WO2015187070A1

WO2015187070A1 - Smart battery module

Info

Publication number: WO2015187070A1
Application number: PCT/SE2014/050671
Authority: WO
Inventors: Björn JERNSTRÖM
Original assignee: Ferroamp Elektronik Ab
Priority date: 2014-06-02
Filing date: 2014-06-02
Publication date: 2015-12-10

Abstract

The invention concerns a battery module (1) comprising: - a battery stack (2) comprising multiple series connected battery cells (3), - a DC/DC converter (4). The DC/DC converter (4) is in connection with the battery stack (2) and in connection with terminals (a, b) of the module (1). The DC/DC converter (4) is adapted to transfer electric energy between the battery stack (2) and the terminals (a, b). Further, the battery module (1) comprises a control system (7) to control said DC/DC converter (4) to discharge the battery stack through the terminals (a, b) when a voltage over the terminals Uab < a voltage U0 and to charge the battery stack through the terminals (a, b) via the DC/DC converter when the voltage Uab > a voltage U1. The invention also comprises an electric energy system, an asset comprising such a system and a method for controlling a battery module.

Description

Smart Battery Module

Technical field

The invention concerns a battery module according to the preamble of claim 1. It also concerns an electric energy system, an asset comprising such a system and a method for controlling a battery module.

Background Art

Arbitrary scaling of battery modules

When designing, building and maintaining energy storage systems, for instance for renewable energy and smart grids, it is desirable to have energy storage modules (battery modules) that can be scaled arbitrarily to reach the desired capacity. It should be possible to increase or decrease the storage capacity at a later time by adding or removing modules and also to mix modules with different state of health, different state of charge (SOC) and with different cell chemistries. It is also desirable to have modules that can be used together with various types of equipment and from various manufacturers.

Prior art solutions

Direct parallel connection of battery cells or battery packs

Prior art solutions achieve modularity by for example direct parallel connection of battery cells or battery packs. This is a very simple approach but it also creates several problems:

If batteries are not identical, will not share load evenly

If the individual batteries are not identical, for example if they have various state of health (SOH), various temperature or differing cell chemistries, they will not share the total load current between them evenly. This will limit the available capacity in the energy storage system.

If batteries have different state of charge, very high currents can occur between them

When batteries are connected together, particularly in the case of high voltage batteries (200 - 1000 V) very high currents can flow if the batteries have different state of charge (SOC) due to low internal resistance of some battery chemistries. These currents can blow fuses and damage cells and connecting relays and switches.

Difficult DC requirements - need custom battery packs

It will be difficult to match the exact DC voltage required by each particular application. Battery packs need to be custom built for each application.

Parallel battery modules with integrated DC-DC converters orchestrated by central controller

Another prior art solution achieve modularity by direct paralleling of battery modules with integrated DC/DC converters controlled from a central controller, as for example described in US2011133556A1, named Dl hereafter. Dl solves some of the above problems but instead introduces new problems related to the control of the DC/DC converters.

Problems with Dl :

Must balance power input/output to/from common DC-link In an energy storage system, as in Dl, where DC/DC and AC/DC converters are connected to a common DC-link it is vital to always maintain a balance between power flowing into the DC-link and power flowing out from the DC-link, otherwise potentially damaging voltages can be built up in the DC-link or the voltage can be reduced such that regulation fails.

DC-link balancing require very robust control system

Balancing in-/output on the DC-link in Dl means that a fast and very robust control system must be used to control the DC/DC converters in the battery modules. For this purpose, a central controller in Dl sends control commands over a digital link. This digital link is therefore part of a time critical control loop. If each module is configured as a constant current source it will also require a very fast control link in order to maintain power balance in the DC-link.

Problem with current sharing

If each module is configured as a constant voltage source this will result in problems with current sharing among the different modules.

Scalability problems

When scaling a system according to Dl by adding more modules, the parameters of the control system need to be updated, which is not trivial.

Summary of Invention

Technical problem

It is an object of the present invention to propose a solution or a reduction of the problems with the prior art. A main object therefore, is to propose an improved battery module that is easier to connect to other battery modules in order to allow an arbitrary scaling of a storage capacity of a battery module system irrespective of state of health or cell chemistry of the battery module.

Solution to Problem

According to the invention this is accomplished with a battery module according to claim 1.

This battery module solves the problem by providing a battery module with an integrated DC/DC converter that is controlled by a controller, internal to the battery module, based on a voltage on the external terminals of the module and an internal nominal voltage.

According to the invention, if the voltage Uab across the battery module terminals rises above the nominal voltage (e.g. due to an external voltage source providing a voltage to the terminals), the battery is charged. If Uab drops below this nominal voltage (e.g. due to an external load connected to the terminals) the battery is discharged.

Compared to a system with centrally controlled DC/DC converters in the packs, the battery module according to the present invention has the following advantages:

- No communication link or central control system required - module charge/discharge rates are controlled internally to the module using the module terminals voltage in a natural way. - Improved safety and stability as the central controller and communication link need not be part in the energy balance control loop.

- Modules can be connected and disconnected during operation. "Hot-swap capability", no need for parameter settings.

Compared to batteries without a DC/DC converter, the battery module according to the present invention has the following advantages:

- Battery modules can be directly connected to the DC-link of the energy storage system.

- Expansion of storage capacity can easily be done by direct parallel connection of new modules.

- No large equalization currents when new packs are connected.

- No need to stop charging/discharging if one pack has been fully charged/discharged

Further advantageous embodiments of the invention are stated in the dependent claims.

The problem is also solved with an electric energy system, an asset comprising an electric energy system and a method for controlling a battery module according to claims 8, 12 and 13. These claims providing benefits corresponding to those of the battery module.

Other art

CN102655346 describes a battery module that has some features in common with the present invention, such as an internal DC/DC controller. However, the problem to be solved is something different compared to the present invention: how to provide a battery module with an auto balance ability (see p. 1, first sentence of automatic translation into English of CN'346). That is, an ability to balance the state of charge over a plurality of series connected modules such that they all have approximately the same charge at a given moment (See fig. 2 and 3, CN '346).

In order to balance the charge over multiple series connected battery modules, CN '346 uses the DC-DC converter in each module in Voltage Mode to control the voltage on its terminals. CN '346 measures the voltage on the terminals and controls this voltage to be a reference voltage Vo ref. Vo ref is dependent on the voltage over the internal battery stack of each module, the current from the internal battery stack and a value (COM) communicated from outside the battery module, dependent on the state of charge of other battery modules that it is connected in series with (see CN '346, fig. 5 and 5).

Auto-balancing of serial batteries is obtained in CN '346 in the following way: If a battery module lags behind the other modules, it can "charge" by lowering the output voltage compared to the other modules. Thus, this module is then depleted more slowly than the other modules and in time this module will have a charge similar to the other modules. In the opposite way, a module that is more charged than other modules may rise the output voltage compared to the other modules and therefore it will be depleted faster than other modules connected in series with it. Lowering or increasing the output voltage from one module in CN '346 necessitates a corresponding increasing or lowering of the output voltage from the other modules in order to retain the total output voltage from the series connected string of modules. Since this is only possible by an outside orchestration of the modules, it is clear that the modules can not function independently from each other.

Even if one would try to use battery modules according to CN '346 for scaling an energy storage capacity, there is no simple way to do that with such modules. Since the modules are connected in series, a given number of modules have to be connected together to attain a required voltage. That is a baseline minimum number of modules in CN '346. To start scaling the storage capacity, in theory one could connect in parallel multiple strings of series connected modules. However, that would entail problems with current sharing, since each string of series connected modules would have a slightly different total voltage compared to any other such string. In theory, one could also connect more battery modules in the series string. However, that would then necessitate a coordinated lowering of the output voltage of all the battery modules to retain the total voltage output from the string. Again, this entails a coordinated orchestration of all modules which makes them dependent on each other. No plug and play extension without this orchestration is therefore possible.

Brief Description of Drawings

Embodiments that exemplify the invention will now be described with reference to the attached drawings on which

Fig. 1 discloses a battery module according to the invention,

Fig. 2 discloses a graph depicting a relationship between an outgoing/incoming current of a battery module and a voltage over external terminals of the battery module according to the invention,

Fig. 3 discloses a graph depicting a relationship between an outgoing/incoming current of a battery module and a voltage over external terminals of the battery module according to the invention,

Fig. 4 discloses an example of an electric equivalent of a programmable characteristic of the battery module according to the invention, and

Fig. 5 discloses a graph depicting a relationship between an outgoing/incoming current of a battery module and a voltage over external terminals of the battery module according to the invention.

Description of Embodiments

Fig. 1 depicts a battery module 1 according to the invention. It comprises a battery stack 2 comprising multiple series connected battery cells 3, a DC/DC converter 4, having a first input-output terminal pair 5 and a second input-output terminal pair 6. The part denoted with "BMS" is a Battery Management System that generally looks after individual battery cells to make sure they are not over charged and may also monitor and estimate the state of charge of cells. The BMS is not strictly needed for the battery module according to the invention. A current I is defined in fig. 1 to be positive when leaving the battery module 1.

The DC/DC converter 4 may be any conventional DC/DC converter with a sufficient power rating for the application. The first terminal pair 5 of the DC/DC converter is in connection with the battery stack 2 and the second pair 6 is in connection with terminals a, b of the module 1. In this arrangement, the DC/DC converter 4 can transfer electric energy between the battery stack 2 and the terminals a, b and vice versa. I.e. electric energy may flow from the battery stack 2, through the DC/DC converter and then out through terminals a, b or the other way around.

The battery module 1 also comprises a control system 7 that is configured to control said DC/DC converter 4 based on a voltage Uab over the terminals a, b of the module 1 and a reference voltage U0 of the module 1. The control system is arranged such that the module 1 discharges the battery stack through the terminals a, b via the DC/DC converter when the voltage Uab < U0 and such that the module charges the battery stack through the terminals a, b via the DC/DC converter when the voltage Uab > U0. In other words: when the external voltage Uab becomes lower than U0, e.g. due to an external load connected to the terminals a,b, the battery module 1 retrieves electric energy from the battery stack 2 and pushes it out via the DC/DC converter as an electric current leaving the terminals a,b. If the external voltage Uab becomes higher than U0, e.g. due to an external electric power source being connected to the terminals a,b, the battery module retrieves an electric current from the terminals a,b and stores it via the DC/DC converter as electric energy in the battery stack 2.

Thus, that the module charges or discharges the battery stack of the module means that the module is working the DC/DC converter in current mode. In current mode, a DC/DC converter imposes an output (or input) current and the voltage that follows depends on the load that is connected to the converter. An example of the current I from the module in relation to Uab of the terminals is shown in fig. 2.

It is understood that the meaning of "an electric current leaving the terminals a,b" means that an electric current leaves one of the terminals, e.g. terminal a, and comes back at the other, e.g. terminal b, at a lower voltage potential. Correspondingly, for the case of an electric current entering the terminals a,b means that an electric current enters one of the terminals, e.g. terminal a, and comes back at the other, e.g. terminal b, at a lower voltage potential.

Since the module of the invention is completely autonomous, it is straightforward to extend an electrical energy storage by simply adding more such modules.

Since the module of the invention uses the DC/DC converter to "hide" the individual battery cells and their chemistries from the outer world and the voltage over the terminals a,b as a control interface, modules do no interfere with each other or other devices connected to a common DC-link. Therefore, the module can also be used in different applications and together with different systems without any adaptations other than ensuring that the module has the right nominal voltage set for the application.

Also, balancing power input/output on a common DC-link to which the module is to be connected would be automatic, since any tendency of a build-up of a voltage on the DC-link would be absorbed by the module starting to charge.

All of this is accomplished without having to resort to a complicated and vulnerable external control system.

To implement a DC/DC converter behaving as described according to the invention, an alternative has been hinted at in figure 1. By applying Pulse Width Modulation (PWM) to two switches (transistors in fig. 1) the electric energy that leaves/enters the module 1 at terminals a,b in the form of an electric current can be shaped arbitrarily. However, there are many other variants that are feasible, for instance resonant converters, continuous mode switching, discontinuous mode switching, etc.

The disposition of the voltage U0 in a graph depicting the relationship between the current entering/leaving the module 1 and the voltage over the terminals a and b of the module 1 is shown in fig. 2. In the figure, the current I is positive, i.e. leaving the module, when Uab is smaller than the reference voltage U0. Vice versa, the current I is negative, i.e. entering the module, when Uab is larger than the reference voltage U0. The make-up of the battery module according to the invention makes it possible not only to design when to charge and when to discharge the battery module (using the voltage Uab over the terminals a, b as a "command interface") but also to shape the charging/discharging currents in relation to Uab. Thus, the electric current going out from or coming into module 1 could be programmed by having the control system 7 control the DC/DC converter 4 such that the discharging of the battery stack occurs according to a predetermined discharging characteristic between a discharging current through the terminals a, b of the module 1 and the voltage Uab over these terminals. Further, charging of the battery stack could occur according to a predetermined charging characteristic between a charging current through the terminals (a, b) and Uab over said terminals. In this way, the behaviour of the module 1 can be tailor made to suit any application.

For instance, any of the discharging and charging characteristics could be any of: linear, quadratic, logarithmic. Thus, the voltage-current response of the module for discharging could be linear and for the same module the voltage-current response for charging could be quadratic. As an example, in fig. 3, the discharge part is exponential while the charging part is linear. Any other combination is also possible including a voltage-current response that does not follow a mathematical function but could be designed with a response chosen for each and every voltage value at an arbitrary precision. In such a case, a lookup table with stored voltage/current value pairs could be used by the control system to decide what the output current should be at any time instance.

As an example, a linear relationship between Uab and the current entering/exiting the battery module would correspond to a Thevenin equivalent as in fig. 4. However, due to the principle of the invention, the resistance R would be lossless since "R" is in fact emulated by the DC/DC control described earlier. This is true for any programmed Uab/current characteristic of the module. Further, the module 1 may comprise a port to set any of the discharging characteristic and the charging characteristic from outside the module 1. The port may for instance be an electric connector over which electric signals signalling the desired setting could be sent. The port must not be an electric connector, it could be something else such as a connector communicating with light or even a wireless port for a wireless connection. The module would receive the signals signalling the desired charging/discharging characteristics and in response the DC/DC converter would be controlled to adhere to this/these new characteristics. The possibility of setting the

discharging/charging characteristics from outside the module makes it more easily adaptable to different applications. Also, setting the discharging/charging characteristics makes it possible to thereby also set the apparent inner resistance of the module. For instance R in fig. 4, if we are emulating a Thevenin equivalent.

Since the module 1 according to the invention is in complete control of the all currents going in or out of it, the control system 7 could be adapted to restrict the energy flow through the DC-DC converter 4 to a preset level in order to prevent currents/power flows that may be harmful to the module 1 or equipment connected to it.

As a further embodiment, the control system 7 of module 1 according to the invention may be adapted to receive information about a state of charge of the battery stack 2 and to set the reference voltage UO to a new value in order to equalise the state of charge of the battery stack 2 in view of any other battery module to be connected to the terminals a, b.

As an example, consider the module 1 and other modules to be connected in parallel with the module 1 all having a UO set to 350 V. If module 1 changes its UO to 346 V it will start charging the battery stack from the other modules when they are connected. When the state of charge is at the desired level, for instance approximately equal to that of other modules, UO could be restored to its original value. Thus, any imbalance in the state of charge between different modules can be removed which will increase battery life by having all modules working at the same state of charge. The balancing of charge can be performed between modules without having to have an external electrical energy source or sink driving a current on a DC-link to which the modules would be connected.

In order to make the module 1 according to invention adaptable to different applications requiring different voltages, the module 1 may comprise a port to set UO from outside the module 1. The port may for instance be an electric connector over which electric signals signalling the desired setting could be sent. The port must not be an electric connector, it could be something else such as a connector communicating with light or even a wireless port for a wireless connection.

The module 1 of the invention may have a reference voltage UO that is higher or lower than the total voltage of the battery cells 3, that is to say that UO may be different from that total voltage of the battery cells . Since the module 1 uses a DC/DC converter it can convert to any desired voltage within specifications of the converter. In fact, since module 1 uses a DC/DC converter in current mode, it will impose a desired current (in or out of the module) and the necessary voltage will follow "automatically" depending on the load connected to the module. However, the specific implementation of a DC/DC converter depicted in fig. 1 must have a reference voltage that is greater than the total voltage of the battery cells.

One of the ideas of the invention is that the voltage on the outer terminals a, b of the module according to the invention is used as an "interface" to control when to charge and when to discharge the module.

However, there is not a strict requirement that the cut between charging and discharging the module must be defined in a point UO. In fact, there maybe a range UO to Ul where the module neither is charged nor discharged, as depicted in fig. 5. In fig. 5, there is a voltage range on terminals a,b between UO and Ul where there is no electric energy leaving or entering the battery module 1. The voltage gap between these reference voltages could be specified to be any suitable voltage for a particular application. It may also be zero, i.e. in that case UO would equal Ul, as shown in fig. 2. This configuration with a range between two reference voltages UO and Ul where there is no charging or discharging going on is applicable to all embodiments of the module described above.

An advantage with such a voltage range UO to Ul is that in this range the battery stack could rest without any charging/discharging. For instance, if an external voltage applied to the terminals a, b of the module 1 during a time period would fluctuate somewhat around UO, it may be the case that an unnecessary, constant charging/discharging cycle is occurring that may only shorten the life of the battery stack. In that case, inserting this voltage range UO to Ul where the module is idle may prolong the life of the battery stack internal to the module.

The invention also concerns an electric energy system comprising:

- a DC-link - that is a common conductor for DC (Direct Current) to which different electric power sources or sinks can interface with a DC interface even though the sources or sinks themselves may in turn work with AC (Alternating Current) with different frequencies or phases behind that interface.

- at least one port to connect an electric energy source to the DC-link - this may for instance be a physical connection directly to the DC-link or an interface to connect AC sources to the DC-link.

- at least one port to connect an electric energy consumer to the DC-link - this may for instance be a physical connection directly to the DC-link or an interface to connect AC energy consumers to the DC-link.

- at least one battery module according to any of the embodiments of the present invention (described previously) in connection with the DC-link.

In an embodiment of the electric energy system, the port to connect an electric energy consumer and the port to connect an electric energy source are both in connection with the DC-link via a bidirectional inverter. The bidirectional inverter serves to convert between DC and AC. Said ports may be different physical ports or may be combined into one common physical port.

An example of an application of the electric energy system with an bidirectional inverter connected to the ports for electrical energy consumers and sources is an energy hub for storing and retrieving electric energy in a mains network. The energy hub could be connected to the mains network, via the inverter, and could release electric energy to AC consumers and store electric energy from AC sources on that network.

For a minimal electric energy system also comprising at least one electric energy consumer, such a consumer would be in connection with the DC-link via the port to connect an electric energy consumer to the DC-link. As mentioned above, such a port could be a direct connection to the DC-link to connect a DC consumer or via an AC interface, such as an inverter.

Such an electric system may for instance be an electric system of a house with various electric energy sinks such as an electric heater, an electric stove, etc. Further, the electric system of the house may have electric energy sources such as a solar panel generating a DC current, a connection to an electricity supply network and a battery module according to any embodiment of the invention described above. All these sources and sinks would be in connection directly or indirectly with the common DC-link. The electric energy storage of such a system could easily be extended by adding another battery module according to the invention. A further example of an electric energy system according to the invention is the electric energy system of an electric car with an electric motor, a battery module according to the invention and a port for connecting an electric power source to the system, for instance for charging the battery module.

Yet another example of an electric system would be an electric power distribution network where the battery module of the invention could be used to buffer energy from intermittent electric power sources such as solar or wind.

The invention also concerns an asset comprising an electric energy system according to the invention. Such an asset may for instance be a house or a car with an electric energy system according to the invention.

Further, the invention also concerns a method for controlling a battery module 1, the battery module 1 comprising:

- a battery stack 2 comprising multiple series connected battery cells 3,

- a DC/DC converter 4, having a first input-output terminal pair 5 and a second input-output terminal pair

6,

the DC/DC converter 4 being at the first terminal pair 5 in connection with the battery stack 2 and at the second pair 6 in connection with terminals a, b of the module 1, wherein the DC/DC converter 4 is adapted to transfer electric energy between the battery stack 2 and the terminals a, b and vice versa. The method comprises the step:

- controlling, by way of a control system 7 internal to the battery module 1, said DC/DC converter 4 based on a voltage Uab over the terminals a,b of the module 1 and a reference voltage U0 of the module 1, such that the module 1 discharges the battery stack through the terminals a, b via the DC/DC converter when the voltage Uab < U0 and such that the module charges the battery stack through the terminals a, b via the DC/DC converter when the voltage Uab > U0.

The method can be further developed with any additional step corresponding to the features of any embodiment of the battery module of the invention described above. For instance, for the feature of the battery module of the invention where the control system 7 is adapted to control the DC/DC converter 4 such that the discharging of the battery stack occurs according to a predetermined discharging characteristic, the corresponding step of the method according to the invention would be: further controlling the DC/DC converter such that the discharging of the battery stack occurs according to a predetermined discharging characteristic.

Reference Signs List

1. Battery module 6. Second terminal pair of DC/DC converter

2. Battery stack 7. Control system

3. Battery cells a, b battery terminals

4. DC/DC converter

5. First terminal pair of DC/DC converter

Claims

1. A battery module (1) comprising:

- a battery stack (2) comprising multiple series connected battery cells (3),

- a DC/DC converter (4), having a first input-output terminal pair (5) and a second input-output terminal pair (6),

the DC/DC converter (4) being at the first terminal pair (5) in connection with the battery stack (2) and at the second pair (6) in connection with terminals (a, b) of the module (1), wherein the DC/DC converter (4) is adapted to transfer electric energy between the battery stack (2) and the terminals (a, b) and vice versa, wherein the battery module (1) is characterised by further comprising

- a control system (7) that is configured to control said DC/DC converter (4) based on a voltage Uab over the terminals (a,b) of the module (1) and a reference voltage UO of the module (1), such that the module (1) discharges the battery stack through the terminals (a, b) via the DC/DC converter when the voltage Uab < UO and such that the module charges the battery stack through the terminals (a, b) via the DC/DC converter when the voltage Uab > UO.

2. Module (1) according to claim 1, wherein the control system (7) is adapted to control the DC/DC converter (4) such that the discharging of the battery stack occurs according to a predetermined discharging characteristic between a discharging current through the terminals (a,b) and Uab over said terminals, and further such that charging of the battery stack occurs according to a predetermined charging characteristic between a charging current through the terminals (a, b) and Uab over said terminals.

3. Module (1) according to claim 2, wherein any of the discharging and charging characteristics is any of: linear, quadratic, logarithmic.

4. Module (1) according to any of claims 2 or 3, wherein the module comprises a port to set any of the discharging characteristic and the charging characteristic from outside the module (1).

5. Module (1) according to any of claims 1-4, wherein the control system (7) is adapted to restrict the energy flow through the DC-DC converter (4) to a preset level.

6. Module (1) according to any of claims 1-5, wherein the control system (7) is adapted to receive information about a state of charge of the battery stack (2) and to set the reference voltage UO to a new value in order to equalise the state of charge of the battery stack (2) in view of any other battery module to be connected to the terminals (a, b).

7. Module (1) according to any of claims 1-6, wherein the module (1) comprises a port to set UO from outside the module (1).

8. Electric energy system comprising:

- a DC-link,

- at least one port to connect an electric energy consumer to the DC-link,

- at least one port to connect an electric energy source to the DC-link,

- at least one battery module according to any of claims 1-7 in connection with the DC-link.

9. Electric energy system according to claim 8, wherein the port to connect an electric energy consumer and the port to connect an electric energy source are both in connection with the DC-link via a bidirectional inverter.

10. Electric energy system according to claim 9, wherein the electric energy system is an energy hub for storing and retrieving electric energy in a mains network.

11. Electric energy system according to any of claim 8-9, wherein

- at least one electric energy consumer is in connection with the DC-link via the port to connect an electric energy consumer to the DC-link.

12. Asset comprising an electric energy system according to any of claims 8-11.

13. Method for controlling a battery module (1), the battery module (1) comprising:

- a battery stack (2) comprising multiple series connected battery cells (3),

the DC/DC converter (4) being at the first terminal pair (5) in connection with the battery stack (2) and at the second pair (6) in connection with terminals (a, b) of the module (1), wherein the DC/DC converter (4) is adapted to transfer electric energy between the battery stack (2) and the terminals (a, b) and vice versa, wherein the method is characterised by the step:

- controlling, by way of a control system (7) internal to the battery module (1), said DC/DC converter (4) based on a voltage Uab over the terminals (a,b) of the module (1) and a reference voltage UO of the module (1), such that the module (1) discharges the battery stack through the terminals (a, b) via the DC/DC converter when the voltage Uab < UO and such that the module charges the battery stack through the terminals (a, b) via the DC/DC converter when the voltage Uab > UO.