Circuits and methods for measuring cell voltages in battery packs

A circuit used to measure cell voltages in a battery pack can include a cell voltage level shifter, a sense block, and a compensation current generator. The cell voltage level shifter selects a cell and shifts the terminal voltages of the selected cell from a first voltage level to a second voltage level. The sense block monitors the current consumed by the level shifter, and generates a signal indicative of the consumed current. The compensation current generator generates compensation currents to compensate the current consumed by the level shifter. Therefore, unbalance of the cell capacities caused by the current consumed by the level shifter can be reduced or eliminated, and thus the overall capacity of the battery pack can be improved.

BACKGROUND ART

Batteries can be used in various applications, such as electric vehicles and hybrid electric vehicles. The workable voltage of a single cell in a battery may be approximately 2˜4 volts, but some systems such as the electric vehicles and the hybrid electric vehicles may require higher voltages, e.g., 40 volts. Multiple cells can be coupled to each other in series to drive the electric vehicles and the hybrid electric vehicles.

In battery management, the status of cells, such as cell voltages, may be detected and measured by a measurement block, e.g., an analog-to-digital converter (ADC). The ADC can be implemented by devices having relatively low voltages. A voltage level shifter can be coupled between the cells and the ADC to shift the terminal voltages of each cell to lower voltages, e.g., from 40 volts to 2 volts, and the ADC detects and measures the cell voltages according to the shifted voltages.

FIG. 1shows a conventional cell voltage detection circuit100. The circuit100includes a battery pack110, a cell voltage level shifter120, and a detection and measurement block140. The battery pack110includes battery cells111-115as shown in the example ofFIG. 1. The level shifter120selects a battery cell according to a cell selection signal180, and shifts the terminal voltages of the selected cell to lower voltages. In such way, the detection and measurement block140can measure the cell voltages for the cells111-115, and output the measured results accordingly.

The level shifter120consumes current from the battery pack110. By way of example, when the cell112is selected, a current IVH1flows from the node H1which is the positive terminal of the cell112, through the level shifter120, the cells115-112, and back to the node H1. Moreover, a current IVL1flows from the node L1which is the negative terminal of the cell112, through the level shifter120, the cells115-113, and back to the node L1. Therefore, the capacities of the cells113-115are degraded by the currents IVH1and IVL1, while the capacity of the cell112is degraded by the current IVH1. Similarly, the level shifter120consumes current when measuring cell voltages for other cells, e.g., cells111and113-115. As a result, the capacities of the cells111-115are unbalanced and the cell115located at the bottom of the battery110may have less capacity compared to other cells111-114, thus affecting the available capacity of the battery pack110.

SUMMARY

In one embodiment, a circuit used to measure cell voltages in a battery pack includes a cell voltage level shifter, a sense block, and a compensation current generator. The cell voltage level shifter selects a cell and shifts the terminal voltages of the selected cell from a first voltage level to a second voltage level. The sense block monitors the current consumed by the level shifter, and generates a signal indicative of the consumed current. The compensation current generator generates a compensation current to compensate the current consumed by the level shifter. Therefore, unbalance of the cell capacities caused by the current consumed by the level shifter can be reduced or eliminated, and thus the overall capacity of the battery pack can be improved.

DETAILED DESCRIPTION

Furthermore, in the following detailed description of embodiments of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the embodiments of the present invention.

Embodiments in accordance with the present invention provide detection or measurement circuits for a battery pack. A detection circuit can include a level shifter. Advantageously, the detection circuit can monitor the current consumed by the level shifter and generate a corresponding compensation current to compensate the current consumed by the level shifter. As a result, unbalance of the cell capacities caused by the current consumed by the level shifter can be reduced or eliminated, and thus the overall capacity of the battery pack can be improved. The battery described in the present invention can be, but is not limited to, a Lithium Ion battery or a Lead Acid battery. Although the invention is described in relation to a battery, the invention is not so limited. For example, the invention may also be used in solar cell applications.

FIG. 2illustrates a block diagram of a circuit200for measuring cell voltages of a battery pack, in accordance with one embodiment of the present invention. In the example ofFIG. 2, the circuit200includes a battery pack210, a cell voltage level shifter220, a detection and measurement block240, a sense block250, and a compensation current generator260. In the example ofFIG. 2, the battery pack210includes cells211-215for illustration purposes but not limitation. The battery pack210can include some other number of battery cells.

The level shifter220is coupled to each cell in the battery pack210and can select a cell according to a cell selection signal280and shift the terminal voltages of the selected cell from a first voltage level to a second voltage level. In one embodiment, the first voltage level is higher than the second voltage level. In one embodiment, one cell is selected at each time. The detection and measurement block240receives the shifted voltages and measures the cell voltages of the cells211-215.

When the level shifter220selects a cell to shift the terminal voltages of the selected cell, the level shifter220may consume currents from the terminals of the selected cell. The sense block250can monitor a current flowing through a terminal of the selected cell and can generate a sense signal251indicative of the current flowing from the terminal of the selected cell to the level shifter220. For example, the sense block250can monitor a current IVH2flowing from the terminal H2of the cell212to the level shifter220and/or a current IVL2flowing from the terminal L2of the cell212to the level shifter220. In one embodiment, the sense signal251is a sense current. The compensation current generator260receives the sense signal251and generates a compensation current ICOMPaccording to the sense signal251. For example, the compensation current generator260can generate the compensation current ICOMPby mirroring and/or scaling the signal251.

Advantageously, the compensation current ICOMPcan flow through the battery pack210to compensate the current consumed by the level shifter220. Therefore, the unbalance of the cell capacities caused by the level shifter220can be reduced or avoided, and thus the lifetime of the battery pack210can be improved.

FIG. 3shows a schematic diagram of a circuit300for measuring cell voltages of a battery pack, in accordance with one embodiment of the present invention.FIG. 3is described in combination withFIG. 2.

In the example ofFIG. 3, the level shifter220can include switches321-330. The switches321-330can be controlled by a cell selection signal280. In addition, the level shifter220includes two proportional amplifiers. One proportional amplifier includes resistors332and334and operational amplifier335, and the other proportional amplifier includes operational amplifier336. Moreover, the level shifter220further includes resistors331and333. The switches321,322,324,326and328are coupled to the positive terminal of the operational amplifier335via the resistor331. The switches323,325,327,329and330are coupled to the negative terminal of the operational amplifier335through the resistor332. In one embodiment, the resistors331and332are identical, and the resistors333and334are identical. The level shifter220can shift the terminal voltages of a selected cell to voltages VOUTPand VOUTN. The shifted voltages VOUTPand VOUTNare the outputs of the amplifier335and the amplifier336, respectively.

By way of example, when the switches322and325are on, and the switches321,323-324and326-330are off, the battery cell212is selected and the terminal voltages VH2and VL2of the cell212are received or sensed by the level shifter220. The level shifter220shifts the terminal voltages VH2and VL2to the voltages VOUTPand VOUTNrespectively. The difference VOUTbetween VOUTPand VOUTNcan be given by:
VOUT=VOUTP−VOUTN=(VH2−VL2)×RB/RA,  (1)
where VH2-VL2is the cell voltage of the battery cell212, RBis the resistance of the resistor333and the resistance of the resistor334, RAis the resistance of the resistor331and the resistance of the resistor332, VH2is the voltage at the node H2which is the positive terminal of the cell212, and VL2is the voltage at the node L2which is the negative terminal of the cell212. The ADC370converts the voltage VOUTto a digital signal DOUT.

In the example ofFIG. 3, the sense block250includes a resistor351and a current mirror including switches352and353. The switches352and353can be N-channel metal-oxide-semiconductor field effect transistors (NMOSFETs) sinking/sourcing the sense current indicative of the current flowing from the terminal of the selected cell to the level shifter220. One terminal of the resistor351receives one terminal voltage of the selected battery cell, and the other terminal of the resistor351receives the gate-source voltage of the switch352in the current mirror. For example, the width-to-length ratio of the NMOSFET352to the width-to-length ratio of the NMOSFET353can be 1:1. However, the invention is not so limited; the width-to-length ratio of the NMOSFET352to the width-to-length ratio of the NMOSFET353can be different from 1:1. The sense block250can monitor a current flowing through a terminal of the selected cell and can generate a sense signal251indicative of the current flowing from the terminal of the selected cell to the level shifter220.

By way of example, when the switches322and325are on, and the switches321,323-324and326-330are off, the battery cell212is selected. A current IVH2flowing from the node H2(positive terminal of the cell212) through the switch322, the resistor331and resistor333can be given by:
IVH2=(VH2−VREF)/(RA+RB),  (2)
where VREFis a predetermined reference voltage. A current IVL2flowing from the node L2(negative terminal of the cell212) through the switch325and the resistors332and334can be given by:
IVL2=IVH2−(VH2−VL2)/RA.  (3)
In other words, the levels of the currents IVH2and IVL2consumed by the level shifter220are dependent on the reference voltage VREF. A current flowing through the sense block250, e.g., a bias current IB, flowing through the resistor351and the NMOSFET352can be given by:
IB≈(VH2−VGS)/RC,  (4)
where VGSis the gate-source voltage of the NMOSFET352and RCis the resistance of the resistor351. RA, RBand RCmeet the following equation:
RC=K×(RA+RB),  (5)
where K can be a constant indicating a proportional coefficient between RCand (RA+RB). Assuming that the predetermined reference voltage VREFis substantially equal to VGSand by combining the equations (2), (3) and (4), the current IVH2can be given by:
IVH2=K×IB.  (6)
Here the term “substantially equal” is used because some difference between the predetermined reference voltage VREFand the gate-source voltage VGSof the NMOSFET352is permitted; however, that difference is small enough to be ignored. Advantageously, the sense block250can sense the current IVH2and can generate a sensing current INindicative of the current IVH2to the compensation current generator260. In one embodiment, the sensing current INcan be equal to the current IB. Thus, the following equation can be obtained:
IN=IVH2/K.(7)

In the example ofFIG. 3, the compensation current generator260can be a current mirror including P-channel metal-oxide-semiconductor field effect transistors (PMOSFETs)361,362and363sinking/sourcing the sense current and the compensation current. The width-to-length ratios of the PMOSFETs361,362and363can have the proportion 1:(K+1):K. In one embodiment, the compensation current generator260receives the sensing current INfrom the sense block250, and generates compensation currents ICN3and ICP3flowing into the battery pack210to compensate the current consumed by the level shifter220.

By way of example, when the switches322and325are on, and the switches321,323-324and326-330are off, the battery cell212is selected. The sensing current INflows through the PMOSFET361. Thus, a current ICP3flowing through the PMOSFET362, the switch322and the cells212-215can be given by:
ICP3=(K+1)×IN=IVH2+IB.  (8)
The current ICN3flowing through the PMOSFET363, the switch325and the cells213-215can be given by:
ICN3=K×IN=IVH2.  (9)
Advantageously, the compensation current ICP3, which is substantially equal to IVH2+IB, flows through the cells212-215in the opposite direction of the currents IVH2and IB. Here the term “substantially equal” is used because some difference between the compensation current ICP3and the current IVH2+IBis permitted; however, that difference is small enough to be ignored. In other words, the currents IVH2and IBcan be compensated by the compensation current ICP3. Therefore, the capacity degradation of the cells212-215caused by the current IVH2can be reduced or eliminated by the compensation current ICP3. Moreover, the compensation current ICN3(given by equation (9)) flows through the cells213-215in the opposite direction of the current IVL2. In one embodiment, the difference between the compensation current ICN3and the current IVL2is small enough to be ignored and can be given by:
ICN3D=ICN3−IVL2=IVH2−IVL2=(VH2−VL2)/RA.  (10)
Therefore, the capacity degradation of the cells213-215caused by the current IVL2can be reduced by the compensation current ICN3.

Similarly, when the cell211,213or214is selected, the sense block250can sense the current consumed by the level shifter220and the compensation current generator260can generate a compensation current to compensate the current consumed by the level shifter220. In one embodiment, when the cell215is selected, the level shifter220does not consume current from the negative terminal of the cell215(ground), and thus no compensation current flows into the negative terminal of the cell215and the current flowing from the PMOSFET363can flow into ground. The current flowing from the PMOSFET362compensates the consumed current flowing from the positive terminal of the cell215.

Furthermore, since the impedance at each cell terminal is relatively low, and the impedances at the outputs of the current mirrors (e.g., nodes A, B, C and D) and the inputs of the level shifter (e.g., nodes C and D) are relatively high, the sense block250and the compensation current generator260can still maintain relatively high accuracy even if there are some slight differences between the current consumed by the level shifter220and the compensation current generated by the compensation current generator260.

FIG. 4shows a schematic diagram of a circuit400for measuring cell voltages of a battery pack, in accordance with another embodiment of the present invention.FIG. 4is described in combination withFIG. 3.

In the example ofFIG. 4, besides the current mirror including the PMOSFETs361,362and363, the compensation current generator260can further include a resistor466having the resistance RBand a voltage follower including an operational amplifier464and an NMOSFET465. In one embodiment, when the switches322and325are on, and the switches321,323-324and326-330are off, the current ICP3compensates the current IVH2+IBin a similar way as in the example inFIG. 3, while a current ICN4flowing through the switch325and the cells213-215compensates the current IVL2. The current ICN4can be given by:
ICN4=ICN3−ICELL,  (11)
where ICELLcan be a current flowing through the NMOSFET465and the resistor466, in one embodiment. By combining the equation (1), the current ICELLcan be given by:
ICELL=(VOUTP−VOUTN)/RB=(VH2−VL2)/RA.  (12)
Thus, by combining the equations (3), (9), (11) and (12), the following equation can be obtained:
ICN4=IVH2−(VH2−VL2)/RA=IVL2.  (13)
Advantageously, the current IVL2can be compensated by the current ICN4having the same level as the current IVL2. Therefore, the capacity degradation of the cells213-215caused by the current IVL2can be reduced or eliminated by the compensation current ICN4.

FIG. 5shows a schematic diagram of a circuit500for measuring cell voltages of a battery pack, in accordance with another embodiment of the present invention.FIG. 5is described in combination withFIG. 4.

In the example ofFIG. 5, the resistors331and333and an amplifier536included in the level shifter220can serve as the sense block250to generate a current IOindicative of the current consumed by the level shifter220. In one embodiment, the current IOcan meet the following equation:
IO=IVH2/K.(14)
The compensation current generator260can include PMOSFETs561,562and563. In one embodiment, the width-to-length ratios of the PMOSFETs561,562and563can meet the proportion 1:K:K. Therefore, both compensation currents ICP5flowing through the PMOSFET562and ICN5flowing through the PMOSFET563are equal to IVH2. In such way, the current ICP5compensates the current IVH2, and the current ICN5-ICELLcompensates the current IVL2. Advantageously, the circuit in the example ofFIG. 5can compensate the consumed currents without dependence on the gate-source voltage of the NMOSFET352in the example ofFIG. 4. Moreover, the circuit in the example ofFIG. 5removes the resistor351in the example ofFIG. 4, and thus further reduces the cost of the circuit.

FIG. 6shows an embodiment of an operational amplifier600which can be used as the operational amplifier536shown inFIG. 5.FIG. 6is described in combination withFIG. 5. In the example ofFIG. 6, the width-to-length ratio of a PMOSFET606to the width-to-length ratio of a PMOSFET609can be 1:1. However, the invention is not so limited; the width-to-length ratio of the PMOSFET606to the width-to-length ratio of the PMOSFET609can be different from 1:1. The width-to-length ratio of a NMOSFET607to the width-to-length ratio of a NMOSFET610can be K:1, and the width-to-length ratio of a NMOSFET608to the width-to-length ratio of a NMOSFET611can also be K:1.

The amplifier600can be used as the amplifier536inFIG. 5and provide a current IO1. In one embodiment, since the total resistance of the resistor331and the resistor333in the example ofFIG. 5is relatively low compared to the on resistance of the PMOSFETs606and609, the difference between the current flowing through the resistor331and the resistor333and the current flowing through the NMOSFETs607and608can be ignored. In other words, the current flowing through the NMOSFETs607and608is approximately equal to IVH2. Accordingly, the current IO1can be approximately equal to IVH2/K.

FIG. 7shows another embodiment of an operational amplifier700which can be used as the operational amplifier536shown inFIG. 5.FIG. 7is described in combination withFIG. 5andFIG. 6.

Compared to the amplifier600inFIG. 6, the PMOSFETs606and609inFIG. 6are removed from the amplifier700inFIG. 7. Therefore, the current flowing through the resistors331and333flows through the NMOSFETs607and608. Advantageously, the current IO2generated by the amplifier700can follow the current IVH2with the proportion 1/K more accurately.

FIG. 8illustrates a flowchart of a method800for measuring cell voltages of a battery pack.FIG. 8is described in combination withFIG. 2. Although specific steps are disclosed inFIG. 8, such steps are examples. That is, the present invention is well suited to performing various other steps or variations of the steps recited inFIG. 8.

At step812, a cell in a battery pack is selected to detect or measure the cell voltage of the selected cell. For example, the level shifter220selects a cell in the battery pack210according to a cell selection signal. At step814, the terminal voltages of the selected cell are shifted from a first voltage level to a second voltage level. At step816, the second voltage level is detected and measured.

At step818, the current consumed during the shifting is monitored, and a sense signal indicative of the consumed current is generated. In one embodiment, the sense signal is proportional to the current consumed. At step820, the compensation current flowing through the battery pack210to compensate the current consumed is generated according to the sense signal. In one embodiment, the compensation current is substantially equal to the current consumed. Here the term “substantially equal” is used because some difference between the compensation current and the current consumed is permitted; however, that difference is small enough to be ignored. Generally speaking, any amount of compensation of the current consumed is beneficial; ideally, the compensation current matches the current consumed. In one embodiment, the compensation current is generated further according to the second voltage level of the terminal voltage of the selected cell. Advantageously, the compensation current can have substantially the same level as the current consumed by the level shifter220. Therefore, unbalance of the cell capacities caused by the current consumed by the level shifter can be reduced or eliminated, and thus the overall capacity of the battery pack can be improved.