed. For example, for certain legislations, if the total weight of lithium is greater than 333 kg in a truck, it must use specific road routes, in particular outside tunnels. The charge level of 240 lithium-ion batteries must also be controlled for transport, especially for air transport where it must be less than 30%.

In addition, 240 lithium-ion batteries also impose storage constraints. For example, a stored lithium battery pack should be recharged every 6 months.

To overcome these constraints, it is possible to use an energy storage element integrating one or more supercapacitors.

Indeed, although supercapacitors have much lower power supply backup times than lithium-ion batteries, typically of the order of 5 seconds, whereas lithium-ion batteries make it possible to guarantee power supply for several tens minutes, these supercapacitors can overcome some of the constraints of lithium-ion batteries because they are less sensitive to aging cycles and transport and storage issues. These supercapacitors are therefore used marginally for certain backed-up power supplies for which the backup time is not an essential criterion.

However, with a particularly low backup time, supercapacitors can be fully discharged faster. In this state, zero voltage is present between their terminals. Thus, for power supplies incorporating one or more supercapacitors, the latter must be charged at the same time as the start of the power supply. Supercapacitors have a major drawback because they cannot be plugged into a running backed up supply.

The technical problem that the invention proposes to solve is therefore to obtain an energy storage element making it possible to meet the constraints previously exposed.

To respond to this technical problem, the invention proposes using at least one resistor in series with at least one supercapacitor such that said resistor limits the current when the energy storage element is connected to a power supply saved in functioning. To prevent this resistor from degrading the behavior of the supercapacitor once it is charged, the invention also proposes using short-circuit means configured to short-circuit the resistor when the voltage across the terminals of this resistor is lower at a threshold value.

To this end, according to a first aspect, the invention relates to an electrical energy storage element comprising:
–a first terminal intended to be connected to a DC voltage;
–a second terminal intended to be connected to a reference voltage; And
–at least one supercapacitor connected between said first terminal and said second terminal.

The invention is characterized in that said at least one supercapacitor is connected in series with at least one resistor between said first terminal and said second terminal; said at least one resistor being connected in parallel with short-circuit means configured to short-circuit said at least one resistor when the voltage across the terminals of said at least one resistor is lower than a threshold value.

The invention thus makes it possible to obtain the advantages of energy storage elements integrating one or more supercapacitors, that is to say their resistance to aging cycles and the simplification of transport and storage, while allowing these energy storage elements to be connected to a power supply saved in operation. To do this, the conventional supercapacitor charging phase when starting the power supply is replaced by the connection of at least one resistor in series with said supercapacitor until the charge level of the latter is sufficient to be able to disconnect the resistor.

The invention therefore makes it possible to combine the advantages of an energy storage element integrating one or more supercapacitors with the installation capacity of an energy storage element integrating one or more lithium ion batteries on a backed up power supply. Operating.

The short-circuit means can be made by any known means. For example, a supervision device can be configured to control a switch according to a voltage measurement at the terminals of said resistor.

Within the meaning of the invention, the measurement of the voltage at the terminals of the resistor can be carried out directly or by estimating this voltage from a remote measurement.

For example, it is possible to measure the voltage at the terminals of the supercapacitor to obtain an estimate of the voltage at the terminals of the resistor without changing the invention.

According to one embodiment, the short-circuit means are made by a first transistor, the source of which is connected to the second terminal, the drain of which is connected between the supercapacitor and the resistor, and the gate of which is controlled as a function of said voltage across the resistor; the gate of said first transistor being connected to a resistor, itself connected between the two resistors mounted in series between said collector and said second terminal; the gate of said second transistor being connected between the supercapacitor and the resistor via at least one diode.

This embodiment makes it possible to electronically connect or disconnect the resistor as a function of the voltage at its terminals.

Preferably, a capacitor is connected between the gate and the source of said first transistor. This capacitor makes it possible to impose the switching of the first transistor after the switching of the second transistor in order to avoid possible fluctuations of the two transistors during the switching phases.

Preferably, a resistor is connected between said at least one diode and said base of said second transistor. This resistor makes it possible to limit the current at the base of the second transistor so that the current is mainly conducted in the supercapacitor and in the resistor when the latter is connected in series with the supercapacitor.

Preferably, the gate of said second transistor is connected between the supercapacitor and the resistor via two Zener diodes having distinct switching thresholds.

When the first Zener diode becomes conductive, the second diode also becomes conductive but this second diode has a lower threshold so that the second transistor remains conductive even when the first diode becomes blocked. The use of Zener diodes makes it possible to avoid that each diode influences one on the other to transmit an undesirable current on the supercapacitor.

To ensure that the second Zenner diode becomes active when the first diode becomes conductive, a third transistor is preferably connected in series with the diode having the lowest switching threshold, the gate of said third transistor being connected via a resistor between said resistors connected in series on said collector of said second transistor.

According to one embodiment, the electrical energy storage element comprises several supercapacitors connected in series, each supercapacitor being connected in parallel with a resistor. This embodiment makes it possible to balance the voltage variation between the supercapacitors due to the capacitance variation which conventionally exists between several distinct supercapacitors.

According to one embodiment, the electrical energy storage element comprises seven supercapacitors connected in series, each supercapacitor having a capacity of between 10 and 20 Farad.

The total capacity provided by these supercapacitors makes it possible to protect a load against cuts of a few seconds or a few tens of seconds depending on the consumption of the load.

According to one embodiment, said at least one resistor has a value between 10 and 200 Ω. If several resistors are connected in series, this value between 10 and 200 Ω corresponds to the total value of the resistors connected in series.

This value of the resistor(s) connected in series with the supercapacitor(s) makes it possible to consume a sufficient quantity of current so that the first voltage converter does not detect a short-circuit at the level of the electrical energy storage element when the energy storage element is installed in an operation-saved power supply.

In addition, this total value of the resistors makes it possible to supply sufficient current to initiate the recharging of the supercapacitors.

According to a second aspect, the invention relates to a backed up power supply comprising:
–an input intended to be connected to an electrical network of a building or an agglomeration at a given voltage;
–an output intended to be connected to a load to supply and protect this load from a power failure;
–a first voltage converter configured to transform said input voltage into a DC voltage;
–a second voltage converter configured to transform said DC voltage and obtain a voltage at said output; And
–an electrical energy storage element, according to the first aspect of the invention, connected between said DC voltage and a reference voltage.

BRIEF DESCRIPTION OF FIGURES

The manner of carrying out the invention as well as the advantages which result from it, will clearly emerge from the embodiment which follows, given by way of indication but not limitation, in support of the appended figures in which FIGS. 1 to 5 represent:

Figure 1 is a schematic representation of the prior art of a backed up power supply;

Figure 2 is a schematic representation of a backed up power supply according to one embodiment of the invention;

Figure 3 is a schematic representation of an energy storage element according to one embodiment of the invention;

FIG. 4 is a temporal representation of the evolution of the currents as resulting from the assembly of FIG. 3; And

Figure 5 is a temporal representation of the evolution of the tensions such as resulting from the assembly of figure 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2 illustrates a saved power supply 10 corresponding to the invention. This saved power supply 10 comprises an AC voltage input 15 and a DC voltage Vs output 16 . The AC voltage input 15 is intended to be connected to an electrical network of a building or an agglomeration. In the saved power supply 10 , the input 15 is connected to a first voltage converter 18 configured to transform the alternating voltage of the input 15 into a direct voltage Vbatt , for example fixed at 16 V.

This DC voltage Vbatt is also connected to the input of a second voltage converter 19 configured to transform the DC voltage Vbatt and obtain the DC voltage Vs at the output 16 .

Output 16 is intended to be connected to a load 12 to supply this load 12 and protect this load 12 from a power failure. In the example of FIG. 2, the first converter 18 is an AC/DC converter while the second converter 19 is a DC/DC converter. As a variant, input 15 can be connected to a DC voltage and first converter 18 can correspond to a DC/DC converter. In the same way, the load 12 can be supplied by the output 16 by means of an AC voltage and the second converter 19 can correspond to a DC/AC converter.

In all cases, voltage Vbatt corresponds to a DC voltage and this DC voltage Vbatt is also connected to an energy storage element 23 . More specifically, this energy storage element 23 comprises a first terminal 20 connected to the DC voltage Vbatt , and a second terminal 21 connected to a reference voltage Vref , for example ground.

Between these terminals 20 and 21 , the energy storage element 23 includes one or more supercapacitors 24 .

In accordance with the invention, supercapacitor 24 is connected in series with at least one resistor 25 , itself connected in parallel with short-circuit means 26 of resistor 25 .

Thus, when the energy storage element 23 is connected to the voltage Vbatt while the first converter 18 is in operation, the short-circuit means 26 are configured to be open during a first phase P1 of charging the supercapacitor 24 , so as to avoid short-circuiting the first converter 18 because the supercapacitor 24 is completely discharged on start-up. When the voltage across resistor 25 is lower than a threshold value, short-circuit means 26 are closed so as to short-circuit resistor 25 .

Thus, the energy storage element 23 can be used to replace an energy storage element integrating a lithium ion battery, and this energy storage element 23 does not modify the behavior of the first converter 18 with respect to to the energy storage element 23 .

The backed up power supply 10 also comprises a supervision unit 17 configured to regularly test the charge level of the energy storage element 23 . To do this, the supervisory unit 17 controls, by the signal Com , the voltage level delivered by the first converter 18 so as to reduce the voltage delivered by the said first converter 18 slightly below the voltage Vbatt , for example to 15.5 V. With this voltage of 15.5 V, even if the energy storage element 23 is completely discharged, the second converter 19 is capable of supplying the voltage level Vs required by the load 12 . This decrease in voltage is used to measure the voltage at the input of the second converter 19 , by the signal Mes (voltage), to ascertain whether the energy storage element 23 is capable of discharging to compensate for the drop in voltage between the voltage nominal of 16 V and the set voltage of 15.5 V.

The energy storage element 23 is considered functional if, during a voltage decrease to 15.5 V, the signal Mes is 16 V. Thus, the energy storage element 23 of the invention behaves like an energy storage element 230 integrating a lithium ion battery 240 of the state of the art, except that the repeated cycles of tests, as imposed by the regulations, degrade less the duration of life of the energy storage element 23 because the latter incorporates one or more supercapacitors 24 .

FIG. 3 illustrates an embodiment of an energy storage element 23 comprising seven supercapacitors C1 to C7 connected in series to increase the total charge and discharge capacity. For example, each supercapacitor C1 to C7 can have a capacitance of between 10 and 20 F, typically 15 F. These supercapacitors C1 to C7 are connected in series on the first terminal 20 of the energy storage element 23 . These supercapacitors C1 to C7 are also connected in series with three resistors R8 to R10 up to the second terminal 21 .

The voltage across the terminals of all the supercapacitors C1 to C7 is denoted Vc , while the current flowing through these supercapacitors C1 to C7 is denoted Ic . Similarly, the voltage across the terminals of the three resistors R8 to R10 is denoted Vr while the current crossing these three resistors R8 to R10 is denoted Ir . Preferably, these resistors R8 to R10 have a total value of between 10 and 200 Ω, for example 66 Ω by means of three resistors R8 to R10 of 22 Ω each connected in series.

Short-circuit means 26 are connected in parallel with resistors R8 to R10 . These short-circuit means 26 can be produced by a controlled switch, as illustrated in FIG. 2. Preferably, the short-circuit means are produced by a first transistor Q1 , for example an N-channel MOSFET transistor, whose source is connected to the second terminal 21 , whose drain is connected to a node No connecting the supercapacitors C1 to C7 and the resistors R8 to R10 , and whose gate is controlled according to the voltage Vr at the terminals of the resistors R8 to R10 .

The voltage of the gate of this first transistor Q1 is controlled by a second transistor Q2 , for example bipolar of the NPN type, whose emitter is connected to the first terminal 20 and the collector is connected to the second terminal 21 by two resistors R14 and R15 mounted in series. The two resistors R14 and R15 preferably have an identical value between 1 and 5 kΩ, for example a value of 2.2 kΩ. More precisely, the gate voltage of transistor Q1 is applied through a resistor R12 connected between the gate of transistor Q1 and between the two resistors R14 and R15 . This resistor R12 can have a value between 50 and 500 kΩ, for example a value of 100 kΩ.

The gate voltage of transistor Q2 is controlled by at least one diode D1 connected in series with a resistor R11 on node No. Resistor R11 makes it possible to limit the current at the level of the base of the second transistor Q2 . Preferably, this resistor R11 has a value between 50 and 200 kΩ, for example 100 kΩ.

To ensure that the first transistor Q1 switches after the second transistor Q2 , a capacitor C8 is connected between the gate and the source of the first transistor Q1 . Preferably, this capacitor C8 has a capacitance of between 0.5 and 2 mF, for example 1 mF. To prevent the transitions of transistors Q1 and Q2 from being controlled solely by the on or off state of diode D1 , it is preferable to use an assembly integrating at least two diodes D1 and D2 so as to present two distinct switching thresholds .

A first switching threshold is controlled by diode D1 , for example at the level of 13 V. A second switching threshold is obtained by a diode D2 connected, on the one hand, between resistor R11 and diode D1 and, on the other hand, on the other hand, on the drain of a third transistor Q3 , for example an N-channel MOSFET transistor. The switching level of this second threshold is for example 11 V. The source of this third transistor Q3 is on node No. This third transistor Q3 is controlled according to a gate voltage via a resistor R16 connected between resistors R15 and R14 . Preferably, this resistor R16 has a value equal to resistors R15 and R14 , for example a value between 1 and 5 kΩ.

When diode D1 becomes conductive, i.e. when voltage Vr across resistors R8 - R10 is less than 13.7 V (13 V + 0.7 V) and the voltage between the gate and the emitter of the second transistor Q2 is 0.7 V, the second transistor Q2 also turns on.

When the second transistor Q2 turns on, the voltage divider bridge formed by the resistors R14 - R16 also drives the third transistor Q3 into the on state, so that the diode D2 turns on.

This assembly makes it possible to form a hysteresis between the switching thresholds of the diodes D1 and D2 . Preferably, the two diodes D1 and D2 consist of Zenner diodes.

A capacitor C9 is mounted between the gate and the source of the third transistor Q3 to filter the disturbances which may occur in the transitions of the third transistor Q3 . Preferably, this capacitor C9 has a capacitance of between 50 and 500 nF, for example 100 nF.

Also, to balance the voltage between the supercapacitors C1 - C7 , each supercapacitor C1 - C7 is connected in parallel with a resistor R1 - R7 . To do this, these resistors R1 to R7 have identical values, preferably between 1 and 10 kΩ, for example 4.7 kΩ.

As shown in Figures 4 and 5, when the supercapacitors C1 to C7 are fully discharged and the energy storage element 23 is connected between a DC voltage Vbatt of 16 V and a reference voltage Vref of 0 V , voltage Vr across resistors R8 to R10 is first of all equal to voltage Vbatt while voltage Vc across supercapacitors C1 to C7 is zero. Currents Ic and Ir flowing through supercapacitors C1 to C7 and resistors R8 to R10 respectively depend on the total value of resistors R8 to R10 . With three resistors R8 to R10 of 22 Ω, the total resistance is equal to 66 Ω and the currents Ic and Ir are approximately 240 mA.

In the first phase P1 , all the transistors Q1 - Q3 and the diodes D1 - D2 are blocked, the supercapacitors C1 to C7 gradually charge, causing an increase in the voltage Vc across the terminals of the supercapacitors C1 to C7 at the same time as a decrease in voltage Vr across resistors R8 to R10 .

This first phase P1 ends when the voltage across the terminals of resistors R8 to R10 is less than 2.3 V (16 V – 13.7 V) while the voltage across the terminals of supercapacitors C1 to C7 is 13.7 V. The currents Ic and Ir are always identical in this first phase P1 and they depend on the voltage across resistors R8 to R10 . Thus, currents Ic and Ir decrease logarithmically from 240 mA to 34 mA.

When the voltage at the terminals of resistors R8 to R10 is 2.3 V, the voltage at the terminals of diode D1 is 13V considering a voltage of 0.7 V between the base and the emitter of the second transistor Q2 . Diode D1 is therefore on, imposing a sufficient voltage on the base of second transistor Q2 for it to switch to the on state. Voltage Vbatt is then connected to the base of transistors Q1 and Q3 respectively via the two voltage divider bridges formed by resistors R12 , R14 - R16 .

The first transistor Q1 therefore also switches to the on state, which has the effect of short-circuiting the resistors R8 to R10 . The voltage across the resistors R8 to R10 is therefore equal to the voltage Vds between the source and the drain of the first transistor Q1 after the switching of the first transistor Q1 from the off state to the on state.

In addition, the switching of the third transistor Q3 has the effect of authorizing the operation of the diode D2 , which has a transition threshold of 11 V instead of the 13 V threshold of the diode D1 .

It follows that diode D2 becomes blocked only if the voltage across its terminals is less than 11V, which corresponds to a voltage of 4.3 V (16 V – 0.7 V – 11 V) across resistors R8 at R10 .

Thus, even if the voltage across resistors R8 to R10 fluctuates during the transitions of transistors Q1 to Q3 , the voltage across resistors R8 to R10 remains below 4.3V and transistors Q1 to Q3 remain on even if diode D1 becomes blocked when the voltage across resistors R8 to R10 is between 2.3 V and 4.3 V.

During the second phase P2 , the current in the supercapacitors C1 to C7 becomes equivalent to the maximum current supplied at the level of the first terminal 20 , for example 1 A. During this second phase P2 , the supercapacitors C1 to C7 are responsible for linearly until the charging voltage Vbatt – Vds is reached.

The second phase P2 ends when the voltage across the terminals of the supercapacitors C1 to C7 is equal to the charging voltage Vbatt – Vds .

In the third phase P3 , the voltage across the terminals of supercapacitors C1 to C7 is constant and equal to Vbatt – Vds .

In the saved power supply 10 of FIG. 2, this energy storage element 23 can therefore be discharged when the first converter 18 fails, that is to say when the voltage supplied by the mains at the level of the input 15 is not sufficient or that the supervision unit 17 commands a decrease in the voltage at the output of the first converter 18 to test the energy storage element 23 . During these phases, the set of supercapacitors C1 to C7 discharges in a conventional manner by meeting the needs of the load 12 to supply a constant voltage Vbatt , whatever the fluctuations of the voltage Vbatt at the output of the first converter 18 .

The invention thus makes it possible to obtain an energy storage element 23 which can replace an energy storage element of the state of the art integrating a lithium ion battery without it being necessary to restart the power supply. saved 10 during the installation of the energy storage element 23 , and without it being necessary to modify the structure of the saved power supply.

Thus, the invention can be used to replace an energy storage element 23 integrating a lithium ion battery of an existing backed-up power supply with an energy storage element 23 in accordance with the invention.

Claims (10)

Element ofstorageof electrical energy (23) comprising:
–a first terminal (20) intended to be connected to a DC voltage (Vbatt);
–a second terminal (21) intended to be connected to a reference voltage (Vref); And
-at least one supercapacitor (24, C1-C7) connected between said first terminal (20) and said second terminal (21);
characterized in that said at least one supercapacitor (24, C1-C7) is connected in series with at least one resistor (25, R8-R10) between the first terminal (20) and the second terminal (21), said at least one resistor (25 , R8-R10) being connected in parallel with short-circuit means (26, Q1) configured to short-circuit said at least one resistor (25, R8-R10) when the voltage (Vr) across said at least a resistor (25, R8-R10) is below a threshold value. Electrical energy storage element according to claim 1, in which the short-circuit means consist of a first transistor (Q1) whose source is connected to the second terminal (21), whose drain is connected between the at least one supercapacitor (24, C1-C7) and the at least one at least one resistor (25, R8-R10), and the gate of which is controlled as a function of said voltage (Vr) across the terminals of said at least one resistor (25, R8-R10);
the gate of said first transistor (Q1) being controlled by a second transistor (Q2) whose emitter is connected to the first terminal (20) and the collector is connected to the second terminal (21) by two resistors (R14-R15) connected in series, the gate of said first transistor (Q1) being connected to a resistor (R12), itself connected between the two resistors (R14-R15) connected in series between said collector and said second terminal (21);
the base of said second transistor (Q2) being connected between said at least one supercapacitor (24, C1-C7) and said at least one resistor (25, R8-R10) via at least one diode (D1). Electrical energy storage element according to claim 2, in which a capacitor (C8) is connected between the gate and the source of said first transistor (Q1). Electrical energy storage element according to claim 2 or 3, in which a resistor (R11) is connected between said at least one diode (D1-D2) and said base of said second transistor (Q2). Electrical energy storage element according to one of Claims 2 to 4, in which the base of said second transistor (Q2) is connected between said at least one supercapacitor (24, C1-C7) and said at least one resistor (25, R8-R10) via two Zener diodes (D1-D2) having separate switching thresholds. Electrical energy storage element according to claim 5, in which a third transistor (Q3) is connected in series with one of the two diodes (D1-D2) having the lowest switching threshold, the gate of said third transistor (Q3) being connected via a resistor ( R16) between said resistors (R14-R15) connected in series on said collector of said second transistor (Q2). Electrical energy storage element according to one of Claims 1 to 6, comprising several supercapacitors (24, C1-C7) connected in series, each supercapacitor (24, C1-C7) being connected in parallel with a resistor (R1- R7). Electrical energy storage element according to one of Claims 1 to 7, comprising seven supercapacitors (24, C1-C7) connected in series, each supercapacitor (24, C1-C7) having a capacitance of between 10 and 20 Farad. Electrical energy storage element according to one of Claims 1 to 8, in which said at least one resistor (R8-R10) has a value between 10 and 200 Ω. Saved power supply (10) comprising:
–an input (15) intended to be connected to an electrical network of a building or an agglomeration at a given voltage;
–an exit (16) intended to be connected to a load (12) to supply and protect this load (12) from a power cut;
–a first voltage converter (18) configured to transform said input voltage (15) into a DC voltage (Vbatt);
–a second voltage converter (19) configured to transform said DC voltage (Vbatt) and obtain a voltage (Vs) at said output (16); And
-an electrical energy storage element (23) according to one of claims 1 to 9 connected between said DC voltage (Vbatt) and a reference voltage.