Active cell balancing

A battery cell balancing system includes N switching circuits connected to first terminals and second terminals of N battery cells, respectively, where N is an integer greater than one. A first transformer includes a first core and N sets of windings wound around the first core. The N sets of windings are connected to the N switching circuits, respectively. A first control module controls switching of the N switching circuits to reverse first connections between the first terminals and the second terminals of the N battery cells and the N sets of windings, respectively, at a first frequency to balance charge levels of the N battery cells.

FIELD

The present disclosure relates to battery systems with multiple battery cells, and more particularly to a cell balancing system for battery systems with multiple battery cells.

BACKGROUND

Electric vehicles and hybrid vehicles typically include a battery system and an electric motor that are used to propel the vehicle. The battery systems typically include a plurality of battery cells that are connected together. During operation of the vehicle, the battery cells are discharged and may be recharged by a utility and/or during operation using regenerative braking.

Battery systems for electric vehicles may cost on the order of $5000 or more, which makes the battery system one of the most expensive components of the vehicle. Given the cost of replacement, the service life of the battery system should be as long as possible. Battery life for most types of battery cells is related to the number of charge/discharge cycles and the depth of discharge. For example only, for Lithium Ion (LiIon) battery cells, the estimated number of cycles is equal to Cycles=20·106·DOD−2, where DOD is depth of discharge as a percentage. When operating a LiIon battery cell between 100% and 15%, the estimated number of cycles is 2768. When operating a LiIon battery cell between 100% and 20%, the estimated number of cycles is 3125, which is approximately 11% greater cycle life as compared to discharging to 15%. Thus, controlling the battery discharge level can impact service life.

Control systems may set a target discharge level to a desired value such as 20% (rather than a lower level such as 15%) to receive the benefit of the longer service life. The battery system will need to be replaced when one of the battery cells in the battery system reaches the end of its service life. If the control system is not accurate in maintaining the desired discharge level for each battery cell, one or more of the battery cells may be regularly discharged to a lower state of charge (i.e. below 20%) than others of the battery cells, which will prematurely end the service life of the battery system. The remaining service life of the other battery cells that were not discharged below the target discharge level will be wasted.

SUMMARY

A battery cell balancing system includes N switching circuits connected to first terminals and second terminals of N battery cells, respectively, where N is an integer greater than one. A first transformer includes a first core and N sets of windings wound around the first core, wherein the N sets of windings are connected to the N switching circuits, respectively. A first control module controls switching of the N switching circuits to reverse first connections between the first terminals and the second terminals of the N battery cells and the N sets of windings, respectively, at a first frequency to equalize charge levels of the N battery cells.

In other features, the N battery cells are connected in series. Each of the N switching circuits includes a first switch and a second switch connected in series between the first terminal and the second terminal of one of the N battery cells and a third switch and a fourth switch connected in series between the first terminal and the second terminal of the one of the N battery cells. A first terminal of one of the N sets of windings is connected between the first switch and the second switch. A second terminal of the one of the N sets of windings is connected between the third switch and the fourth switch.

In other features, the first control module switches the first switch and the fourth switch on and the second switch and the third switch off during one portion of a switching period. The first control module switches the first switch and the fourth switch off and the second switch and the third switch on during another portion of the switching period.

A battery system comprises M battery subgroups, wherein M is an integer greater than one. Each of the M battery subgroups includes the battery cell balancing system and the N battery cells.

In other features, M switching circuits are connected to a first terminal and a second terminal of M battery subgroups, respectively. A second transformer includes a second core and M sets of windings wound around the second core. The M sets of windings are connected to the M switching circuits, respectively. A second control module controls switching of the M switching circuits to reverse second connections between the first terminal and the second terminal of the M battery subgroups and the M sets of windings, respectively, at a second frequency to equalize charge levels of the M battery subgroups.

In other features, the first frequency is equal to the second frequency. Each of the M switching circuits includes a first switch and a second switch connected in series between the first terminal and the second terminal of one of the M battery subgroups and a third switch and a fourth switch connected in series between the first terminal and the second terminal of the one of the M battery subgroups. A first terminal of one of the M sets of windings is connected between the first switch and the second switch and a second terminal of the one of the M sets of windings is connected between the third switch and the fourth switch.

In other features, the second control module switches the first switch and the fourth switch on and the second switch and the third switch off during one portion of a switching period. The control module switches the first switch and the fourth switch off and the second switch and the third switch on during another portion of the switching period.

In other features, the first transformer includes N printed circuit boards each including a substrate defining a central opening. A conductive trace is formed on the substrate and is arranged around the central opening. The first core includes a first “E”-shaped core section and a second “E”-shaped core section. Middle legs of the first “E”-shaped core section and the second “E”-shaped core section are inserted into the central openings of the N printed circuit boards. Outer legs of the first “E”-shaped core section and the second “E”-shaped core section are arranged outside of the N printed circuit boards.

In other features, the second transformer includes M printed circuit boards each including a substrate defining a central opening. A conductive trace is formed on the substrate and is arranged around the central opening. The second core includes a first “E”-shaped core section and a second “E”-shaped core section. Middle legs of the first “E”-shaped core section and the second “E”-shaped core section are inserted into the central openings of the M printed circuit boards and outer legs of the first “E”-shaped core section and the second “E”-shaped core section are arranged outside of the M printed circuit boards.

A method of balancing battery cells in a battery subgroup includes connecting N switching circuits to first and second terminals of N battery cells, respectively, where N is an integer greater than one; connecting N sets of windings of a first transformer having a first core to the N switching circuits, respectively; and switching the N switching circuits to reverse first connections between the first terminal and the second terminal of the N battery cells and the N sets of windings, respectively, at a first frequency to equalize charge levels of the N battery cells.

In other features, the N battery cells are connected in series. Each of the N switching circuits includes a first switch and a second switch connected in series between the first terminal and the second terminal of one of the N battery cells and a third switch and a fourth switch connected in series between the first terminal and the second terminal of the one of the N battery cells. A first terminal of one of the N sets of windings is connected between the first switch and the second switch and a second terminal of the one of the N sets of windings is connected between the third switch and the fourth switch.

In other features, the switching of the N switching circuits includes switching the first switch and the fourth switch on and the second switch and the third switch off during one portion of a switching period and switching the first switch and the fourth switch off and the second switch and the third switch on during another portion of the switching period.

In other features, the method comprises providing M of the battery subgroups, where M is an integer greater than one; connecting M switching circuits to a first terminal and a second terminal of the M battery subgroups, respectively; connecting M sets of windings of a second transformer having a second core to the M switching circuits, respectively; and switching the M switching circuits to reverse second connections between the first terminal and the second terminal of the M battery subgroups and the M sets of windings, respectively, at a second frequency to equalize charge levels of the M battery subgroups.

In other features, the method includes switching the first connections at a first frequency and the second connections at a second frequency. The first frequency is equal to the second frequency.

DESCRIPTION

A cell balancing system according to the present disclosure balances the charge levels of the battery cells in a battery system during charging, charging during regeneration, discharging, quiescence and/or storage. By accurately performing cell balancing, the cell balancing system ensures that all of the battery cells will achieve the desired number of cycles during their service life.

According to the present disclosure, cell balancing is achieved in part by connecting the battery cells in parallel to a transformer. More particularly, sub-groups of battery cells are switchably connected together using the transformer, such as a high coupling coefficient, multi-winding transformer. The sub-groups are also switchably connected in parallel to another transformer to perform balancing between the sub-groups.

Referring now toFIG. 1, an example vehicle10is shown. The vehicle includes a powertrain control module16and a battery system22including a plurality of battery cells24. A cell balancing module28according to the present disclosure balances the battery cells in the battery system22during charging, charging during regeneration, discharging, quiescence and/or storage. The battery system22supplies power to an electric motor30, which drives one or more wheels32of the vehicle10. While an electric vehicle is shown, the cell balancing system of the present disclosure may be applied to a variety of other applications such as hybrid vehicles and/or other non-automotive applications.

Referring now toFIG. 2, a first cell balancing system110for a battery system including one or more sub-groups102of battery cells104-1,104-2, . . . , and104-N (collectively battery cells104) is shown, where N is an integer greater than 1. The battery cells104are connected in series.

For example, the switching circuit114-1includes series-connected switches126-1and128-1that are connected in parallel with series-connected switches130-1and132-1. A second terminal of the switch126-1and a first terminal of the switch128-1are connected to one input of a transformer116(via one end of windings118-1wound around a core120). A second terminal of the switch130-1and a first terminal of the switch132-1are connected to another input of the transformer116(via another end of windings118-1wound around the core120).

First terminals of the switches126-1and130-1are connected to a first battery terminal of a corresponding one of the battery cells104. For example, the first battery terminal may be a positive battery terminal of the battery cell104-1. Second terminals of the switches128-1and132-1are connected to a second battery terminal of a corresponding one of the battery cells104. For example, the second battery terminal may be a negative battery terminal of the battery cell104-1.

In use, the switching circuits114switch the polarity of connection of the battery cell104to the transformer116at a first desired frequency, which equalizes the charge of the battery cells104in the sub-group102.

Referring now toFIG. 3, a second cell balancing system140for plurality of sub-groups (SGs)102of battery cells is shown. In this example, there are M sub-groups of battery cells, where M is an integer greater than one.

For example, the switching circuit142-1includes series-connected switches146-1and148-1that are connected in parallel with series-connected switches150-1and152-1. A second terminal of the switch146-1and a first terminal of the switch148-1are connected to one input of a transformer156(via one end of windings158-1wound around a core160). A second terminal of the switch150-1and a first terminal of the switch152-1are connected to another input of the transformer156(via another end of windings158-1wound around the core160).

First terminals of the switches146-1and150-1are connected to a first terminal of a corresponding one of the sub-groups102. For example, the first battery terminal may be a positive terminal of the sub-group102-1. Second terminals of the switches148-1and152-1are connected to a second terminal of a corresponding one of the sub-groups104. For example, the second terminal may be a negative terminal of the sub-group102-1.

In use, the switching circuits142switch the polarity of the connection from the sub-group104to the transformer156at a second desired frequency, which equalizes the charge of the sub-groups102. The second desired frequency can be the same as or different than the first desired frequency.

In some examples, the switches can be n-channel or p-channel MOSFET transistors, although other transistors may be used. In some examples, each of the transistors is arranged in an isolation pocket that is insulated from remaining portions of the integrated circuit. In some examples, the control module and the switching circuits for one or more sub-groups are implemented as an integrated circuit or an integrated circuit package.

Referring now toFIGS. 4 and 5, example timing diagrams for the switching circuits114for the battery cells102and for the switching circuit142for the sub-groups102, respectively, are shown. InFIG. 4, during a first half period, switches126and132are ON and switches128and130are OFF. During a second half period, switches128and130are ON and switches126and132are OFF. InFIG. 5, during a first half period, switches146and152are ON and switches148and150are OFF. During a second half period, switches148and150are ON and switches146and152are OFF.

As can be appreciated, while synchronous switching of the switching circuits for the battery cells inFIG. 4and for the subgroups inFIG. 5is shown, the switching does not need to be synchronous. Asynchonous switching of the battery cells inFIG. 4relative to the subgroups inFIG. 5can be performed. In some examples, the switching may be performed at a desired frequency. For example, the frequency can be in the range of 50 kHz to 150 kHz. For example, the desired frequency can be 100 kHz or another suitable frequency. In some examples, the switching of the battery cells is performed at a different frequency than switching of the sub-groups. In some examples, the switching circuit142is switched at an integer multiple or divisor of the switching frequency of the switching circuit114.

Referring now toFIG. 6, an example of a suitable transformer is shown. The transformer190includes a plurality of printed circuit boards (PCBs)200-1,200-2, . . . , and200-N (collectively PCBs200). Each of the PCBs200includes one or more windings204(such as winding204-1shown inFIG. 6). The windings204may include traces formed on the PCBs200. The PCBs200also include a central opening210. A core212may include first and second core sections214A and214B.

In some examples, the first and second core sections214A and214B may have an “E”-shaped cross section, although other cores may be used. Each of the center legs of the first and second core sections214A and214B may be inserted approximately half-way into the central opening210of the PCBs200. Outer legs of the first and second core sections214A and214B are arranged adjacent to opposite sides of the PCBs200. In some examples, the PCBs214may have a surface area of 1 to 2 cm2and the core212may be made of Ferrite or another suitable core material. While a single winding is shown, multiple windings may be used. In some examples, the windings may be arranged on both sides of the PCBs.

The cell balancing system according to the present disclosure has relatively high electrical efficiency and topological and computational simplicity. There is no accurate measurement requirement required to implement the cell balancing system. Nonetheless, current and/or voltage balancing may be performed at the battery cell level and/or the sub-group level. The current and/or voltage feedback can be used to adjust the switching frequency or other parameters if needed. Furthermore, the cell balancing system according to the present disclosure has minimal reactive energy storage, either inductive or capacitive.

Referring now toFIG. 6, drive signals for the switches may be generated by a circuit including a signal generator such as an oscillator250and a driver circuit252that generates the drive signals for switches254based on an output frequency of the signal generator250. Alternately, the drive signals for the switches can be generated by a controller such as an existing vehicle controller. Examples of suitable existing controllers include a powertrain controller, a battery controller, an engine controller or another similar controller.