WO2010029356A1

WO2010029356A1 - Cell balancing with dc-dc converters

Info

Publication number: WO2010029356A1
Application number: PCT/GB2009/051152
Authority: WO
Inventors: Peter Miller
Original assignee: Ricardo Uk Limited
Priority date: 2008-09-09
Filing date: 2009-09-09
Publication date: 2010-03-18
Also published as: GB2463120A; GB0816464D0

Abstract

Apparatus for balancing a plurality of connected cells in a battery which battery comprises a first module containing a first plurality of groups of cells and second module containing a second plurality of groups cells, the apparatus comprising one or more processors, one or more inputs connected to the one or more processors, a first set of switches configured to be connected to the first plurality of groups of cells, a first DC to DC converter connected to the first set of switches and configured to be connected to the second plurality of groups of cells of the second module, a second set of switches configured to be connected to the second plurality of groups of cells, a second DC to DC converter connected to the second set of switches and configured to be connected to cells of a module different to the second module, wherein one or more of the processors are programmed to calculate which groups of cells of the first plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, such as from a voltmeter or state of charge estimator, and is programmed to open one or more of the first set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack power from the second module charges one or more groups of cells of the first module, and wherein one or more of the processors is programmed to calculate which groups of cells of the second plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, and is programmed to open one or more of the second set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack, power from a different module to the second module charges one or more groups of cells of the second module.

Description

CELL BALANCING WITH DC-DC CONVERTERS

The present invention relates to apparatus for balancing the charge levels of cells within a battery pack. In particular but not exclusively it relates to charging and monitoring a battery pack for use within an electric or hybrid vehicle.

Battery packs used in hybrid fuel cell or electric vehicles are typically strings of batteries/cells coupled together.

There is a tendency, especially after a significant length of time and/or number of uses, for the cells within individual battery packs to have different energy storage capacity to each other. This is caused by many variants such as temperature material impunities etc. Because each cell can naturally acquire a different energy storage and capacity, and because the amount of useful energy that can be extracted form the battery pack depends on the weakest cell, the ability to balance the energy contained in each of the cells improves the life and capacity of the battery pack. If the same cells are repeatedly being charged and discharged more often than others then this will lead to them losing capacity quicker thus reducing the lifetime of the battery pack.

With many traditional battery technologies the conventional way of balancing the battery pack is by a method known as 'equalization charging'. This involves passing a low current through the battery pack over a long period of time. This will charge the lower charged cells while the fully charged cells slowly evolve gas through electrolysis. This is performed at a deliberately low current to minimise damage to good cells. Whilst this method is suitable for many conventional battery technologies such as lead/acid it is less useful for others. In particular it has become more common to use Li-ion cells for which this method is not suitable. Firstly, it may not effectively at balancing, but more importantly Li-ion cells are more vulnerable to over-charging and have been known to overheat or even explode on such over-charging. Accordingly regularly charging them for a large period of time with a low current which runs through already fully charged cells, is not advisable. Additionally there is a problem with equalisation charging if it is wished to balance during discharge such as during the use of an electric vehicle. In these situations quick recharging is needed and the conventional slow processes is not fast enough to be much use in real time.

In vehicles such as hybrid vehicles where such real time charge is more important and where chemistry other than lead/acid may be used, alternative systems have been developed. Many of these systems require complex circuitry which uses all of the cells in the battery pack to charge a capacitor then attempt to feed that stored charge to those determined to be weak cells based on monitoring information. US5498950 describes a system in which instead of equalisation charging over the entire battery pack, individual cells are identified via battery voltage sensors and temperature sensors and then by using a controller and a set of switches the chosen cells are charged by the isolated current source by closing the switches corresponding to the desired cells. This was done to overcome the long duration and problems of equalisation charges and though it appears that the possibility of using its focussed cell approach for other battery chemistries such as Li-ion has not been appreciated. However, even if it was to be used in such a new Li-ion setting it would still suffer from problems. The isolated current source is in fact provided from the battery pack via some isolating converter using separate induction loops or an opto-coupler. This means that all of the cells including the weakest cells are collectively used to power a few of the cells. That is, the weak cells themselves provide a percentage of the power used to charge the weak cells. Where the number of cells in the battery is small, the cell being charged provides a significant proportion of the power used to recharge it leading to significant inefficiency.

Additionally if systems such as those in US5498950 were applied to common commercial battery management systems in which these cells are divided into modules, the system could either use a great number of interconnections across all of these modules to provide a global battery management system or alternatively if it were individually provided separately to each module it would suffer from the small number of cells per battery pack problem discussed above. An object of the current invention is to at least mitigate some of the problems discussed above. In some forms of the invention this object may be achieved with incorporation of some of the features of the system of US5498950 with a module based battery management system adapted to be more effective and better suited to modern cell chemistries.

According to a first aspect of the invention as provided apparatus for balancing a plurality of connected cells in a battery which battery comprises a first module containing a first plurality of groups of cells and second module containing a second plurality of groups cells, the apparatus comprising one or more processors, one or more inputs connected to the one or more processors, a first set of switches configured to be connected to the first plurality of groups of cells, a first DC to DC converter connected to the first set of switches and configured to be connected to the second plurality of groups of cells of the second module, a second set of switches configured to be connected to the second plurality of groups of cells, a second DC to DC converter connected to the second set of switches and configured to be connected to cells of a module different to the second module, wherein one or more of the processors are programmed to calculate which groups of cells of the first plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, such as from a voltmeter or state of charge estimator, and is programmed to open one or more of the first set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack power from the second module charges one or more groups of cells of the first module, and wherein one or more of the processors is programmed to calculate which groups of cells of the second plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, and is programmed to open one or more of the second set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack, power from a different module to the second module charges one or more groups of cells of the second module.

According to another aspect of the invention there is provided a method of balancing a plurality of connected cells in a battery which battery comprises a first module containing a first plurality of groups of cells and second module containing a second plurality of groups cells, using a processor to calculate which groups of cells of the first plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, such as from a voltmeter or state of charge estimator, and to opening one or more of the first set of switches corresponding to one or more groups of cells it is desired to charge, so that power from the second module charges one or more groups of cells of the first module, and wherein calculating which groups of cells of the second plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, and opening one or more of the second set of switches corresponding to one or more groups of cells it is desired to charge, so that power from a different module to the second module charges one or more groups of cells of the second module.

Preferably wherein one or more an preferably each group only comprises a single cell.

Preferably wherein one or more and preferably each of the DC to DC converters comprises an isolating converter such as a fly back converter.

Preferably wherein the one or more processors comprises a first processor connected to the first set of switches and a second processor connected to the second set of switches, the first processor being the processor programmed to calculate which groups of cells of the first plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, such as from a voltmeter or state of charge estimator, and is programmed to open one or more of the first set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack power from the second module charges one or more groups of cells of the first module, the second processor being the processor programmed to calculate which groups of cells of the second plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, and is programmed to open one or more of the second set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack, power from a different module to the second module charges one or more groups of cells of the second module.

Preferably for a battery comprising more than two modules wherein there are more than two sets of switches and/or processors and/or converters corresponding to separate modules, processors programmed so that in use power for each module of groups of cells receives power form a different module and not from class within its own module.

Preferably wherein processors for different sets of switches are connected and programmed to balance the battery pack across multiple modules.

Embodiments of the invention will now be described by way of example only and with reference to the following figures in which :

Figure 1 is a perspective view of battery pack divided into two modules;

Figure 2 is a schematic view of connections between two modules of cells in accordance with the invention;

Figure 3 is more detailed but still schematic view of some of the features of a control charging system for a single module in accordance with the invention;

Figure 4 is an illustration of connections between two modules;

Figure 5 is an example of connections between modules in a multi module system;

Figure 6 is a more detailed circuit diagram for a single module; of an implementation of balancing apparatus

Figure 7 details schematic apparatus in accordance with the invention which includes hardware protection to ensure that certain switches cannot be set. Referring to figure 1 there is shown a battery pack 10 for an electric vehicle comprising a set of batteries 16 side circuit boards 28 and 30 and two end components 24 and 26.

The set of batteries 16 comprises thirty two cells 18 organised into two adjacent modules of 16 cells each.

Each circuit board 28 or 30 fits over one module/side of the set of batteries 16 therefore enclosing 16 cells each. The circuit board 28, 30 includes micro processors etc. for controlling the batteries along with the wiring for connecting batteries to relevant sensors. Since each of the cells 26 and 28 only attaches to 16 of the cells this battery pack is considered to be in two modules, each modules comprising 16 cells and the circuit board including micro processors. There may be a separate micro processor and/or sensors for each of the cells in the portions. As can be seen from figure 1 circuit boards 28 and 30 comprise sixteen sections 32 each of which contains electronics relating to that cell.

End components 24 and 26 simply fit over the two modules to mechanically attach them together.

Each cell 18 comprises terminals 22 from which power drives the electric vehicle. These can connect with the top of the battery pack 10 going with connections between the two circuit boards 28 and 30 or may connect through the circuit boards 28 and 30 via any designed electronics contained within it.

In figure 2 is shown a schematic view of some of the features which of the battery pack 10. The two modules of cells are shown separately as set 52 and set 54 within a module, the first module 46 and the second module 48.

Each module also contains a programmed micro processor 56, sensors such as temperature sensors 64 and fly back converter 58 or other means for generating an isolated current supply. Each of the battery sets 52, 54 contains 16 cells 18 preferably each with their own sensor 64. The cells of battery set 52 collectively connect to the fly back converter 58 whilst the cells 18 of battery set 54 collectively connect to flyback converter 59 of first module 46.

The programmed micro-processor 56 takes inputs from each of the sensors 64 of its module 46 or 48 and connects to and controls the fly back converter 58, 59.

There is also a connection between the two modules. Part of the intention of using modules is to minimise the amount of wiring between lots of multiple cells. This can have mechanical advantages in terms of fitting the battery pack within a suitable place in the vehicle. For these reasons there can be a single connection 72 between the two modules 46 and 48 which may be for instance a simple serial link eg., C A N. In the embodiment shown this connects directly between the micro processors 56 with the connections from the other components connecting into this link allowing the micro processor in the opposite module to also send data or control the flow of power into the other module as described in detail below.

Referring to figure 3 there is shown more detail of the electronics within each module 48. Here there is shown the micro processor 56 and fly back converter 59 sensors 64 along with a state of charge provider 80 and a series of sixteen switches 70 which are control by the micro processor 16 and each individually connect to one cell 18 within the module 48 on one end and on the other end connect to the fly back converter 59. From the components described so far this has similarities to US5498950.

Two notable differences are that the state of charge provider instead of simply being a battery voltage sensor for each individual cell may be any other known means for either measuring or estimating the state of charge of the cells. In the case of Li- ion batteries it is not always advisable to use voltage sensors since pretty much of the use of voltage sensor when attempting to balance during use, i.e., during discharging since when a Li-ion cell the amount of charge only correlates to the voltage for a limited amount of the time of the discharge. Of most significance however is that the fly back converter 59 is connected to the other module 46 rather than an isolated current source being provided from the entire battery pack 10.

The functional connection between the modules is shown in more detail on figure 4.

Here the two sets of module cells 52 and 54 are shown along with their corresponding fly back converters 58, 59 and micro processors and other electronics 56. As shown, within each module 46 and 48 the fly back converter 58, 59 connects to the micro processor 56 and the micro processor then connects to the module cells 52 or 54.

Significantly, however, the inputs to the fly back converters 58, 59 come from the cells 52 or 54 from the opposite module 46 or 48. That is, that the fly back converter 58 of module 46 is powered by cell set 54 whilst the fly back converter 59 of the module 48 is powered by the cell set 52. Once the isolated current supply is then provided by the fly back converter, the micro processor 56 this then uses the state of charge temperature sensors 64 and SOC provider 80 to know which cells within the module set 52 it is desired to charge and control the switches 70 accordingly.

Beneficially this means these weak cells are never charged by themselves since all of the energy for any given cell being charged comes from cells from a different module.

Additionally the micro processor 56 and other electronics contained within the module 46 are also powered by the cells from a different module such as module 58.

The more modules that the system is broken into the further the advantages of the invention become apparent. Where there are multiple modules the weak cells in particular modules will be charged from pre determined other modules. This also has the advantage that unlike in US5498950 where only one weak cell can be charged at a time a weak cell per module can be charged and where there are multiple modules this can mean that several weak cells are being charged simultaneously without having to draw on the same power source. Additionally due to the serial connection between the micro processors 56 the battery management system can balance globally across all of the modules rather than merely within an individual module.

A desired implementation for a multiple modular battery pack is shown in figure 5. In figure 5 is shown a modular battery sets 90, 92, 94, 96, 98 each sending power to an adjacent balancing apparatus (comprising the fly back converter, micro processor etc.,) 91, 93, 95, 97 and 99 which provide power to the weak cells in a different module. Accordingly each of the modules provides power for balancing the cells in the adjacent module with the end module 90 providing the power to balance the opposite end module via its balancing apparatus 99.

In figure 6 is shown a particular implementation of balancing apparatus (with the power for the fly back supply coming from another module) with more components shown using switches CSl to CS 17 to connect to the cells 18 of a battery set 52/.

In figure 7 is shown a further development of this embodiment to ensure that there is no invalid combination of switches CSl to CS 17 activated.

Claims

1. Apparatus for balancing a plurality of connected cells in a battery which battery comprises a first module containing a first plurality of groups of cells and second module containing a second plurality of groups cells,

the apparatus comprising one or more processors, one or more inputs connected to the one or more processors, a first set of switches configured to be connected to the first plurality of groups of cells, a first DC to DC converter connected to the first set of switches and configured to be connected to the second plurality of groups of cells of the second module, a second set of switches configured to be connected to the second plurality of groups of cells, a second DC to DC converter connected to the second set of switches and configured to be connected to cells of a module different to the second module, wherein one or more of the processors are programmed to calculate which groups of cells of the first plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, such as from a voltmeter or state of charge estimator, and is programmed to open one or more of the first set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack power from the second module charges one or more groups of cells of the first module, and wherein one or more of the processors is programmed to calculate which groups of cells of the second plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, and is programmed to open one or more of the second set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack, power from a different module to the second module charges one or more groups of cells of the second module.

2. Apparatus according to claim 1 wherein one or more an preferably each group only comprises a single cell.

3. Apparatus according to any preceding claim wherein one or more and preferably each of the DC to DC converters comprises an isolating converter such as a fly back converter.

4. Apparatus according to any preceding claim wherein the one or more processors comprises a first processor connected to the first set of switches and a second processor connected to the second set of switches, the first processor being the processor programmed to calculate which groups of cells of the first plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, such as from a voltmeter or state of charge estimator, and is programmed to open one or more of the first set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack power from the second module charges one or more groups of cells of the first module, the second processor being the processor programmed to calculate which groups of cells of the second plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, and is programmed to open one or more of the second set of switches corresponding to one or more groups of cells it is desired to charge, so that in use when connected to the battery pack, power from a different module to the second module charges one or more groups of cells of the second module.

5. Apparatus according to any preceding claim for a battery comprising more than two modules wherein there are more than two sets of switches and/or processors and/or converters corresponding to separate modules, processors programmed so that in use power for each module of groups of cells receives power form a different module and not from class within its own module.

6. Apparatus according to any preceding claim wherein processors for different sets of switches are connected and programmed to balance the battery pack across multiple modules.

7. A method of balancing a plurality of connected cells in a battery which battery comprises a first module containing a first plurality of groups of cells and second module containing a second plurality of groups cells, using a processor to

calculate which groups of cells of the first plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, such as from a voltmeter or state of charge estimator, and to opening one or more of the first set of switches corresponding to one or more groups of cells it is desired to charge, so that power from the second module charges one or more groups of cells of the first module, and wherein calculating which groups of cells of the second plurality of groups of cells should be charged to balance the battery pack based on signals from one of the inputs, and opening one or more of the second set of switches corresponding to one or more groups of cells it is desired to charge, so that power from a different module to the second module charges one or more groups of cells of the second module.

8. A method according to claim 7 comprising features or steps equivalent to the features of any of claims 1 to 7.

9. Apparatus or method according to any preceding claim mechanically divided into multiple, preferably more than two modules, with each module including a set of switches, a converter and preferably a processor, the processors programmed and the connected configured so that cells in any module are powered by a different module and not by cells in that module.

10. A battery pack comprising a first module containing a first plurality of groups of cells and second module containing a second plurality of groups cells, apparatus according to any preceding claim wherein switches configured for connection to a plurality of groups of cells are connected to that plurality.