WO2009146952A1

WO2009146952A1 - Electrical energy store

Info

Publication number: WO2009146952A1
Application number: PCT/EP2009/050153
Authority: WO
Inventors: Volker Doege; Mario Roessler; Markus Backes; Philipp Kohlrausch
Original assignee: Robert Bosch Gmbh
Priority date: 2008-06-03
Filing date: 2009-01-08
Publication date: 2009-12-10
Also published as: DE102008002179A1

Abstract

The invention relates to an energy store containing a plurality of identical storage elements, wherein there are at least two strands of storage elements connected in series and the strands are connected in parallel, wherein the parallel connection is present at a plurality of points in the strands and each strand has at least two storage elements, wherein at least one active or passive component is provided for the purpose of connecting the strands in parallel.

Description

Title Electric Energy Storage

The invention relates to an energy storage device comprising a plurality of nominally identical storage elements, wherein at least two strings of serially interconnected storage elements are present and the strands are connected in parallel, wherein the parallel interconnection is present at several points of the strands and each strand has at least two storage elements. Energy storage of the type mentioned are used for example for driving wireless tools, portable data processing equipment or electric vehicles.

It is known from the prior art to assemble energy stores from a plurality of identical storage elements. In particular, rechargeable electrochemical cells or capacitors come into consideration as memory elements. If the energy store should have a higher voltage than a single memory element, the voltage of the energy store is increased by series connection of individual memory elements. If the energy storage device should have a higher total capacity or a higher power supply capability than a single storage element, several storage elements are interconnected in parallel. Furthermore, combinations of series and parallel connection, are common, in which, for example, three elements connected in series with each other are connected in parallel to three other elements connected in series with each other (3s2p configuration). Such an energy storage device has three times the voltage and twice the capacity of a single storage element. In this way, the provided capacity, the power supply capability and the electrical voltage of the energy storage for each application can be optimized.

However, nominally identical storage elements also have a production-related variance with respect to their internal resistance and their capacity. Furthermore, the parameters of the storage elements can vary as a result of the different temperatures of the individual storage elements which are set during the operation of a network. This relates in particular to the internal resistance of the memory element. This scattering causes individual storage elements to be discharged at different depths under load and the current intensity to vary between several parallel lines.

From WO 06/03080 A2 it is known to provide the parallel connection at several points of the strands. The parallel connection is carried out as low as possible, i. by an ohmic line connection. The multiple parallel connection makes it possible, once the charge or discharge has been completed, to equalize the charge between differently discharged storage elements to the same desired voltage level. As a result, variations in the capacity of individual memory elements can be compensated.

A disadvantage of such a configuration, however, that make the scattering of internal resistances more noticeable. During the charging or discharging of the energy storage, the current at each node splits according to the inverse ratio of the internal resistances. This causes the current to seek the path of least resistance through the energy store. As a result, the currents in the individual storage elements of the energy store vary. This different current load leads to a different thermal load and thus to an additional aging of individual storage elements. Accordingly, the object of the present invention is to reduce or avoid an uneven current load on individual storage elements of an energy store and at the same time to enable a charge equalization of differently deeply discharged storage elements.

The object is achieved by an energy storage device containing a plurality, in particular identical, memory elements, wherein at least two strands of series-connected memory elements are present and the strands are connected in parallel, wherein the parallel interconnection is present at several points of the strands and each strand has at least two memory elements, wherein for the parallel connection of the strands at least one active or passive component is provided.

The solution proposed according to the invention is based on the principle of controlling the cross-currents via the bridges used for parallel connection by means of at least one active or passive component. On the one hand, the cross-currents can be kept low during the load, on the other hand, however, a charge balance between individual memory elements are made possible in the resting phases without stress of the energy storage. This can be done for example by means of a switching element, for example a field effect transistor. This can take on a higher resistance during the charge or discharge of the electrical energy storage and have a lower electrical resistance during the resting phases of the energy storage.

In a particularly simple manner, the parallel connection of at least two memory elements for charge equalization can have an electrical resistance as a passive component. Such an electrical resistance limits the cross-currents across the bridges particularly advantageous when the electrical resistance is greater than the internal resistance of the Storage elements. Nevertheless, such a resistance in the resting phases can allow a substantial charge equalization between different memory elements when the time required for the charge equalization is short against the average duration of the rest period. The electrical resistance can be formed from one or more discrete components. Alternatively, a line connection with a desired increased electrical resistance can be used.

The energy store according to the invention can be used particularly advantageously for accumulator-operated or battery-operated power tools and vehicle batteries, in particular for electric drives. For example, individual storage elements may comprise a lithium ion accumulator, a lithium polymer accumulator, a nickel metal hydride accumulator, a nickel cadmium accumulator, a nickel / zinc accumulator or a double layer capacitor. The invention does not teach the use of a special memory element. Rather, the skilled person will select the memory element according to the required capacity, the necessary current carrying capacity, the required voltage and the number of serially and in parallel interconnected memory elements.

The invention will be explained with reference to exemplary embodiments and figures without limiting the general inventive concept.

1 shows a first embodiment of a

Energy storage device in which three passive components are used for parallel connection of two strands.

Figure 2 shows another embodiment of an energy storage device, wherein the parallel connection is performed with at least one active device. 1 shows an energy storage device 1. The energy storage device 1 consists of eight individual storage elements 10, 11,

12, 13, 20, 21, 22 and 23. The individual storage elements include, for example, lithium-ion batteries or nickel-metal hydride batteries. In some cases, the person skilled in the art can also provide for any other storage element which fulfills its requirements in terms of capacity, voltage and power supply capability. Although the memory elements 10, 11, 12,

13, 20, 21, 22 and 23 are nominally identical storage elements, have these manufacturing tolerances, which for

Scattering of the electrical capacitance and the internal resistance lead.

Four of the eight memory elements are connected in series with each other. Thus, a first strand A consists of the memory elements 10, 11, 12 and 13. A second strand B consists of the memory elements 20, 21, 22 and 23. In this way, each strand has four times the electrical voltage of the rated voltage of a single memory element 10, 11, 12, 13, 20, 21, 22 and 23. Both strands A and B, each with four memory elements 10, 11, 12, 13 and 20, 21, 22, 23 are connected in parallel with each other. This causes a doubling of capacity and power delivery capability compared to the values of a single memory element.

It should be noted that the invention is not limited to the use of 8 memory elements 10, 11, 12, 13, 20, 21, 22 and 23 or two parallel strands A and B. In some cases, the person skilled in the art will provide a larger or smaller number of memory elements and / or a greater number of strings connected in parallel. The exact number is determined based on the requirement profile of the energy storage 1. According to the invention, the two strands A and B are connected in parallel not only via the respective connection contacts 60, 61 of the energy store. Rather, the energy storage 1 has a parallel connection at several points of the strands A and B. In the embodiment of Figure 1, a parallel connection by means of three resistive elements 40, 41 and 42 between each two adjacent memory elements 10, 11, 20, 21 and 11, 12, 21, 22 and 12, 13, 22, 23 is provided. This allows a charge balance between all the memory elements 10, 11, 12, 13, 20, 21, 22 and 23. It should be noted that the expert from time to time a smaller number of parallel connections or additional parallel connections without resistance element 40, 41, 42nd can provide. In particular, the person skilled in the art will weigh up the deteriorated charge balance between individual memory cells 10, 11, 12, 13, 20, 21, 22 and 23 and the lower production costs of the energy store 1.

The value of the resistance elements 40, 41 and 42 is so great that during the load of the energy storage, i. during a charging or discharging process, the current through the resistive elements 40, 41 and 42 remains as low as possible.

On the other hand, the resistance of the resistive elements 40, 41 and 42 chosen so small that in the resting phases between individual load cycles as complete as possible charge balance between individual, parallel-connected memory elements.

In order to estimate the time required for the charge equalization, it can be assumed that there is a relationship between its cell voltage U, the discharged charge Q and the capacitance of the storage element C for each individual storage element. This relationship causes the cell voltage U to fall monotonically with the amount of charge Q taken out, and the smaller the capacitance of the storage element, the greater the magnitude of the slope of the function U (Q). Since nominally identical storage elements also have a different capacitance C, the cell voltage U changes differently for the same discharged charge Q for each storage element 10, 11, 13, 20, 21, 22 and 23 of the energy store 1.

For a simplified consideration of the charging or discharging process of the energy accumulator 1, it is assumed that no current flows through the resistance elements 40, 41 and 42 during the charging or discharging process and thus no charge Q between the individual storage elements 10, 11, 13, 20 , 21, 22 and 23. In this case, upon charging or discharging from an initially identical voltage level at the same current flow I for a period of time t through each of the parallel strands A and B, the memory elements 10, 11, 13, 20, 21, 22 and 23 will end up with a different voltage U, which is given for each memory element 10, 11, 13, 20, 21, 22 and 23 by the function U (Q), the amount of charge Q starting from an initial charge Qo decreases linearly with the time t, ie

Q = Q ₀ + i * t,

The sign of the current changes for a loading and unloading situation. The voltage difference between parallel memory elements will be referred to below as ΔU _P. After the charging or discharging process, each memory element 10, 11, 13, 20, 21, 22 and 23 of the energy store 1 should have an identical cell voltage U within a typical idle time, ie ΔU _P soll 0. The amount of charge required for this purpose can be calculated from the inverse function of U (Q), the extracted charge Q and the respective individual capacities, the initial charge state and the final charge state of the storage elements. In the exemplary embodiment considered here, it is assumed that the

Scattering σ (C) of the capacitances C of the memory elements 10, 11, 13, 20, 21, 22 and 23 with a normal distribution around one Mean value μ (C). This means that 67% of the capacitance values C of the memory elements 10, 11, 13, 20, 21, 22 and 23 lie in a range μ (C) -σ (C) <C <μ (C) + σ (0) is the required amount of charge ΔQ to compensate for the voltage differences ΔU _P in about σ (C).

To compensate for the voltage difference ΔU _P within a

Time interval tL must be a compensation current

flow.

According to Ohm's law, this compensating current I _A also results from the quotient of the voltage difference .DELTA.U _P and the resistance R of the resistive elements 40, 41 and 42. Thus it follows that

R o (C) t _L = - AU _C is.

In one embodiment, an energy storage 1 to

Use can be considered in a battery-powered drilling and screwing tool. Such a device is characterized in particular by the fact that this is operated for a few seconds and then not operated for several minutes to hours. By way of example, the energy storage device 1 is to be composed of lithium-ion storage elements. These have an internal resistance R ₁ of about 0.04 to 0.07 Ω. Due to the scattering σ (C) of the capacitances C of the individual memory elements 10, 11, 13, 20, 21, 22 and 23 of about 0.1 Ah (corresponding to 360 As), a voltage difference between parallel memory elements of approximately results after completion of a load phase 0.1 to 0.5 volts.

If the voltage difference after completion of the loading phase should be balanced within 10 minutes, must Accordingly, the electrical resistance of each resistive element 40, 41 and 42 be less than 0.8 Ω. At the same time, this value corresponds approximately to 10 to 20 times the internal resistance of the storage elements. As a result, the current flow through the resistance elements 40, 41 and 42 during the load, ie during a charging or discharging process of the energy storage device 1, negligible.

FIG. 2 shows a further embodiment of an energy store 1 according to the invention. The energy store 1 according to FIG. 2 has a total of 6 storage elements 10, 11, 12, 20, 21 and 22. The memory elements 10, 11, 12, 20, 21 and 22 are nominally identical, but have differences in their storage capacity C and their internal resistance R ₁ due to manufacturing tolerances.

By way of example, the six memory elements from FIG. 2 are again divided into two strands A and B. The strand A contains three serially interconnected memory elements 10, 11 and 12. The strand B contains another three serially interconnected memory elements 20, 21 and 22. Both strands A and B are connected in parallel with each other to form an energy storage 1. The parallel connection is carried out not only at the start and end contacts 60 and 61 of the strands A and B, but at least at a further point. Preferably, there is a parallel connection between each memory element 10, 11, 12, 20, 21 and 22. With respect to the selection of the memory elements, their number, the number of parallel strands and the number of parallel connections, the above statements with respect to Figure 1 apply unchanged.

The parallel connection is carried out in the embodiment of Figure 2 by means of a respective field effect transistor 50 and 51. The channel region of a field effect transistor 50, 51 serves as a variable resistor, with which the Resistor of the parallel connection during operation of the energy storage can be changed. In the limiting case, the resistance of a channel of a field effect transistor 50, 51 can be so large that it can be regarded as a switch which completely separates the parallel connection between two adjacent strands, A and B. This resistance change takes place as a function of the applied voltage at the gate terminal 5. In this case, both such field effect transistors are known, which when applied an electrical voltage to the gate terminal 5 the

Decrease resistance in the channel, as well as those in which the resistance is increased by applying a voltage to the gate terminal 5.

As an embodiment, an embodiment of a

Considered energy storage device 1, in which each field effect transistor 50, 51 has a low electrical resistance, when no voltage is applied to the gate electrode 5. This is for example always the case when the electrical energy storage is not used in an electrical appliance or a charger.

During such a storage time compensation currents, for example, from the memory cell 12 via the field effect transistor 50 to the memory cell 22 can flow. In the same way, equalizing currents flow from the memory cell 11 via the field effect transistor 51 to the memory cell 21 and from there via the field effect transistor 50 to the memory cell 11. These equalizing currents cause a discharge of a storage element which has a larger charge quantity Q and a charge of a further storage element with a smaller charge quantity Q. The equalizing currents come to a standstill when the voltage difference AU _P = 0 has become. At the time period during which the charge equalization has taken place, the explanation given with respect to FIG. 1 applies further. During the charging or discharging process, an electrical voltage is applied to the gate electrode 5. This can for example be done by the gate electrode 5 is coupled to the connection of an electric motor of a power tool, in which the energy storage 1 is used. When the electric motor is switched on, ie when the energy store 1 is discharged, a voltage is always applied to the gate electrode 5.

The application of an electrical voltage to the gate electrode 5 causes a change in resistance of the field effect transistors 50 and 51. Due to the increased resistance during the charging or discharging process, no equalizing currents flow between the parallel connected strands A and B. Thus, each strand and each storage element within the strand is loaded with the same charge or discharge current. As a result, it is reliably avoided that a single storage element within the energy store is loaded with a larger current and thus has to absorb a larger thermal load.

The different charge quantities .DELTA.Q in the individual memory elements 10, 11, 12, 20, 21 and 22 which are produced by loading with the same discharge current are compensated again in the subsequent service break by the

Gate electrodes 5 are disconnected from the power supply again. This reduces the resistance again so that equalizing currents can flow between the storage elements.

By such active, d. H. switched, components, the electrical resistance of the parallel connections of the two strands A and B can be adjusted in each case to the operating state of the energy storage device 1. This allows shorter compensation times of the charge differences, without the

Current capacity or the total capacity of the energy storage suffers. Furthermore, a different thermal heating of the individual memory elements 10, 11, 12, 20, 21 and 22 reliably prevented due to different current load during the charging and discharging process.

Claims

claims

1. Energy storage (1), containing a plurality, in particular identical, storage elements, wherein at least two strands (A, B) of serially interconnected memory elements (10, 11, 12, 13, 20, 21, 22, 23) are present and the strands parallel wherein the parallel connection is present at several points of the strands (A, B) and each strand (A, B) has at least two memory elements, characterized in that for parallel connection of the strands (A, B) at least one active or passive component ( 40, 41, 42) is provided.

2. Energy storage according to claim 1, characterized in that each memory cell (10, 11 or 11, 12 or 12, 13) of the one strand (A) with the memory cell (20, 21 or 21, 22 and 23 , 24) of the other strand (B) is connected via the component (40, 41, 42).

3. Energy store according to claim 1 or 2, characterized in that the component (40, 41, 42) comprises a resistor.

4. Energy storage according to claim 3, characterized in that the resistor has a value of about 0.5 ohms to about 500 ohms.

5. Energy store according to one of claims 3 or 4, characterized in that the resistance value of the formula t _L ^■ AU _n

R «R < ^p - σ (C) obey, with

R ₁ : internal resistance of a memory element, t _L : time required for charge equalization AU _P : voltage difference between parallel memory elements σ (C): dispersion of the capacitance of the memory elements

6. Energy store according to one of claims 1 to 5, characterized in that the component (40, 41, 42) has a variable resistor.

7. Energy store according to one of claims 1 to 6, characterized in that the storage elements are selected from lithium-ion cells or lithium polymer cells or nickel metal hydride cells or nickel-cadmium cells or

Nickel / zinc secondary cells or double-layer capacitors or BatCaps.

8. Power tool with an energy store according to one of claims 1 to 7.