WO2009085658A2

WO2009085658A2 - Integrated cell voltage measurement and equalization system for series-connected batteries and method for operating same

Info

Publication number: WO2009085658A2
Application number: PCT/US2008/086584
Authority: WO
Inventors: Thomas A. Stuart
Original assignee: University Of Toledo
Priority date: 2007-12-31
Filing date: 2008-12-12
Publication date: 2009-07-09
Also published as: WO2009085658A3

Abstract

A battery pack management system comprises a power supply adapted to be connected to a battery pack. The battery pack includes a plurality of cells, where each cell is interconnected in a series arrangement to other cells in the battery pack. Each cell has a cell charge characteristic and a voltage potential characteristic. An electronic control unit (ECU) measures the voltage potential characteristic of each cell. An equalizing circuit (EQU) equalizes the voltage of each cell individually relative to the other cell voltages in the battery pack. A common harness connects the plurality of cells both to the electronic control unit (ECU) and to the equalizing circuit (EQU).

Description

INTEGRATED CELL VOLTAGE MEASUREMENT AND

EQUALIZATION SYSTEM FOR SERIES-CONNECTED BATTERIES AND

METHOD FOR OPERATING SAME

STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001 ] This invention was made with Government support under research grant N62306-07-P-7035 and N62306-07-P-9005 awarded by the U.S. Department of the Navy. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0002] Rechargeable batteries come in many forms and are generally well-known in the art. These batteries may be small or large, have single or multiple cells, and have multiple cells connected in parallel or series configurations. Many large, rechargeable battery packs contain numerous cells that are connected in series. When certain types of these large battery packs, such as, for example, lithium-ion batteries, are recharged, monitoring of various cell characteristics is necessary in order to provide improved functional life and safe operating conditions. This monitoring process is necessary because slight differences in construction between the various cells will cause their respective voltages to drift apart as the battery pack is charged and discharged. The slight differences between the cells can produce large imbalances in the cell voltages after several charge/discharge cycles. Additionally, the relative voltage variations between cells tend to increase as the battery pack ages and/or cells are replaced.

[0003] Battery pack charging is typically limited by the highest voltage cell, while battery pack discharging is limited by the lowest voltage cell. Thus, the battery pack capacity will be determined by the voltage or charge characteristics of these two cells. In order to monitor the charge state of these various series-connected cells, various circuits are used to measure the cell voltages and temperatures and to monitor the relative voltage adjustments required between the cells.

[0004] In many battery systems, an electronic battery management system (BMS) is employed to measure and/or control the voltage, temperature, and charge differences between cells. The BMS may include an electronic control unit (ECU) to control the various parameter measurements and make operational determinations. The BMS may also include an equalizer circuit (EQU) which is used to adjust, or equalize, each of the cell voltages individually relative to the other cell voltages in the battery pack. A common problem with these BMS systems is that the ECU and the EQU must have separate wiring harnesses that connect to the cells, even though the connection points are identical. The separate harnesses are used to prevent the current in the EQU wires from causing errors in the ECU voltage measurements. [0005] The adjustment, or equalization, process is conducted by the EQU under instructions from the ECU, after measurements and comparisons of the various cell voltages have been made. The equalization process essentially means keeping all cells at the same voltage while the pack is in operation. Most EQUs operate by using a small bleed resistor for each cell to reduce its voltage until all cell voltages equal the lowest one in the pack. Another fairly simple method that can be used for some types of large batteries, e.g., lead acid or Nickel Metal Hydride (NiMH), is to trickle charge the battery at a low current until all cells reach full charge. However, this means that some cells will receive at least a slight overcharge.

[0006] Some types of batteries, such as lithium ion (Li-ion), cannot be trickle charged since this would slightly overcharge some cells and cause these cells to ignite. These cells, therefore, must be diligently monitored by the BMS to determine the state of charge prior to initiating and during a recharging operation sequence. This ignition problem means that the EQU for volatile battery cell chemistries, such as Li-ion or lithium sulfide (LiS), must use an alternate method to equalize the various cell voltages. Some alternate methods include a charge transfer method that is effected between cells or an individual cell equalization method which is applied to a single cell. [0007] Charge transfer methods involve transferring an excess differential charge from a higher charged cell to a lower charge cell in order to balance the battery pack. Often times, there is an intermediary capacitor to absorb and transfer the charge differential from one cell to the next. Charge transfer methods provide an appealing concept, but they tend to be complex and rather inefficient for low voltage cells, such as Li-ion. These methods may be advantageous for batteries that do not need to measure the individual cell voltages, but such measurements should not be omitted for large Li-ion packs due to the need to avoid cell overcharge. [0008] Individual cell equalization can be used to adjust cell charge values. Many of these systems use a discharge EQU that consists of a small resistor for each cell in order to discharge all cells to the lowest cell voltage in the pack. Although effective, discharge EQUs can produce excessive charge dissipation, especially if one cell voltage tends to lag the others. Another problem is the excessive time required to achieve equalization since the EQU current should be kept fairly low.

SUMMARY OF THE INVENTION

[0009] There is provided herein a charging circuit having an electronic control unit, an equalization circuit, and a charge/discharge circuit connected to a plurality of series-connected battery cells by way of a wiring harness and a plurality of relays. The equalization circuit includes a single charge/discharge circuit, and is selectively connected to any of a plurality of cells via a set of relays. The equalization circuit and the electronic control unit use portions of the same wiring harness. A cell discharge transistor and a cooperating transistor- switched resistor selectively couple the charge/discharge circuit to the plurality of series-connected battery cells. [0010] Various aspects of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[001 1 ] Fig. IA is a schematic view of an embodiment of the battery management system including a relay circuit.

[0012] Fig. IB is an enlarged, schematic view of the relay circuits of Fig. IA. [0013] Fig. 2 is a schematic view of a voltage measurement and control circuit of a battery management system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT [0014] Fig. IA illustrates an embodiment of a targeted charge/discharge equalization (targeted EQU) circuit, shown generally at 100. The targeted EQU circuit 100 includes a battery pack, shown generally at 130. Though shown as having six battery cells 131-136, the battery pack 130 may have any number of cells or cell sets. The targeted EQU 100 further includes a plurality of relay actuation circuits 141-146, shown generally at 140. The relay actuation circuits 141-146 are connected to the corresponding battery cells 131-136, through a common harness, shown generally at 110, in order to selectively couple the particular cell requiring equalization to a charge circuit, shown generally at 150, or a discharge circuit, shown generally at 160, as further explained below. The common harness 110 includes a plurality of wires 101-107 that form a part of the selective connections between each of the battery cells 131-136 and the respective battery management circuits utilized in monitoring or altering individual cell charge conditions.

[0015] Fig. IB is a schematic view of each of the relay actuation circuits 141- 146 which have power supply inputs 111-116 and control points 121-126. The relay actuation circuits 141-146 are similar in construction and will be described in terms of relay circuit 141. The relay actuation circuit 141 includes an electromagnetic coil 14 IA, a freewheeling diode 14 IB and a transistor 141C. The relay actuation circuit 141 further includes the power supply input 111 and the control point 121. The control point 121 is connected to a microprocessor, shown generally at 120 in Fig. 2. The electromagnetic coil 141 A provides the actuating force to cause the mechanical contacts of the relay actuation circuit 141 to open and close, thus breaking or completing the selected circuit. The freewheeling diode 14 IB prevents the stored energy in the energized relay actuation circuit 141 from damaging the transistor 141C. The transistor 141C is connected to the microprocessor 120 at the control point 121.

[0016] The microprocessor 120, shown in Fig. IA, controls the operation of the plurality of relay actuation circuits 140, the charge circuit 150, and the discharge circuit 160. In operation, the relay actuation circuit 141 is actuated by energizing the transistor 141C through the control point 121. The charge circuit 150 is connected to each selected battery cell; for example, battery cell 131, via the energized relay actuation circuit 141.

[0017] The charge circuit 150 includes a charging unit 152 that is actuated by an optical coupler 154 that provides a protective, electrical isolation function between the charge circuit 150 and the microprocessor 120. The charging unit 152 is connected to a power supply 180 to provide a voltage and current input. Though shown as an optical coupler, the charging unit 152 may be activated by any means such as a relay, transistor, or other suitable switching element if so desired. In the embodiment shown, the charging unit 152 is activated by the optical coupler 154 through a control point 128 that is connected to the microprocessor 120. [0018] The discharge circuit 160 is likewise connected by an optical coupler 166 to a transistor 164. The transistor 164 may be any suitable electronic switch, such as for example a field effect transistor, bipolar transistor, and the like. In certain embodiments, the discharge circuit 160 can include a driver 165 operatively connected to the transistor 164 and the optical coupler 166.

[0019] The microprocessor 120 further receives voltage or charge measurement inputs from a measurement circuit, shown generally at 220, that is part of an electronic control unit 200 shown in Fig. 2. The measurement circuit 220 connects to a multiplexer 210 which receives the various voltage measurements from the battery cells 131-136 and transmits the voltage measurements individually to an analog to digital (AIO) converter 129. Though shown as part of the microprocessor 120, the A/D converter 129 may be a separate device, if so desired. The electronic control unit (ECU) circuit 200 also includes the measurement circuit 220 which is operatively connected to wires 101-107, respectively, of the common harness 110 at connection points HOA through HOG.

[0020] In operation, the voltage measurement information is sent via the common harness 110 to the ECU 200 which sets the relay actuation circuits 141-146 of the targeted EQU 100 to target any particular battery cell 131-136 that needs equalization. When the ECU 200 is activated to measure the cell voltages, the ECU 200 turns off both the cell charger 152 and the cell discharge transistor 164 (see Figs. IA and IB), thereby driving the equalization current to zero so that the current will not interfere with the voltage measurements. The voltage measurements take a fraction of a second, after which the cell charger 152 or the cell discharge transistor 164 is turned back on. Though the cell charge is the parameter that must be adjusted, typically voltage is used as the control variable. Voltage is used because charge is proportional to voltage for several types of batteries, and voltage is generally an easier parameter to measure. The ECU 200 may apply a proportionality factor to the voltage measurement to determine the charge state. Further, the ECU 200 may operate on, or respond to, one or more of: the voltage measurement, the calculated charge state, the measured charge state, or another measurement correlated to the cell charge state. [0021 ] In the embodiment shown in Fig. 1, the EQU 100 uses solid state components to activate the charge circuit 150 or the discharge circuit 160; therefore, no switching is required for the electro-mechanical portion of the relay actuation circuits 141-146 of the EQU 100 when voltage measurements are performed by the ECU 200. The solid state components provide a distinct advantage since voltage measurements taken by the ECU 200 must be measured every 5-10 sec, whereas the relay actuation circuits 141-146 of the EQU 100 only operate sporadically every few minutes, and some may not need to operate at all. The monitoring and equalization circuit described herein provides an advantage over the typical relay lifetime of 10⁷ operations of previous charging circuits which may be more than sufficient for equalization, but may not be sufficient for the much greater number of voltage measurement operations that occur every 5-10 sec. [0022] The targeted EQU cell equalization circuit 100 provides faster equalization and higher efficiency compared to circuits requiring a separate equalization wiring harness and a separate electronic control unit wiring harness. For example, a wiring harness for an 80-cell pack would normally require 162 wires to connect the electronic control unit and equalization circuits to. In contrast, the EQU 100 and the ECU 200, utilizing the common harness 110, only require 81 wires. [0023] The circuits shown in Figs. IA, IB, and 2 work together to provide a functionalized battery management system. The EQU 100 of Fig. IA equalizes the voltage across the cells 131-136 by adding or removing a small amount of charge, as required. The measurement circuit 220 in Fig. 2 measures the voltages across the cells 131-136 and the ECU 200 then directs the EQU 100 of Fig. IA which of the cells 131- 136 needs to be equalized next. The wires 101-107, comprising the common harness 110, connects the circuits of Figs. IA, IB, and 2 to the cells 131-136. The EQU 100 and the ECU 200 provide an improved equalization performance at the system level, more so than the operation of the individual parts of the circuits. These individual circuits are combined to produce a system that equalizes the cells much faster, is relatively simple, and is more reliable than previous systems. The EQU 100 provides a relatively high equalizing current for the cell being serviced. In addition, the electro-mechanical relays 141-146 only need to operate relatively infrequently, thus prolonging the relay useful life.

[0024] The battery management system of Figs. IA, IB, and 2 provides operational, cost, and packaging advantages over previous circuit constructions. The number of relay operations is much lower than for previous circuit constructions. In certain embodiments of the battery management circuit, the probability of activating any given relay is only about 18%, thereby extending the lifetime of the electromechanical relays, and requiring less current to activate the relay coils. All of the relays in Figs. IA and IB can be switched when there is no current flowing through their contacts. This switching mode extends the relay lifetime by a factor of about 100, i.e. 10,000,000 operations instead of 100,000 operations. [0025] Although modeled as ideal lossless switches, relay contacts actually have a significant amount of resistance. For any of the cells in Fig. IA the charge current only needs to pass through 2 contacts, regardless of the number of cells in the pack. This means the cell charging process will be more efficient and reliable than for other configurations. Many existing and future applications that utilize volatile chemistry batteries may benefit from improved charge management efficiencies. Large lithium ion batteries are used in several vehicular applications, for example autonomous underwater vehicles (AUVs) such as those used by the U. S. Navy and oil companies in off-shore drilling and exploring applications. The automotive industry's development efforts toward commercializing hybrid electric vehicles may use lithium- based battery systems because of their high energy density. Commercial electric vehicles, such as fork lifts, tow motor lifts, golf and utility carts, and the like are also capable of utilizing the benefits of lithium battery systems with improved charging and monitoring capabilities. Even stationary power systems and large scale power standby systems for computer networks and hospitals have applicability for lithium- based storage units and the supporting battery maintenance circuit. [0026] The principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.

Claims

CLAIMSWhat is claimed is:

1. A battery pack management system comprising: a power supply adapted to be connected to a battery pack, the battery pack including a plurality of cells, each cell being interconnected in a series arrangement to other cells, each cell having a cell charge characteristic and a voltage potential characteristic; an electronic control unit (ECU) adapted to measure the voltage potential characteristic of each cell; an equalizing circuit (EQU) adapted to equalize the voltage of each cell individually relative to the other cell voltages in the battery pack; and a common harness connecting the plurality of cells both to the electronic control unit (ECU) and to the equalizing circuit (EQU).

2. The battery pack management system according to Claim 1, wherein the equalizing circuit (EQU) is disposed between the battery and the power supply.

3. The battery pack management system according to Claim 2, wherein the equalizing circuit (EQU) includes a charge/discharge circuit and a plurality of relays for selectively connecting each of the cells to the charge/discharge circuit.

4. The battery pack management system of Claim 3, wherein the plurality of relays is connected to the common harness, each of the relays selectively connecting the respective cell to one of the equalizing circuit (EQU) or the electronic control unit (ECU).

5. The battery pack management system of Claim 1, wherein the battery is a lithium ion battery.

6. A circuit for equalizing cell voltages in a battery having a plurality of cells as shown in Fig. IA.

7. The equalizing circuit of Claim 6, wherein a plurality of relays as shown in Fig. IB are connected to the plurality of cells.

8. A circuit for equalizing cell voltages having an electronic control unit (ECU) circuit of Fig. 2.

9. A battery management system for equalizing cell voltages having an equalizing circuit (EQU) of Fig. IA and Fig. IB and an electronic control unit (ECU) circuit of Fig. 2.

10. A method for monitoring and adjusting cell charge states of a battery pack comprising the steps of: a) providing a battery pack having a plurality of cells, each of the cells being interconnected in a series arrangement to other cells, each of the cells having a cell charge characteristic and a voltage potential characteristic; b) providing a power supply for selective connection to the cells of the battery pack; c) selectively connecting at least one of an electronic control unit (ECU) and an equalization circuit (EQU) with the plurality of cells by a common harness; d) measuring at least one of the cell charge characteristic and the voltage potential characteristic of each cell with the electronic control unit (ECU); and e) altering the cell charge characteristic with the equalization circuit (EQU), such that the voltage characteristic of each cell is substantially equal to the other cell voltage characteristics in the battery pack.

11. The method of Claim 10 wherein the battery pack provided in step a) is a lithium ion battery pack

12. The method of Claim 10 wherein the selective connection of step c) made in part by a plurality of relays.

13. The method of Claim 10, wherein the cell charge characteristic of step d) is determined by measuring the voltage potential characteristic and applying a proportionality factor thereto.