Battery voltage detector having pull-up resistor

1. Field of the Invention

The present invention relates to a battery voltage detector.

Priority is claimed on Japanese Patent Application No. 2011-075305, filed Mar. 30, 2011, the content of which is incorporated herein by reference.

2. Description of the Related Art

As known, a vehicle, such as an electric car or a hybrid car, is provided with a motor, and a high-voltage and high-capacity battery that supplies power to the motor. The battery includes multiple battery cells connected in series. The battery cell includes a lithium-ion battery, a nickel-metal hydride battery, or the like. In related art, a voltage balance control of monitoring and equalizing cell voltages of the battery cells is performed to maintain the performance of the battery.

It is mainstream to use a flying-capacitor-type voltage detection circuit in order to detect a cell voltage because dedicated insulating power or insulating elements are unnecessary for the flying-capacitor-type voltage detection circuit. The flying-capacitor-type voltage detection circuit has a demerit in that a cell voltage of a battery cell to be measured is detected lower than a true value if a leak current of a flying capacitor or a sampling switch increases due to time degradation. Consequently, a precise cell voltage cannot be obtained, thereby causing a decrease in the precision of cell balance control in some cases.

To overcome the above demerit of the flying-capacitor-type voltage detection circuit, Japanese Patent Laid-Open Publication No. 2002-291167 discloses technique of detecting a cell voltage of each battery cell to be measured in different timing using a flying-capacitor-type voltage detection circuit, and estimating a voltage immediately after the start of charging the flying capacitor (i.e., a cell voltage regarded as a true value), based on the voltage attenuation characteristics of the cell voltage obtained as a result of the detection.

The detection value of the cell voltage, which is obtained from the flying-capacitor-type voltage detection circuit, includes not only errors caused by the aforementioned increase in leak current due to time degradation of the flying capacitor and each switch, but also drift errors caused by variations with time of resistance values of an input resistance of the flying capacitor, an on-resistance and an input circuit resistance of each switch, and the like. The drift errors are factors that cause the cell voltage to be detected to be lower than the true value.

The present invention has been made in view of the above situations. An object of the present invention is to provide a battery voltage detector that reduces drift errors caused by variations with time of resistance values of various resistances included in the cell voltage detection circuit, thereby enhancing precision of detecting a cell voltage.

To solve the above problem, a battery voltage detector according to one aspect of the present invention includes, but is not limited to: a voltage detection circuit; and a voltage processor. The voltage detection circuit includes, but is not limited to: a capacitor configured to be charged by a battery cell; a pair of output terminals; an output switch; and a voltage processor. While the capacitor is charged, the output switch is configured to be off-state and insulate the capacitor from the pair of the output terminals. After the capacitor is charged, the output switch is configured to be on-state and connect the capacitor to the pair of the output terminals. The voltage processor is configured to obtain, as a cell voltage, a voltage between the output terminals of the voltage detection circuit while the output switch is on-state. A high-potential output terminal of the pair of the output terminals is connected to a power line via a pull-up resistor.

The above features and advantages of the present invention will be more apparent from the following description of certain preferred embodiments taken in conjunction with the accompanying drawings, in which:

FIG. 1 schematically illustrates a configuration of a battery voltage detector 1 according to an embodiment of the present invention;

FIG. 2A illustrates a circuit configuration of the cell voltage detection circuit D 1 when SW 1 is on-state and SW 2 is off-state;

FIG. 2B illustrates a circuit configuration of the cell voltage detection circuit D 1 when SW 1 is off-state and SW 2 is on-state; and

FIG. 3 is a timing chart illustrating operation of the battery voltage detector 1 .

The present invention will now be described herein with reference to illustrative embodiments. The accompanying drawings explain a battery voltage detector. The size, the thickness, and the like of each illustrated portion might be different from those of each portion of an actual battery voltage detector.

Those skilled in the art will recognize that many alternative embodiments can be accomplished using the teachings of the present invention and that the present invention is not limited to the embodiments illustrated herein for explanatory purposes.

Hereinafter, an embodiment of the present invention is explained with reference to the drawings.

FIG. 1 schematically illustrates a battery voltage detector 1 according to the present embodiment. As shown in FIG. 1 , the battery voltage detector 1 is an ECU (Electronic Control Unit) that detects cell voltages of twelve battery cells C 1 to C 12 included in a battery and performs voltage balance control on the battery cells C 1 to C 12 (equalization of the cell voltages). The battery voltage detector 1 includes: twelve bypass circuits B 1 to B 12 ; twelve cell voltage detection circuits D 1 to D 12 ; a microcomputer (voltage processor) M; and an insulating element IR.

Each of the bypass circuits B 1 to B 12 includes a bypass resistor and a switching element which are connected in series. The switching element includes a transistor or the like. The bypass circuits B 1 to B 12 are connected in parallel to the battery cells C 1 to C 12 , respectively. In FIG. 1 , reference symbols R 1 to R 12 denote bypass resistors included in the bypass circuits B 1 to B 12 , respectively. Reference symbols T 1 to T 12 denote switching elements included in the bypass circuits B 1 to B 12 , respectively.

The cell voltage detection circuits D 1 to D 12 are provided respectively for the battery cells C 1 to C 12 connected in series, which are called single-flying-capacitor-type voltage detection circuits. FIGS. 2A and 2B illustrate circuit configurations of the cell voltage detection circuit D 1 that detects a cell voltage of the battery cell C 1 . The other cell voltage detection circuits D 2 to D 12 have the same configuration as that of the cell voltage detection circuit D 1 . Therefore, only the cell voltage detection circuit D 1 is explained in detail, and explanations of the other cell voltage detection circuits D 2 to D 12 are omitted, here.

In FIGS. 2A and 2B , reference symbol Pi 1 denotes a first input terminal connected to a positive electrode terminal of the battery cell C 1 . Reference symbol Pi 2 denotes a second input terminal connected to a negative electrode terminal of the battery cell C 1 . Reference symbol Po 1 denotes a first output terminal connected to an input port of the microcomputer M (i.e., a port connected to an A/D (analog to digital) convertor). Reference symbol Po 2 is a second output terminal connected to a common potential line (for example, a ground line SG) in the battery voltage detector 1 . The first input terminal Pi 1 and the second input terminal Pi 2 correspond to a pair of input terminals according to the present embodiment. The first output terminal Po 1 and the second output terminal Po 2 correspond to a pair of output terminals according to the present embodiment.

Reference symbol FC is a flying capacitor connected between a high potential line L 1 and a low potential line L 2 . The high potential line L 1 connects the first input terminal Pi 1 and the first output terminal Po 1 . The low potential line L 2 connects the second input terminal Pi 2 and the second output terminal Po 2 . Reference symbol SW 1 is an input switch inserted on the left side of the flying capacitor FC connected between the high potential line L 1 and the low potential line L 2 .

Specifically, the input switch SW 1 includes a first input switch SW and a second input switch SW 1 b . The first input switch SW 1 a is inserted onto the high potential line L 1 on the side of the first input terminal Pi 1 with respect to the flying capacitor FC. The second input switch SW 1 b is inserted onto the low potential line L 2 on the side of the second input terminal Pi 2 with respect to the flying capacitor FC. Additionally, an input resistor Ra is inserted onto the high potential line L 1 between the first input terminal Pi 1 and the first input switch SW 1 a . An input resistor Rb is inserted onto the low potential line L 2 between the second input terminal Pi 2 and the second input switch SW 1 b.

The input switch SW 1 turns on while the flying capacitor FC is charged, and connects the flying capacitor FC to the input terminals (i.e., to the battery cell C 1 ). Then, the input switch SW 1 turns off after the flying capacitor FC is charged, and insulates the flying capacitor FC from the input terminals (i.e., from the battery cell C 1 ).

Reference symbol SW 2 is an output switch inserted on the right side of the flying capacitor FC connected between the high potential line L 1 and the low potential line L 2 . Specifically, the output switch SW 2 includes a first output switch SW 2 a and a second output switch SW 2 b . The first output switch SW 2 a is inserted onto the high potential line L 1 on the side of the first output terminal Po 1 with respect to the flying capacitor FC. The second output switch SW 2 b is inserted onto the low potential line L 2 on the side of the second output terminal Po 2 with respect to the flying capacitor FC.

The output switch SW 2 turns off while the flying capacitor FC is charged, and insulates the flying capacitor FC from the output terminals (i.e., from the microcomputer M). Then, the output switch SW 2 turns on after the flying capacitor FC is charged, and connects the flying capacitor FC to output terminals (i.e., to the microcomputer M).

A high-potential output terminal of the pair of output terminals (the first output terminals Po 1 and the second output terminal Po 2 ) included in the cell voltage detection circuit D 1 , which is the first output terminal Po 1 , is connected to a power line (for example, a 5V Vcc line) in the battery voltage detector 1 via a pull-up resistor Rp. Although not shown, the power line is connected to a reference power source that generates a stable reference voltage (Vcc) to be a reference for circuit operation. Thus, the first output terminal Po 1 is connected to the power line via the pull-up resistor Rp between the cell voltage detection circuit D 1 and the microcomputer M.

With reference back to FIG. 1 , the microcomputer M is a microcomputer including a memory such as a ROM, a RAM, or the like, a CPU (Central Processing Unit), an A/D (analog to digital) conversion circuit, an input/output interface, and the like.

The microcomputer M obtains, as a cell voltage, a voltage between the output terminals of each of the cell voltage detection circuits D 1 to D 12 (i.e., a voltage between the first output terminal Po 1 and the second output terminal Po 2 ) while the output switch SW 2 of each of the cell voltage detection circuits D 1 to D 12 is on-state. Then, the microcomputer M stores, in an internal memory (for example, a RAM), data indicating a relationship between an identification number and the cell voltage of each of the battery cells C 1 to C 12 , as results of the cell voltage detection.

Additionally, the microcomputer M can communicate with the battery ECU 2 that is an upper controller, via the insulating element IR. The microcomputer M transmits, to the battery ECU 2 , the results of the cell voltage detection stored in the internal memory. The battery ECU 2 monitors the states of the battery cells C 1 to C 12 (the balance states of the cell voltages) based on the results of the cell voltage detection received from the microcomputer M. If the battery ECU 2 finds a battery cell that is higher in cell voltage than the other battery cells, the battery ECU 2 specifies that battery cell as a cell requiring discharge (discharge requiring cell), and transmits the result of the specification to the microcomputer M.

Upon receiving the result of specifying the discharge requiring cell from the battery ECU 2 , the microcomputer M controls the switching element of the bypass circuit connected to the discharge requiring cell (i.e., the cell balance control).

Hereinafter, operation of the battery voltage detector 1 having the aforementioned configuration (cell voltage detection operation) is explained in detail with reference to FIGS. 2A , 2 B, and 3 . For simplification of explanations, only the cell voltage detection circuit D 1 that detects a cell voltage of the battery cell C 1 is explained. Operations of the other cell voltage detection circuits D 2 to D 12 are similar to that of the cell voltage detection circuit D 1 , and therefore explanations thereof are omitted here.

FIG. 3 is a timing chart illustrating time relationship among a voltage V between the output terminals of the cell voltage detection circuit D 1 which is obtained by the microcomputer M, the on/off state of the input switch SW 1 (the first input switch SW 1 a and the second input switch SW 1 b ), and the on/off state of the output switch SW 2 (the first output switch SW 2 a and the second output switch SW 2 b ).

As shown in FIG. 3 , charging of the flying capacitor FC with a voltage detection period Td is assumed to start at time t 1 . The input switch SW 1 (SW 1 a and SW 1 b ) of the cell voltage detection circuit D 1 becomes on-state at the time t 1 . On the other hand, the output switch SW 2 (SW 2 a and SW 2 b ) becomes off-state at the time t 1 . Thereby, the battery cell C 1 starts the charging of the flying capacitor FC (see FIG. 2A ).

Then, the input switch SW 1 becomes off-state at time t 2 which is a given time after the time t 1 . The given time is a duration that is enough for the flying capacitor FC to be sufficiently charged. The flying capacitor FC is insulated from the output terminal (the first output terminal Po 1 and the second output terminal Po 2 ) for the charging duration from the time t 1 to the time t 2 (i.e., while the output switch SW 2 is off-state).

After the flying capacitor FC is charged, the output switch SW 2 is on-state from time t 3 to time t 4 (the input switch SW 1 is off-state). The flying capacitor FC is electrically connected to the output terminal (the first output terminal Po 1 and the second output terminal Po 2 ) for the duration from the time t 3 to the time t 4 (i.e., while the output switch SW 2 is on-state). For the duration from the time t 3 to the time t 4 , the microcomputer M can obtain the voltage V between the output terminals of the cell voltage detection circuit D 1 (i.e., the voltage between the terminals of the flying capacitor FC).

At this time, the first output terminal Po 1 is connected to the power line (Vcc line) via the pull-up resistor Rp. For this reason, the flying capacitor FC is recharged by the reference voltage Vcc (5V), and thereby the voltage between the terminals of the flying capacitor FC becomes slightly higher (for example, approximately +1 mV) than the voltage value immediately after being charged by the battery cell C 1 (see FIG. 2B ).

For the duration from the time t 3 to the time t 4 (while the output switch SW 2 is on-state), the microcomputer M obtains the voltage V between the output terminals of the cell voltage detection circuit D 1 , as a cell voltage V_FC of the battery cell C 1 . Then, the microcomputer M causes the A/D conversion circuit to convert the cell voltage V_FC into digital data which can be processed by the CPU. Then, the microcomputer M stores in the internal memory (for example, a RAM), the digital data as the result of the cell voltage detection for the voltage cell C 1 .

The cell voltage V_FC finally obtained from the cell voltage detection circuit D 1 in this manner is higher than the true value of the cell voltage (the voltage between the terminals of the flying capacitor FC immediately after the charging by the battery cell C 1 ). In other words, even if the voltage between the terminals of the flying capacitor FC immediately after the charging by the battery cell C 1 decreases due to drift errors caused by variations with time of resistance values and becomes lower than the true value, the decreased amount of voltage is cancelled when the voltage between the terminals of the flying capacitor FC (the voltage V between output terminals) is obtained by the microcomputer M, thereby obtaining the cell voltage V_FC close to the true value.

The battery voltage detector 1 repeatedly performs the series of operation explained above with the voltage detection period Td, thereby obtaining results of the cell detection for the respective cells C 1 to C 12 . Then, the battery voltage detector 1 transmits the results of the cell voltage detection to the battery ECU 2 . Upon receiving the result of specifying a discharge requiring cell from the battery ECU 2 , the battery voltage detector 1 performs cell balance control. Here, the on/off states of the input switch SW 1 and the output switch SW 2 may be controlled by the microcomputer M or another control circuit provided separately.

As explained above, according to the present embodiment, the drift errors caused by variations with time of resistance values of an input resistance of the flying capacitor, an on-resistance and an input circuit resistance of each switch, and the like can be reduced with a simple configuration. Consequently, a cell voltage close to the true value can be precisely detected. Additionally, the amount of current from the power line Vcc can be limited by setting a resistance value of the pull-up resistor Rp. Therefore, the resistance value of the pull-up resistor Rp is set so as not to affect sampling of a cell voltage for the sampling duration, thereby confining the small amount of current in the flying capacitor FC.

It is apparent that the present invention is not limited to the above embodiments, and may be modified and changed without departing from the scope and spirit of the invention. For example, it has been explained in the first embodiment that the battery voltage detector 1 performs cell voltage detection with respect to the twelve battery cells C 1 to C 12 . However, the number of the battery cells C 1 to C 12 is not limited to twelve.

Additionally, it has been explained in the first embodiment that the twelve cell voltage detection circuits D 1 to D 12 are provided respectively for the battery cells C 1 to C 12 . However, only one cell voltage detection circuit may be provided. In this case, both terminals of each of the battery cells C 1 to C 12 may be sequentially connected, using a multiplexor or the like, to the input terminals (the first input terminal Pi 1 and the second input terminal Pi 2 ) of each of the cell voltage detection circuits, thereby sequentially detecting a cell voltage of each of the battery cells C 1 to C 12 . If a multiplexor is used, the input switch SW 1 is unnecessary.

As used herein, the following directional terms “left” and “right” as well as any other similar directional terms refer to those directions of an apparatus equipped with the present invention. Accordingly, these terms, as utilized to describe the present invention should be interpreted relative to an apparatus equipped with the present invention.

The term “configured” is used to describe a component, section or part of a device includes hardware and/or software that is constructed and/or programmed to carry out the desired function.

The terms of degree such as “substantially,” “about,” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. For example, these terms can be construed as including a deviation of at least ±5 percent of the modified term if this deviation would not negate the meaning of the word it modifies.