TECHNICAL FIELD

The present disclosure generally pertains to testing of secondary batteries. More specifically, the disclosure relates to a bi-directional DC/DC converter that can operate in both directions on one channel. The disclosure also relates to a cycler comprising the bi-directional DC/DC converter and to a micro grid.

BACKGROUND

In addressing climate change there is an increasing demand for rechargeable batteries, e.g. to enable electrification of transportation and to supplement renewable energy. Currently, lithium-ion batteries are becoming increasingly popular. Lithium-ion batteries represent a type of rechargeable battery in which lithium ions move from the negative electrode to the positive electrode during discharge, and back when charging.

A rechargeable battery, also referred to as a secondary battery, comprises one or more cells, for example lithium-ion cells. During production of cells, test procedures need to be performed in order to obtain essential characteristics of the lithium-ion cells regarding capacity, power density, energy density, storage life and cycle life. These tests are commonly referred to as Performance and Life-cycle, P&L, tests and Formation and Aging, F&A, tests.

During testing, batteries are typically charged and discharged in a controlled environment. In presently used systems a cycler is typically charging the cells using power from a power source and then dissipating cell energy into a number of dump loads (heaters) during discharging, which is of course power inefficient. In addition, the cooling challenge associated with dealing with the heat is also a huge problem, which may cause environmental impacts and also risk exposing electronics to ambient heat temp of more than 45°C leading to premature failure.

Due to mass scale production and increased quality requirements, it is foreseen that within a near future mass scale testing will be required at the manufacturing site, whereby there is a need to mitigate this power inefficiency. SUMMARY

It is an object of this disclosure to provide techniques that enables efficient recycling of power during testing of secondary cells. In particular, it is an object to provide a bidirectional power electronic solution that is configured to charge and discharge cells over one channel at extremely high currents and that are suitable for mass production.

According to a first aspect the disclosure relates to a bi-directional DC/DC converter. From a first end, the bi-directional DC/DC converter forms a boosted forward converter circuit for discharging a connected cell to the bi-directional DC/DC converter. From a second end, the bi-directional DC/DC converter forms a forward converter circuit for charging the connected cell over the same cell test channel. The boosted forward converter circuit and the forward converter circuit are morphed together with a transformer. The proposed bi-directional DC/DC converter is configured to provide a relatively smooth and continuous current for both charging and discharging a cell at extremely high currents, over one single channel, with a minimum component count for cost effectiveness. The transformer provides electrical isolation between the cell during test and a reversible DC source connected at the second end, whereby the cell, and the components at the second end, are protected from harmful voltages and currents.

The boosted forward converter circuit comprises the transformer, an inductor, an energy storing component and a boost switch. A first end winding of the transformer forms a primary winding of the boosted forward converter circuit. The inductor is arranged in series with the cell and the first end winding of the transformer and the energy storing component arranged to block continuous DC current from flowing through the first end winding from the cell. The boost switch is configured to alternately switch on and switch off. When the boost switch is switched on the inductor is charged with current from the cell, and when the boost switch is switched off a first end winding of the transformer is charged by the cell and the charged inductor, whereby current is generated in a second end winding of the transformer and fed to a reversible DC source connected at the second end. The boosted forward converter circuit enables discharging a cell delivering an extremely high current and recycle energy.

The forward converter circuit is formed by the transformer, the inductor, the energy storing component, a first diode and a forward converter switch. The second end winding of the transformer forms a primary winding of the forward converter. The first diode is arranged to bypass the energy storing component and the first end winding of the transformer. The forward converter switch is configured to alternately switch on and switch off. When the forward converter switch is switched on current flows from the reversible DC source to the second end winding and causes a current to flow from the first end winding to the cell, whereby the cell is charged. When the forward converter switch is switched off, current flow from the reversible DC source to the second end winding is blocked, whereby the first diode closes while the inductor acts as a low pass filter that maintains the current flow to the cell via the first diode. In addition, a demagnetising current is allowed to flow out of the first winding and into the energy storing component, circulating via the first diode. The forward converter enables charging the cell over the same channel as the cell is discharged. Thereby, power generated during discharging can be recycled to the reversible DC source and reused during charging, without additional power conversion or channel switching.

In some embodiments, the bi-directional DC/DC converter comprises a current blocking component preventing current from flowing back from the reversible DC source. Thereby, current that would otherwise flow from the reversible DC source is blocked.

In some embodiments, the current blocking component comprises a second diode arranged to rectify current generated in the secondary end winding. Hence, the current blocking component may also act as a rectifier during discharging, while also blocking DC current from flowing from the reversible DC source.

In some embodiments, the forward converter circuit is arranged such that upon the forward converter switch being switched on the energy storing component is discharged by the current flowing from the first end winding to the cell and upon the forward converter switch being switched off the transformer is allowed to demagnetise via current path that recharges the energy storing component via the first diode. Thereby, the transformer is properly reset during cell charging and the magnetising energy is stored in the capacitor. In some embodiments, the energy storing component comprises a capacitor, a battery or reversible voltage source. Hence, a capacitor, such as an infinite capacitor is typically a suitable choice, but other implementations are also possible.

In some embodiments, the transformer is a 1 : N step up transformer, with respect to the boosted forward converter circuit. Hence, the transformer steps up the boosted voltage of the boosting part of the booster forward converter even further.

In some embodiments, a transformation turn ratio of the transformer is tuneable or adjustable. Thereby, the circuit may be used for cells of different voltage.

In some embodiments, the reversible DC source is connected via a DC bus. By connecting a plurality of bi-directional DC/DC converters to a common DC bus, power may be recycled between connected cells. In other words, bi-directional DC/DC converters may act as power sources and loads of each other.

In some embodiments, the boost switch and the first diode are implemented as one electrical component. In some embodiments, the forward converter switch and the second diode are implemented as one electrical component. Thereby, number of components are reduced.

According to a second aspect the disclosure relates to a bi-directional cycler configured to charge and discharge a cell on one channel. The bi-directional cycler comprises one or more bi-directional DC/DC converters according to the first aspect, one or more channels, a power connector and a control arrangement. The one or more channels configured to electrically connect a first ends of the one or more bi-directional DC/DC converters to a respective cell and the power connector configured to electrically connect second ends of the bi-directional DC/DC converters to a reversible DC source. The control arrangement is configured to control testing of the connected cells, by interchangeably charging cells using power from reversible DC source over the respective channels and discharging the cells over the same channels, while recycling power generated during the discharging to the reversible DC source. By using cyclers comprising individual bi-directional DC/DC converters offers an energy efficient solution is offered, where energy generated during charging can be recycled and stored in a reversible DC source for reuse during charging. According to a third aspect the disclosure relates to micro grid for powering a facility for manufacturing cells. The micro grid comprises at least one power source, the facility forming a load of the power grid, and an energy storing cell testing system forming an energy storage of the micro grid. The energy storing cell testing system comprises, a reversible DC source (for example a battery bank) and a plurality of bidirectional cyclers (for example, cyclers according to the second aspect). The reversible DC source and the bi-directional cyclers are connected to a common bidirectional DC bus. The bi-directional cyclers are arranged to test cells by charging and discharging the cells and to recover energy generated during the discharging and the control system configured to control the bidirectional cyclers to initiate tests of the cells in an ongoing manner, wherein the tests are started at individual points in time different from each other. At one point in time a first subset of the bi-directional cyclers are charging cells and a second subset of the bi-directional cyclers are discharging cells. Power recovered during the discharging is used as a primary power source and the reversible DC source as a secondary power source for the charging. Any surplus power generated in the discharging which is not used for charging other cells is sent to the reversible DC source. Hence, storing capacity of cells under test can be utilised as an energy storage of microgrid powering a facility. Thereby, an energy efficient solution is achieved, while at the same time the need for power storing components in the microgrid is reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments disclosed herein are illustrated by way of example, and by not by way of limitation, in the figures of the accompanying drawings. Like reference numerals refer to corresponding parts throughout the drawings, in which

Fig. 1 A illustrates a power grid micro grid for powering a facility for manufacturing cells according to the third aspect,

Fig. 1 B illustrates the power grid micro grid for powering a facility for manufacturing cells with a cell testing system comprising a movable walk in chamber,

Fig. 2 conceptually illustrates a bi-directional cycler according to the second aspect,

Fig. 3 illustrates a bi-directional DC/DC converter according to the first aspect. Figs. 4a-4b illustrates operation of the bi-directional DC/DC converter during discharging of a cell.

Figs. 5a-5b illustrates operation of the bi-directional DC/DC converter during charging of a cell.

Fig 6a shows example current and voltage during discharging of a cell.

Fig 6b shows example current and voltage during charging of a cell.

DETAILED DESCRIPTION

In a facility for manufacturing cells, there may be one or more labs performing various types of testing involving charging and discharging cells, such as Performance and Life-cycle, P&L, testing and Formation and Aging, F&A, testing. During mass scale production, the number of cells being under test will typically be constantly high, such as hundreds or even thousands. The proposed technique is partly based on the insight that the cells under test form a power storage that can be used for powering the facility.

In other words, a significant problem of the existing system is inefficient use of energy due to cell energy typically being dissipated into a number of dump loads (heaters) during cell testing. In is herein proposed to replace of the dump loads with individual regenerative DC-DC converters feeding into a DC bus, also having an energy buffer (a large battery bank) connected thereto. In this way power can be extracted from the DC bus and fed to a power grid, thereby offsetting power consumed by the factory.

More specifically, it is herein proposed that in a facility for manufacturing cells, a cell testing lab may play the role of a power storage in a micro grid powering the facility. However as opposed to one battery pack with cells operating in uniform, which is normally used for this type of power storage, the cell testing lab would be made up of hundreds or thousands of cells passing energy between each other, i.e. some cells that are charging and some that are discharging. In other words, the cell testing lab will form an energy storing cell testing system.

Fig. 1A illustrates an example micro grid 1000 for powering a facility 1003 for manufacturing cells 200. The micro grid 1000 comprises at least one power source 1001 , the facility 1003 which forms a load of the micro grid 1000, and an energy storing cell testing system 1002.

In the illustrated example the micro grid 1000 comprises two renewable power sources 1001 (wind power and sun power). The facility 1003 is primarily powered by the power sources 1001 . If the power sources 1001 generate more power than used by the facility 1003, a surplus is stored in the energy storing cell testing system 1002. If the power sources 1001 generate less power than used by the facility 1003, power from the energy storing cell testing system 1002 is used. Hence, energy may be stored in the energy storing cell testing system 1002 also when no testing is ongoing. The micro grid 1000 may also comprise one or more back-up power sources such as diesel generators, a connection to a utility grid or an additional power storage device (not shown).

The energy storing cell testing system 1002 is formed by a huge number (hundreds or even thousands) of cells under test managed by a control system 10. More specifically, the energy storing cell testing system 1002 comprises a reversible DC source 300, a DC bus 301 , a plurality of bi-directional cyclers 1 (herein simply cyclers) and a control system 10. The cyclers 1 and the reversible DC source 300 are all connected to a common bidirectional DC power bus, herein called a DC bus 301 . The DC bus 301 will distribute power between connected components acting as sources and loads of the DC bus 301 . The cyclers 1 will either form a load or a power source of the DC bus 301 , depending on whether cells 200 under test are charging or discharging. Surplus or net demand of power from the testing can be exchanged with the reversible DC source 300, such as a high voltage battery storage system. Hence, the reversible DC source 300, has the purpose of smoothing out supply and demand of power. Connected to DC bus 301 is also a grid tied inverter 20 for net import or export of power with the microgrid 1000 should the state of charge of the reversible DC source 300 become fully recharged, or conversely, close to being depleted. The grid tied converter 20 is basically a bi-directional AC/DC converter. The grid tied converter enables power exchange between the micro grid and the energy storing cell testing system 1002. Hence, if the voltage of the reversible DC source 300 goes below a certain level it can be charged by power delivered by the micro grid 1000. Also if voltage of the reversible DC source 300 goes below a certain level it can deliver excess power to the micro grid 1000. In the illustrated embodiment the microgrid comprises one energy storing cell testing system 1002. However, it must be appreciated that one micro grid may comprise several energy storing cell testing systems 1002, whereby power storing capabilities will be even more flexible. When no testing is ongoing, energy may still be stored in the energy storing cell testing systems 1002. Cells 200 may then be connected to the cyclers 1 only for the purpose of storing energy, for example at night.

The plurality of bi-directional cyclers 1 , for simplicity called cyclers, are arranged to test cells 200 by charging and discharging the cells 200, while recovering energy generated during the discharging. The cyclers 1 will be described in further detail in Fig. 2. The cyclers 1 are bidirectional in the sense that they can charge and discharge one or more cells 200 over one respective channel 400, wherein one channel corresponds to a charging and discharging channel of one single cell 200. In other words, power for charging the cell is supplied via the channel 400. Power generated when discharging is also recycled over the same channel 400. In Fig. 2 each cycler comprises one single channel 400. In other words, energy produced during discharging a cell 200 is fed back on the same channel used for feeding energy to the cell 200 during charging. Hence, power from one cycler 1 can effectively be diverted into a neighbouring cycler 1 , via the DC bus 301 , without the need to be converted to AC for feeding onto the micro-grid and back again.

In one example embodiment, a number, N, (typically N = 16 or more) of single channel cyclers 1 are housed in racking units to form an N channel system. The number of channels, N, is up to implementation and depends on a number of factors: e.g. primarily the physical size and number of racking/cabinets housing N cyclers 1 , and the power limitations of the reversible DC source 300. Other limitations may include digital/software, physical distance between individual channels and the supervisory controller, and finally cost.

All individual channels are typically connected to the DC bus 301 , and also to a common communications bus e.g. a Controller Area Network, CAN, bus. An overall supervisory monitoring system, herein called control system 10, provides an interface 501 configured to control each channel. In other words, the control system 10 is basically a test management system. For example, the control system 10 is configured to set parameters of upper and lower voltage limits, maximum charge and discharge currents, number of charge/discharge cycles, cell temperatures, charging rates etc. Each channel may be independently configured, or the channels may be configured in groups of channels. The control system 10 may also have autonomy over when each channel (or each group of channels) starts in an attempt to balance the number of cyclers discharging cells with the number of cyclers 1 recharging cells at any given moment, and to minimize the net import or export of power to/from each channel. In other words, the control system 10 configured to control the bidirectional cyclers 1 to initiate tests of the cells in an ongoing manner. In other words, new tests of cells 200 are continually started on demand. The tests are started at individual points in time different from each other, whereby at one point in time a first subset of the bi-directional cyclers are charging cells 200 and a second subset of the bi-directional cyclers are discharging cells. In other words, tests of individual cells may be started at arbitrary points in time. Thereby, at a certain point in time some cells under test will be charging and some will be dis-charging. Alternatively, a certain schedule may be used so that the number of cells charging is similar to the number of cells discharging. Alternatively, a schedule may be formed such that the difference from consumed and recycled energy is as small as possible over time.

The facility 1003 is for example a facility for manufacturing cells 200 and forms a load of the power grid 1000. During operation of the energy storing cell testing system 1002 power recovered during discharging of cells 200 is used as a primary power source and the reversible DC source 300 as a secondary power source for charging other cells 200.

The cell testing system 1002 is typically dimensioned for mass scale testing. In some embodiments, the cell testing system 1002 is designed as a movable test lab, that is easy to install and that enables fast upscaling. The movable test lab is in some embodiments a walk-in chamber 2000 for testing cells 200.

Fig. 1 B illustrates a power grid micro grid for powering a facility for manufacturing cells with a cell testing system 1002 comprising a movable walk-in chamber 2000 for testing cells. Several movable walk-in chamber 2000 may be attached to one single micro grid 1000, but for simplicity only one is shown in Fig 1 B. The movable walk-in chamber 2000 may be located at the facility 1003. Fig. 1 B conceptually illustrates the movable walk-in chamber 2000, obliquely from above with one short side removed, when connected to a plurality of cyclers 1 . The movable walk-in chamber 2000 is connected to the power grid 1000 of Fig. 1A. In the illustrated example, the movable walk-in chamber 2000 comprises an enclosure 2101 which forms one (huge) thermally isolated temperature chamber. In other words, the enclosure 2101 forms a “super chamber” that can accommodate thousands of cells

200 (herein also simply called cells 200) in racking 2105 arranged along inner walls of the enclosure 2101 . Inner walls herein refers to the inside, or inner face, of the walls of the enclosure 2101. In the illustrated embodiment the enclosure is rectangular prismatic and has six walls, herein also referred to as sides. However, other shapes are also possible. In other words, in some embodiments, the cell testing system 1002 is designed as one single unit.

The racking is configured or constructed to enable fast connection and disconnection of cells 200. In the illustrated example, the racking 2105 is positioned along the long sides of the enclosure 2101 along a walking aisle 2102. Thereby, the racking 2105 is accessible for someone standing in the walking aisle 2102. The racking 2105 is typically configured to receive a large number of cells 200, such as at least 500 cells 200, at the same time and possibly even thousands (such as at least 1000 or 2000 cells). The more cells 200 that can be attached, the higher power storing capacity. The racking 2105, may be adapted or arranged to fit different types of cells 200, such as pouch cells, coin cells, prismatic cells and/or cylindrical cells. In some embodiments, different parts of the racking 2105 may be designed or constructed to receive different types of cells 200. Furthermore the racking 2105 may be is designed to receive individual cells and/or groups of cells, such as cells 200 arranged in bundles, such as packs of batteries. The external cyclers 1 are connected to cells 200 inserted in the racking via cycler interfaces arranged in the walls of the enclosure 2101 .

The movable walk-in chamber 2000 is movable, which means that it can be moved or transported without or with minor disassembly. In some embodiments, the movable walk-in chamber 2000 is manufactured off-site in advance, in standard sections that can be easily shipped and assembled. For example, the movable walk-in chamber

201 comprises a floor section. The sections are assembled on-site with a clamping arrangement (such as a bracket or strapping bar) that is for example screwed or welded to keep the movable walk-in chamber 2000 in a permanent state. This type of assembly may facilitate retrofitting into existing buildings. Prefabrication is typically desirable due to health and safety, as the number of construction workers needed in the already crowded factories is reduced.

The size of the enclosure 2000 is for example 20 feet (length) x 8 feet (width) x 8.5 feet (height), which corresponds to a standard freight container suitable for stacking. Hence, in some embodiments, the movable walk-in chamber 2000 is modular such that a plurality of movable walk-in chambers 2000 can be stacked on top of or next to each other. The enclosure 2101 may also be slightly smaller or larger. In some embodiments, the enclosure 2101 has a size of at least 10(l)*6(w)*2(h) meters or at least 5(l)*3(w)*2(h) meters. Hence, the enclosure forms a chamber where people (e.g. staff) can enter to insert or remove cells 200 from the racking 2105. Cyclers 1 used for performing the testing are arranged outside the enclosure. In other words, the cyclers 1 are external cyclers in relation to the enclosure 2101. In this example, they are attached on outer walls of the enclosure 2101.

The enclosure 2101 is typically made from one or more solid and heat isolating materials, such as mineral wool, fiberglass, polystyrene, cellulose or polyurethane foam. Thermal insulation implies that transfer of heat through walls of the enclosure 2101 is reduced. The movable walk-in chamber 2000 further comprises a temperature and/or climate control mechanism 2109 configured to control the temperature inside the enclosure 2101. The temperature and/or climate control mechanism 2109 may be located inside or outside the enclosure 2101. The temperature and/or climate control mechanism 2109 comprises for example one or more fans and/or coolers, heating elements, compressors, cooling circuits, HVAC, frost free heater and thermostats. Hence, the temperature may be held at a constant temperature such as about 25 °C or 45 °C, which is typically sufficient for P&L testing.

In some embodiments, the movable walk-in chamber 2000 comprises at least one central power connector 2110 used to power all the cyclers 1 from a common power source, such as from the micro grid 1000 or the reversible DC source 300. In some embodiments, the reversible DC source 300 comprises a battery arranged in the movable walk-in chamber 2000. Fig. 2 conceptually illustrates a bi-directional cycler 1. Cyclers are commonly used to analyse the cell function through charge/discharge cycles, by measuring a response of the cell over time. During cell cycling, a number of parameters can be measured, including capacity, temperature, current, voltage, efficiency of the battery and selfdischarge. The cycler 1 is being designed and modelled to discharge and recharge one or more cells 200 (in the range of 100-1000 Ah) at full current. At the heart of the cycler 1 is one or more bi-directional power converters 100 that not only deliver very high current (such as hundreds of A) efficiently to the cells 200 under test, but that can also discharge the cells 20 with currents in the range of 300, and at the same time recover this energy to a DC bus 301 via a power connector 701 .

The cycler will now be described in more detail. The cycler 1 comprises one or more bi-directional DC/DC converters 100, one or more channels 400, a power connector 700 and a control arrangement 500.

Each bi-directional DC/DC converter 100 is configured to provide a relatively smooth and continuous current for both charging and discharging a connected cell 200 over one respective channel 400, at extremely high currents (such as thousands of A). The bi-directional DC/DC converter 100 is described in further detail in Fig. 3.

The one or more channels 400 are configured to electrically connect first ends 101 of the one or more bi-directional DC/DC converters 100 to a respective cell 200. Each channel 400 is used for both charging and discharging one respective cell 200.

The power connector 700 is configured to electrically connect second ends 102 of the bi-directional DC/DC converters 100 to a reversible DC source 300. More specifically, the power connector 700 is connected to the one or more bi-directional DC/DC converters 100 via respective power cabling 701 . For a multi-channel cycler the power cabling 700 may comprise an internal DC bus, that feeds cells 200 being charged and is fed by cells 200 being charged, whereby a surplus or shortage is handled by the DC bus 301 . Alternatively, the power cabling 701 connects each individual channel directly to the DC bus 301 .

The control arrangement 500 comprises one or more processors 501 and memory 502. It may also comprise further components such as pulse width modulating chips. The control arrangement 500, or more specifically the processor 501 of the control arrangement 500, is configured to cause the cycler 1 to perform testing of cells 200. This is typically done by running computer program code ‘P’ stored in the data storage or memory 502 in the processor 501 of the control arrangement 10. For example, the control arrangement 500 control the bi-directional DC/DC converters 100 to charge and discharge connected cells 200 using pulse width modulated switching. Typically, the control arrangement comprises one dedicated controller circuit (such as a processor) for each bi-directional DC/DC converter 100. For example, each bidirectional DC/DC converter 100 has its own dedicated controller circuit with pulse width modulating functionality. Thereby, each channel 400 can be individually controlled.

In other words, the control arrangement 500 is configured to control testing of the connected cells 200. The testing comprises interchangeably charging cells over the respective channels 400 using power from the reversible DC source 300 and discharging the cells over the same channels 400. In other words, during the charging power supplied by the reversible DC source 300 is converted to a lower voltage by a connected bi-directional DC/DC converter 100 and fed back to a cell 200. During the discharging the bi-directional DC/DC converts power supplied by the cell 200 to a higher voltage, whereby the power can be fed back to the reversible DC source 300. During the charging and the discharging, the one or more cells 200 are analysed by monitoring for example their current, voltage, temperature etc. Hence, by using the bidirectional DC/DC converters 100, power generated during the discharging can be recycled back to the reversible DC source 300.

The memory 102 may additionally be configured to store various relevant parameters, such as test parameters. In the illustrated example the cycler 1 also comprises a user interface 600, that enables a user to configure operational parameters for the testing. It must be appreciated that the illustration in Fig. 2 is conceptual and that in a real implementation the cycler 1 will comprise more components, such as sensors, converters, etc.

The proposed bi-directional DC/DC converter 100 configured to provide a relatively smooth and continuous current for both charging and discharging a connected cell 200 over one respective channel 400 at very high currents with a minimum component count for cost effectiveness will now be described with reference to Figs. 3, 4a-b and 5a-b. It should be anticipated that other bi-directional DC/DC converter with similar performance may also be used in the proposed cycler 1 and micro grid 1000.

Fig 3 illustrates an example bi-directional DC/DC converter forming, from a first end, a boosted forward converter circuit for discharging a connected cell and, from a second end a forward converter circuit for charging the connected cell 200. The bidirectional DC/DC converter 100 basically from two different circuits, one for boosting voltage during the charging and one for down conversion of voltage supplied by the reversible DC source during charging. The circuits are morphed together with a transformer 110.

More specifically, from a first end 101 (or first side), the bi-directional DC/DC converter 100 forms a boosted forward converter circuit 100a (Fig. 4a-4b) for discharging a connected cell 200. Operation of the boosted forward converter circuit 100a is illustrated in Figs 4a-b. From a second end 102 (or second side), the bi-directional DC/DC converter 100 forms a forward converter circuit 100b (Fig. 5a-5b) for charging the connected cell 200. The first and second ends 101 , 102 are arranged at opposite directions with respect to a corresponding channel 400. Operation of the forward converter circuit 100b is illustrated in Figs 5a-b. The boosted forward converter circuit 100a and the forward converter circuit 100b are morphed together with the transformer 110. In other words, the boosted forward converter circuit 100a and the forward converter circuit 100b are not physically different circuits but rather different operation modes of the bi-directional DC/DC converter 100.

In some embodiments, the transformer 110 is a 1 : N, N>1 , step up transformer in the direction from the first end 101 to the second end 102. A step-up transformer is a type of transformer that converts the low voltage and high current from the primary side of the transformer to the high voltage and low current value on the secondary side of the transformer. In other words, in these embodiments the cell voltage is stepped up, which contributes to a total conversion ratio of the bi-directional DC/DC converter 100

In some embodiments, a transformation turns ratio N of the transformer 110 is variable, i.e. tuneable or adjustable. Variable transformers take in utility line voltage and provides continuously adjustable output voltage in the range of zero to or above line voltage. In other words, the transformation ratio of the bi-directional DC/DC converter 100 can be tuned to be adapted to different cell voltages.

Components of the bi-directional DC/DC converter 100 will first be described with reference to Fig. 3. The circuit in Fig. 3 is one example embodiment, and it should be appreciated that some components may be exchange and/or moved while maintaining the same main functionality.

The bi-directional DC/DC converter 100 comprises a first end 101 and a second end 102. The first end 101 of the bi-directional DC/DC converter 100 is the side facing the cell 200, here the left side. The second end 102 of the bi-directional DC/DC converter 100 is the side facing the reversible DC source 300, here the right side. In some embodiments, the reversible DC source 300 is connected to the second end via a DC bus 301 , see Fig. 1. The transformer winding of the first end 101 is herein referred to as the first end winding 111. The transformer winding of the second end 102 is herein referred to as a second end winding 112. The first and second end windings 111 ,112 will interchangeably act as primary and secondary windings of the transformer 110, depending on whether the cell 200 is charging or discharging as will be described in further detail below. Hence, “first” and “second” herein refers to a position of the winding, not to a function with regards to the power transformation.

The first end 101 of the bi-directional DC/DC converter 100 comprises an inductor 120, an energy storing component 160, a boost switch 130 and a first diode 140. The inductor 120 is arranged in series with the cell 200, the energy storing component 160 and the first end winding 111 of the transformer 110. The energy storing component 160 comprises for example a capacitor, a battery or reversible voltage source, or any suitable chargeable component that can block continuous DC current. The boost switch 130 is arranged to by-pass the energy storing component 160 and the first end winding 111 of the transformer 110, whereby the inductor 120 can be charged by the cell 200 when the boost switch 130 is closed, as explained below.

The second end 102 of the bi-directional DC/DC converter 100 comprises a forward converter switch 150 and a current blocking component 170, here a second diode. The second end winding 112 is connected in series with the current blocking component 170 and the reversible DC source 300. The current blocking component 170 is arranged to prevent current from flowing from the reversible DC source 300. The forward converter switch 150 is arranged to (when closed) bypass the current blocking component 170, whereby the second end winding 112, and thereby also the cell 200, can be charged by current from the reversible DC source 300, upon the forward converter switch 150 being closed.

It must be appreciated that all the functional components described above may be implemented by several physical component to achieve a certain size, which is commonly known to a skilled person. For example, the energy storing component 160 may be implemented by several energy storing components in parallel or an inductor 120 may be implemented by several components in series.

Operation of the boosted forward converter circuit 100a will now be described in further detail with reference to Figs 4a-b. In other words, operation of the bi-directional DC/DC converter 100 while discharging a connected cell 200 will now be described.

A boost converter, sometimes called step-up converter, is a commonly used type of DC-to-DC power converter that steps up voltage (while stepping down current) from its input (i.e. supply) to its output (i.e. load). A forward converter is another commonly used type of DC-to-DC power converter that uses a transformer to increase or decrease the output voltage (depending on the transformer ratio) and provide galvanic isolation for the load. These circuits are commonly known in the art. During discharging, the bi-directional DC/DC converter 100 operates as what one may call a boosted forward converter. One may also consider it being a boost converter with a DC blocked output transformer 110.

The boosted forward converter circuit 100a is mainly formed by the transformer 110, the inductor 120, the energy storing component 160, and the boost switch 130. The cell 200 forms an input, or supply, of the boosted forward converter circuit 100a, while the reversible DC source 300 forms the output, or load of the boosted forward converter circuit 100a. In this mode of operation, the forward converter switch 150 is constantly opened, and therefore not shown. Discharging operation is controlled by controlling the boost converter switch 130, based on a pulse width modulated signal.

When discharging the cell 200, the first end winding 111 of the transformer 110 forms a primary winding of the boosted forward converter circuit 100a and the second end winding 112 of the transformer 110 forms a secondary winding of the boosted forward converter circuit 100a. The inductor 120 has the function of boosting the cell voltage. The energy storing component 160 is arranged to block continuous DC current from flowing through the first end winding 111 from the cell 200.

The boost switch 130 is configured to alternately switch on and off, whereby power is transferred from the cell 200 to the reversible power switch. More specifically, the boosted forward converter circuit 100a is switched between two different states. Fig. 4a illustrates what happens when the boost switch 130 is switched on, i.e. closed. When the switch is closed, the inductor 120 is charged with current (here called boost current) I _boost from the cell 200. The inductor 120 initially has a high reactance which limits the rate of rise of boost current, as energy is building up as an increasing magnetic field, which results in a rising boost current I _boost . In the illustrated embodiment, the boosted forward converter circuit 100a comprises a current blocking component 170, here a second diode. The second diode is arranged to rectify current generated in the secondary end winding 112, such that it can only flow into the reversible de source 300. The second diode is also arranged to prevent current from flowing back from the reversible DC source 300.

However, the transformer 110 may be charged with magnetic energy due to a charge current I _boost that flew in the first winding 111 when the boost switch was off, see Fig. 4b. When the charge current I _boost is suddenly removed, the magnetic field in the transformer 110 causes the polarity of the transformer 110 to immediately reverse, whereby the upper (dot) side becomes negative. Due to the current blocking component 170 demagnetizing current is blocked in the secondary side (i.e. second end winding 112) of the transformer 110. Hence, the transformer cannot be demagnetised on the second end 102 and the energy storing component is not unloaded. However, the boost switch 130 provides a path for a de-magnetising current l _mag from the first winding 111 , whereby the transformer is de-magnetising. The energy storing component 160 is discharged by the de-magnetising current l _mag . In the illustrated embodiment, the energy storing component 160 is an infinite capacitor with a constant voltage v _c , which is not affected by the discharging.

Fig. 4b illustrates what happens when the boost switch 130 is switched off, i.e. open. When the boost switch 130 is opened, current immediately starts to flow from the secondary cell 200 to the first winding 111 and the transformer 110 is magnetising. The sudden current change reverses the polarity of the inductor 120, whereby the left side becomes negative. Hence, the inductor 120 now forms an additional voltage source that acts in series with the cell 200. Hence, the first end winding 111 of the transformer 110 is charged by the cell 200 and the charged inductor. In other words, the transformer 110 and the energy storing component 160 are charged by a boost circuit formed by the cell 200 and the inductor 120.

The transformer 110 itself and the reversible DC source 300 forms a load of the boost converter (formed by the inductor 120, the boost switch 130 and the cell 200) that consumes a load current that corresponds a sum of a magnetising current I _mag of the transformer 110 and a charge current I _cfiarge of the energy storing component 160. The magnetising current I _mag establishes the magnetic field in the transformer. The current l _charge + l _mag discharges the inductor 120. Hence, a charge current I _charge is generated in a second end winding 112 of the transformer 110 and fed to a reversible DC source 300 connected at the second end. The current blocking component 170 rectifies the charge current I _charge . The charging voltage of the reversible DC source 300 corresponds to the sum of the cell voltage v _ce u plus the inductor voltage V _ind minus the voltage V _c of the energy storing component, stepped up by the turn ratio W ₁₁₁ /W ₁₁₂ of the transformer (where N ₁₁ N ₁₁₂ are the count of first and second windings 111 , 112 respectively).

When the boost switch 130 is open energy stored in the inductor 12 and the cell 200 slowly dissipates into the reversible DC source 300. Before the inductor is completely discharges the boost switch 130 is closed again and the inductor 20 is re-boosted, and so on. The on/off switching of the boost switch 130 is typically controlled by a pulse width modulated signal that controls the time for loading and unloading the inductor 120, based on components sizes, such as reset time of the transformer 110. Generation of the pulse width modulated signal is for example controlled by the control arrangement 500.

Operation of the forward converter circuit 100b will now be described in further detail with reference to Figs 5a-b. In other words, operation of the bi-directional DC/DC converter 100 while charging a connected cell 200 will now be described. In this mode of operation the boost converter switch 130 is constantly open, therefore not shown. Charging operation is controlled by controlling the forward converter switch 170 based on a pulse width modulated signal.

The forward converter circuit 100b is mainly formed by the transformer 110, the inductor 120, the energy storing component 160, a first diode 140 and the forward converter switch 150. The reversible DC source 300 forms an input, or supply, of the forward converter circuit 100b, while the cell 200 forms the output, or load, of the forward converter circuit 100b.

During charging of the cell 200, the second end winding 112 of the transformer 110 forms a primary winding of the forward converter circuit 100b and the first end winding 111 forms a secondary winding of the forward converter circuit 100b. The inductor 120, used for boosting the boosted forward converter circuit 100a operates as a filter in the forward converter circuit 100b. The first diode 140 acts as a free-wheeling diode both for a charge current l _charge flowing into the inductor 120 and the cell 200, and a de-magnetising current I _charge flowing from the transformer 110 into the energy storing component 160.

The forward converter switch 150 is configured to alternately switch on and off. Thereby, the forward converter circuit 100b is switched between two different operation states. Fig. 5a illustrates what happens when the forward converter switch 150 is closed, and Fig. 5b illustrates what happens when the forward converter switch 150 is open.

Fig. 5a illustrates what happens when the forward converter switch 150 is switched on, i.e. closed. The forward converter is now said to be in powering mode. When the forward converter switch 150 is closed, current can flow from the reversible DC source 300 to the secondary winding 112, whereby the transformer is magnetised. The first diode 140 is immediately biased by the voltage induced in the first winding of the transformer 110. In other words, both the windings 111 , 112 of the transformer 110 start conducting simultaneously. The output at the secondary side (first end) of the transformer depends upon the turn ratio (N:1 ) of the transformer 110. This output voltage is applied to the first end, which basically consists of L-C filter. Hence, the current in the first winding 112 causes charge current I _cfiar g _e ^to flow from the first end winding 111 to the cell 20, whereby the cell 200 is charged. The charge current I _cfiarge discharges the energy storing component 160, but as the energy storing component 160 is an infinite capacitor there is no voltage drop. Hence, the cell 200 forms a load of the transformer 110 and the energy storing component 160 and consumes a charge current I _charge , which is a rising current due to an initially high resistance in the inductor 120.

Current drawn from the reversible DC source 300, corresponds to a sum of the charge current I _cfiarge and a less significant magnetising current I _mag required to establish a magnetic field in the transformer 110. The charging voltage of the cell 200 comprises the voltage over the energy storing component 160 plus the voltage from the voltage source 300 transformed voltage across the transformer 110 stepped down by a transformer turn ratio of N ₁₁₂ /N _{lll t} where N is the number of windings. In other words, the forward converter circuit 100b is arranged such that the energy storing component 160 is discharged by the current flowing from the first end winding 111 to the cell 200, upon the forward converter switch 150 being switched on.

Fig. 5b illustrates what happens when the forward converter switch 150 is switched off, i.e. open. The forward converter is now said to be in freewheeling mode. When the forward converter switch 150 is open current flow from the reversible DC source 300 to the second end winding 112 is immediately blocked. Hence, no current can flow in the circuit at the second end 102. The sudden current change causes the voltage over the transformer 110 to reverse, whereby the first diode 140 immediately closes (or switches on), i.e. current path is closed and current can flow through. The closed first diode 140 provides a path that enables a continuous current flow to the cell 200 to be maintained, as the inductor 120 mitigates the current change by dropping voltage in the polarity necessary to oppose the change. Hence, the cell 200 is now charged by a decaying current from the inductor 120. In other words, the inductor 120 acts as a low pass filter that maintains the current flow to the cell 200 also when the forward converter switch 150 is open. The first diode 140 also provides a path for removal of energy stored in the transformer 110. Hence, a decaying demagnetising current flows out of the lower side (no dot) of the first winding 111 and into the energy storing component 160 and circulate via the first diode 140. Thus, the energy storing component 160 is charged by the demagnetising current of the transformer 110, upon the forward converter switch 150 being switched off. A key feature here is that the transformer 110 is allowed to demagnetise via a current path that recharges the energy storing component 160 via the first diode 140. Stored energy in the inductor 120 and the energy storing component 160 slowly dissipates into the load. Before it dissipates completely the again the forward converter switch 150 is switched on again to end the freewheeling mode and to maintain the magnitude of load voltage within the required tolerance band. In some embodiments, the forward converter switch 150 is controlled by pulse width modulated signal. For example, the boost switch 130 (Fig. 4a-b) and the forward converter switch 150 are controlled by a closed loop control system designed to control the output voltage or current, based on for example a target maximum and minimum cell voltage. The closed loop control system is for example implemented by the control arrangement 500.

As an alternative to using an infinite capacitor the energy storing component 160 is in some embodiments a capacitor configured to resonantly commutate the boost switch 130. Depending on the size, e.g. capacitance, of the energy storing component and the switching frequency of the boost switch 130, the reversible DC source could be used (or tuned) to resonantly turn-off the boost switch 130. This is not a new concept, resonant switching to achieve either zero-current or zero-voltage is commonly used in current converters.

In some embodiments, the boost switch 130 and the first diode 140 are implemented as one electrical component. For example, the switch is a mosfet transistor and the first diode is a body diode of the transistor. The same could be applied to the forward converter switch 150 and the current blocking component 170.

Simulation have been performed for various states of charges. Fig 6a shows example current in ampere (y-axis, upper diagram) and voltage in volts (y-axis, lower diagram) of a 161 Ah cell during discharging with an example implementation of the bidirectional power converter, at a medium state of charge with a 2.7 volts open circuit voltage. Fig 6b shows example current in ampere (y-axis, upper diagram) and voltage in volts (y-axis, lower diagram) of a 161 Ah cell during charging with an example implementation of the bi-directional power converter, at a medium state of charge with a 2.7 volts open circuit voltage. The proposed technique has been described with reference to lithium-ion cells, but it should be appreciated that the proposed technique may be used to test other types of cells including cells made from solid state materials, such as graphene. Such cells are expected to be more commonly used in the future. The technique may be used for any type of cells such as prismatic cells or pouch cells etc.

The terminology used in the description of the embodiments as illustrated in the accompanying drawings is not intended to be limiting of the described method, control arrangement or computer program. Various changes, substitutions and/or alterations may be made, without departing from disclosure embodiments as defined by the appended claims.

The term “or” as used herein, is to be interpreted as a mathematical OR, i.e. , as an inclusive disjunction; not as a mathematical exclusive OR (XOR), unless expressly stated otherwise. In addition, the singular forms "a", "an" and "the" are to be interpreted as “at least one”, thus also possibly comprising a plurality of entities of the same kind, unless expressly stated otherwise. It will be further understood that the terms "includes", "comprises", "including" and/ or "comprising", specifies the presence of stated features, actions, integers, steps, operations, elements, and/ or components, but do not preclude the presence or addition of one or more other features, actions, integers, steps, operations, elements, components, and/ or groups thereof. A single unit such as e.g. a processor may fulfil the functions of several items recited in the claims.