DE102015116371A1 - BALANCING A BATTERY - Google Patents

Abstract

The present invention relates to the balancing of a battery (100) comprising: - a first cell (1) and a second cell (3) connected in series; - A measuring device (14) for determining state of charge values (11, 12) of the first and second cell (1, 3) and a charging current (9) in the battery (100) - a control device (10) having a difference value (5) the charge state values (11, 12) of the first and the second cell (1, 3), and - a charging device (15) which initiates a charge equalization between the first cell (1) and the second cell (3), if this Difference value (5) exceeds a first threshold value (6) and - the charge state value (11, 12) of the first or second cell (1, 3) to be discharged exceeds a second threshold value (7), and - the charge current (9) into the battery (100) exceeds a third threshold (8).

Description

Technical area

The present invention relates to the balancing of a battery, in particular a lithium-ion battery.

State of the art

To increase the nominal voltage of batteries, usually several single cells are connected in series. Due to different aging and manufacturing methods of the single cells of a battery, however, there are fluctuations in the capacity, internal resistance and self-discharge of these cells in the series-connected single cells. In the case of series-connected battery cells, in addition to a different charge or discharge, this leads to the cells being charged and discharged differently and therefore each being able to assume critical states of charge. Different voltage levels of the individual battery cells within a series circuit of cells of a battery, however, lead to a risk of deep discharge of individual cells in a discharge of cells.

Another problem is that cells connected in series when charging the battery, for example due to manufacturing tolerances, do not collectively reach the same end-of-charge voltage, which can lead to overloading of individual cells.

To address these known problems, so-called balancers are used in the prior art. A balancer is an electrical device that brings all battery cells within a battery pack to the same end-of-charge voltage, thus enabling full charging of all cells.

Of great importance is the use of these balancing devices in batteries based on lithium-ion cell technology. One problem with the use of lithium-ion cells is that overcharging of individual cells-which may be caused, for example, by different states of charge of individual cells within the series-connected battery cells-immediately leads to cell damage through decomposition of the electrolyte Some types even cause excessive heating up to the risk of fire.

Different charge states of the series-connected battery cells can occur, for example, if the series-connected cells differ in terms of different initial capacitances or different capacitive aging, but also because the series-connected cells discharge differently, for example due to an additional external connection of these cells can be caused.

The state of charge of a battery is usually determined by the total voltage at the ends of the series circuit. However, it is not possible to determine whether stronger, ie more charged or weaker, ie less charged, cells are present in between. This often leads to the following scenario: When reaching the final discharge voltage, weak cells may already be in the deep discharge area, while the stronger cells still contain energy. During charging, lower capacity cells and cells that were not fully discharged reach the end of charge end earlier and are therefore overcharged. The remaining battery cells, however, are not fully charged. Generally, it is known that over-or under-discharged battery cells age faster and lose their capacity, which in turn damages the overall battery. Balancers are designed to prevent this.

A problem of balancers, as known in the art, is that they operate near the battery end of charge, that is, in an area where the individual cells are practically fully charged. In the prior art, therefore, a full charge balancing is performed: If a series-connected cell thus reaches the end-of-charge voltage or a certain threshold voltage, the charge equalization is started. However, this has the disadvantage that, if the battery is not fully charged, this can lead to unsymmetrical cells. In the prior art, it is therefore not possible to perform a balancing of lithium-ion cells over a wide state of charge state. In addition, a conclusion from the voltage of the cells on their state of charge is possible only with relatively high inaccuracies.

Description of the invention

The object of the present invention is to develop a battery for the balancing of series-connected single cells of a rechargeable battery or a balancing such that a charge equalization between the series-connected cells over a wide charge state range is made possible.

This object is achieved by the features of independent claim 1.

This object is achieved by a battery having a first cell and a second cell, which are connected in series, a measuring device for determining state of charge values of the first and second cell and a charging current into the battery, a control device which determines a difference value of the state of charge values of the first and the second cell, and a charging device which initiates a charge equalization between the first cell and the second cell, if this difference value exceeds a first threshold value, and the state of charge value of the first or second cell to be discharged exceeds a second threshold value, and the charging current into the battery exceeds a third threshold value.

An advantage of the present invention is that for a charge balance (= cell symmetry) no full charge of the battery is necessary. The balancing can thus take place in the present battery according to the invention over a wide state of charge state. Balancing is thus also possible with cyclization in a flat region of the voltage characteristic of the cells. In this way, overcharging and over-discharging of individual series-connected battery cells is prevented.

Furthermore, a different capacitive aging of the individual cells due to different states of charge of the individual cells is at least slowed down by the battery according to the invention.

Another advantage of the battery according to the invention is that the charge balance (= balancing) can be done faster.

Another advantage of the battery according to the invention is that a capacity decrease of the battery is avoided by different states of charge of the individual cells of the battery.

Another advantage of the battery according to the invention is that just lithium-ion memory can be used in the automotive sector, which can serve as a replacement for the conventional lead-acid batteries. The use of lithium-ion batteries is well suited for this application due to its flat voltage characteristic over a wide charge state. The challenge in the balancing of lithium-ion cells is namely to carry out the balancing in a flat voltage characteristic range of the individual cells over a wide charge state range. This is now possible with the battery according to the invention.

Preferred embodiments of the invention

In a preferred embodiment of the present invention, the charge balance between the first cell and the second cell occurs in that the cell discharge control means switches a capacitor in parallel with the first cell to be discharged and then switches the charged capacitor in parallel with the second cell to be charged. This charges the second cell to a state of charge state.

In a preferred embodiment of the present invention, the state of charge value is a voltage value.

In a preferred embodiment of the present invention, the second threshold value is a charge end voltage. As a result, damage to the cells can be avoided by overcharging. In particular, when using lithium-ion cells irreparable cell damage may occur by overcharging, which are avoided in this way.

In a further preferred embodiment of the present invention, the charge balance between the first and second cell takes place in that the cell to be discharged, ie the cell with the higher state of charge, is connected in parallel to a balancing resistor and thereby discharged to a state of charge value.

In a preferred embodiment of the present invention, the second threshold is in a range between 65 and 90 percent of a full charge state of charge. This has the advantage that the time for a start of charge equalization does not have to be made until the series-connected individual cells each have reached their end-of-charge voltage, ie they are practically fully charged. For this reason, charge balancing can take place earlier and does not have to take place until the individual cells are fully charged.

In a preferred embodiment of the present invention, the control device terminates the charge compensation between the first and the second cell if the difference value of charge state values of the first and second cell falls below a fourth threshold value, or the charge state value of the first or second cell to be discharged falls below a fifth threshold value. or the charging current into the battery falls below a sixth threshold.

In a preferred embodiment of the present invention, the battery is a lithium iron phosphate battery or a lithium ion battery. The use of lithium iron phosphate batteries has the advantage that this type of battery has a very flat voltage characteristic over a wide charge state range.

Short description of the figures

The invention will be described with reference to an embodiment which is explained in more detail with reference to a drawing. Hereby shows:

1 a schematic illustration of the battery according to the invention.

1 shows a battery according to the invention 100 with a first cell 1 and a second cell 3 , which are interconnected in series. The battery according to the invention 100 further comprises a measuring device 14 for determining state of charge values 11 . 12 the first and second cell 1 . 3 and a charging current 9 in the battery 100 , Furthermore, the battery includes 100 a control device 10 that has a difference value 5 the state of charge values 11 . 12 the first and the second cell 1 . 3 determined. In addition, the battery according to the invention comprises 100 a charging device 15 that charge a balance between the first cell 1 and the second cell 3 initiates. The charging device 15 can be designed as a charge transfer device or a charge compensation device. The battery according to the invention 100 further includes a voltage connection 50 , as in 1 shown.

It should be noted here in general that the representation of the battery according to the invention 100 in the 1 the inventive principle illustrated by way of example and simplified. In practice, for example, a 12 volt automotive battery includes four lithium ion cells connected in series with one another. A 24 volt battery for a truck includes, for example, eight lithium-ion cells or lithium-iron-phosphate cells) connected in series with each other.

The charge balance between the first cell 1 and the second cell 3 is from the charging device 15 initiated when the difference value 5 a first threshold 6 exceeds, and the state of charge value 11 . 12 the first or second cell to be unloaded 1 . 3 a second threshold 7 exceeds, and the charging current 9 in the battery 100 a third threshold 8th exceeds. The difference value 5 is thus formed from a voltage difference between the cell to be balanced 1 . 3 and a cell 1 . 3 which has the lowest cell voltage.

The charge balance between the first cell 1 and the second cell 3 So a balancing, done by the controller 10 for the cell discharge a capacitor 13 parallel to the first cell to be unloaded 1 switches to the first cell to be unloaded 1 to a state of charge value 11 to unload. This charge state value 11 the first cell to be unloaded 1 For example, by enabling balancing, it may drop from 3.3 volts to a voltage of 2.8 volts.

To charge the cell with the lower state of charge value, the control unit switches 10 by a discharge of the cell 1 charged capacitor 13 parallel to the second cell to be charged 3 to the second cell 3 to a state of charge value 12 charge. This allows, for example, the second cell 3 from an initial state of charge of 2.3 volts to a new state of charge of 2.8 volts.

In this way it is thus achieved that the charge state values of the first cell 1 and the second cell 3 adjust each other by a charge balance. Thus, an overloading of the first cell to be balanced becomes 1 prevents, but at the same time also a deep discharge of the second cell 3 , which is the cell with the lowest cell voltage, also prevents it. This will be a capacity uptake of the individual cells of the battery 100 avoided by different charge states of the two cells.

The charge balance between the first cell 1 and the second cell 3 However, it can also take place in that the cell to be discharged, ie the cell with the higher state of charge, connected in parallel to a so-called balancing resistor (not shown) and thereby discharged to a new or different state of charge state. This approach is also referred to as passive balancing.

Preferably, the second threshold is 7 in a range between 65 and 90% of a state of charge of the cells 1 . 3 at full charge. The second threshold 7 is a charge end voltage.

Thus, when charging the battery, the start of charge balancing (= balancing) takes place between the series-connected cell 1 and the cell 3 at a very early stage, that is, in a shallow region of the quiescent voltage characteristic of the cells. This lengthens the possible balancing time, allowing sufficient charge equalization before reaching a final charging voltage. The advantage is that the charging device 15 can be dimensioned smaller and thus less thermal losses in the battery 100 arise. The charging current 9 is divided during charge equalization as follows: During cell discharge through the capacitor 13 parallel to the first cell to be discharged 1 is switched, a portion of the charging current flows through the cell 1 and to the capacitor connected in parallel with it 13 , A second part of the charging current 9 continues to the second cell 3 and thus loads the second cell 3 on.

By contrast, during cell charging, part of the charging current flows from the capacitor 13 , now parallel to the second cell to be loaded 3 is switched to the second cell 3 , In this way, the state of charge values of the first cell become 1 and the second cell 3 aligned with each other.

The charge balance between the first cell 1 and the second cell 3 is from the controller 10 ended when the difference value 5 of state of charge values 11 . 12 the first cell 1 and the second cell 3 a fourth threshold 16 below or the state of charge value 11 . 12 the first or second cell to be unloaded 1 . 3 a fifth threshold 17 falls below, or the charging current 9 in the battery 100 a sixth threshold 18 below.

In practice, it has been proven that the charge equalization then takes place or by the control device 10 is initiated when the state of charge of the first cell 1 and the second cell 2 differ by about 1%. In this case, the balancing is activated if there is a voltage difference between the individual cells. The difference between the associated state of charge values of the individual cells is also dependent on the charging current and the state of charge of the two cells.

The charging current 9 can vary in its amount. This occurs, for example, when the second cell 3 has a very low voltage value. Then can be by a higher charging current 9 the second cell 3 bring faster to their final charge voltage. However, it must be ensured that the first cell to be unloaded (= to be symmetrized) 1 it is ensured that the charging current flowing through them 9 also from the capacitor 13 can be dissipated to overload the cell 1 to prevent. The capacitor 13 must therefore be dimensioned accordingly.

It should be noted here that the charging current is limited by the charger or connected to the battery generator. If the cells have strong non-symmetrical states of charge and must be charged with a high current, the charging device must be dimensioned accordingly in order to be able to compensate the charge quickly enough or, alternatively, there should be a turn-off device which prevents an overcharging of a cell. By the battery according to the invention 100 But should just be avoided that arise too large asymmetries in the charge states of the cells connected in series.

By the battery according to the invention 100 So the advantage is achieved that a balancing even with cyclization of the individual cells 1 . 3 in a shallow region of the respective voltage characteristics of the cells 1 . 3 is possible. It is therefore no (interval) full charge of the individual cells 1 . 3 necessary to perform a cell symmetry. The charge balance between the first cell 1 and the second cell 3 In addition, it is preferably only dependent on the direction of the current, which means that the charge balance is preferably during a charging process of the battery 100 he follows.

In the following, some examples of possible threshold values are given, as they may occur in particular in the cell symmetrization of LFP cells (= lithium iron phosphate):
The first threshold 6 provides, as already mentioned, a difference value 5 out of state of charge values 11 . 12 the first and the second cell 1 . 3 dar. This difference value 5 , which is a voltage difference between the first cell 1 and the second cell 3 can be between 1 mVolt and 200 mVolt.

The second threshold 7 , which is a state of charge value 11 . 12 the first or second cell to be unloaded 1 . 3 indicates, and in which a cell symmetry is initiated when this second threshold 7 can be at about 3.0 volts to 3.8 volts.

The third threshold 8th can be a value of about 200mAmpere. Preferably, the third threshold 8th in a range between 20mA and 1 Amps lie. The charging current exceeds this third threshold 8th , then let the cell balance between the first cell 1 and the second cell 3 to be started.

The cell balancing is terminated when either the voltage difference between the first cell 1 and the second cell 3 a fourth threshold 16 falls below about 5mVolt, or if the fifth threshold 17 is below, which may be at about 3.3 volts, or if the sixth threshold 18 , that is, the charging current into the battery, falls below a value that is at about 500mAmpere or less, to end the cell balancing when only a very small charging current flows into the battery.

LIST OF REFERENCE NUMBERS

1

first cell

3

second cell

5

difference value

6

first threshold

7

second threshold

8th

third threshold

9

charging current

10

control device

11, 12

State of charge value

13

capacitor

14

measuring device

15

loader

16

fourth threshold

17

fifth threshold

18

sixth threshold

50

battery voltage

100

 battery

Claims (7)

Battery ( 100 ) with: - a first cell ( 1 ) and a second cell ( 3 ), which are connected in series; - a measuring device ( 14 ) for determining state of charge values ( 11 . 12 ) of the first and second cells ( 1 . 3 ) and a charging current ( 9 ) into the battery ( 100 ); A control device ( 10 ), which has a difference value ( 5 ) of the state of charge values ( 11 . 12 ) of the first and second cells ( 1 . 3 ), and - a charging device ( 15 ), which charge balance between the first cell ( 1 ) and the second cell ( 3 ), if - this difference value ( 5 ) a first threshold ( 6 ), and - the state of charge value ( 11 . 12 ) of the first or second cell to be discharged ( 1 . 3 ) a second threshold ( 7 ), and - the charging current ( 9 ) into the battery ( 100 ) a third threshold ( 8th ) exceeds. Battery ( 100 ) according to the preceding claim, wherein the charge balance between the first cell ( 1 ) and the second cell ( 3 ) is effected in that the control device ( 10 ) for a cell discharge a capacitor ( 13 ) parallel to the first cell to be unloaded ( 1 ) and then the charged capacitor ( 13 ) parallel to the second cell to be charged ( 3 ) switches. Battery ( 100 ) according to one of the preceding claims, wherein the state of charge value ( 11 . 12 ) is a voltage value. Battery ( 100 ) according to one of the preceding claims, wherein the second threshold value ( 7 ) is a charge end voltage. Battery ( 100 ) according to one of the preceding claims, wherein the second threshold value ( 7 ) is in a range between 65 and 90 percent of a full charge state of charge. Battery ( 100 ) according to one of the preceding claims, wherein the control device ( 10 ) the charge balance between the first and the second cell ( 1 . 3 ), if the difference value ( 5 ) of state of charge values ( 11 . 12 ) of the first and second cells ( 1 . 3 ) a fourth threshold ( 16 ), or - the state of charge value ( 11 . 12 ) of the first or second cell to be discharged ( 1 . 3 ) a fifth threshold ( 17 ), or - the charging current ( 9 ) into the battery ( 100 ) a sixth threshold ( 18 ) falls below. Battery ( 100 ) according to one of the preceding claims, wherein the battery ( 100 ) is a lithium iron phosphate battery or a lithium ion battery.

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