WO1995030905A1

WO1995030905A1 - Monitory system for a battery

Info

Publication number: WO1995030905A1
Application number: PCT/FI1995/000241
Authority: WO
Inventors: Teuvo Suntio
Original assignee: Finlandia Interface Oy
Priority date: 1994-05-09
Filing date: 1995-05-08
Publication date: 1995-11-16
Also published as: FI942142A; FI99170C; FI99170B; FI942142A0

Abstract

The invention relates to a battery monitoring system comprising a battery and a monitoring device (L), the battery in the system comprising at least four voltage units (B1-B4) connected in series, each unit comprising a terminal with a higher potential and a terminal with a lower potential, an electric coupling (S) being arranged at the terminals, and electric conductors (J) being connected to the electric couplings (S) of each voltage unit, the conductors being connected in electric contact with the monitoring device (L) for monitoring the voltages provided by the voltage units (B1-B4). The monitoring device (L) comprises, for monitoring the four voltage units (B1-B4) connected sequentially in series, a first difference voltage measuring member for measuring the difference voltage of the voltages across two adjacent voltage units (B1, B2), a second difference voltage measuring member for measuring the difference voltage of the voltages across the other two adjacent voltage units (B3, B4), a third difference voltage measuring member for measuring the difference voltage between the sum voltage across the two first-mentioned adjacent voltage units (B1, B2) and the sum voltage across said other two adjacent voltage units (B3, B4), and comparing means for comparing said difference voltages with a preset alarm limit value and for providing an alarm signal when at least one difference voltage exceeds the alarm limit value.

Description

MONITORY SYSTEM FOR A BATTERY

The present invention relates to a battery monitoring system comprising a battery and a monitoring device, the battery in the system comprising at least four voltage units connected in series, each unit comprising a terminal with a higher potential and a terminal with a lower potential, electric couplings being arranged into the terminals. Electric con- ductors are connected to the electric couplings of each vol¬ tage unit, the conductors being connected in electric contact with the monitoring device to monitor the voltages provided by the voltage units.

Electrochemical units, i.e., accumulators or batteries, generally consist of cells connected in series and/or in parallel. As sources of electric energy, the batteries generally use, e.g., galvanic cells and fuel cells, both of which convert chemical energy into electric energy. The output voltage provided by each cell depends on the cell type and the materials used inside it. A voltage produced by an electrochemical element is generally 1-1.5 V. A maximum voltage provided by a lead cell can be up to 2.75 V, however. Voltages provided by alkaline cells, such as nickel-iron cells, nickel-cadmium cells and silver-zinc cells are considerably lower than those of a lead cell, i.e., about 1.3 V. In contrast, the voltage provided by a phosphoric fuel cell is only c. 0.6 V.

In order to produce an electrochemical element, such as a battery providing the required voltage, several cells are connected in series. A short circuit or a disturbance in a single cell, or a fault in the contact between two cells can cause disturbances in the battery. Such disturbances decrease the efficacy of the battery and may cause disturbances in other cells. Therefore, it is important to recognize and localize such disturbances as soon as possible. For instance, when the battery becomes old, the impedance level of one of the cells can vary causing local heating in the cell in question, which may damage the cell permanently.

In order to utilize the battery to its maximum and to obtain a reliable voltage supply from the battery, all kinds of rechargeable batteries, both conventional and so-called maintenance-free batteries, i.e., valve-controlled batteries, must be monitored. The state of the battery's capacity is estimated with the aid of monitoring to detect faults. The monitoring methods can be divided into manual and automatic ones.

The manual methods include, e.g., the measuring of cell voltages, cell densities and water consumption and the recording of the same at regular intervals. This manual method can be applied in conventional batteries which enable the measuring of the cell density and water consumption. In contrast, the method cannot be applied in valve-controlled, i.e., so-called maintenance-free batteries because the electrolyte in them is not in liquid form and they cannot be refilled. Testing the capacities of batteries have also been effected manually.

In the automatic method the battery is continuously connected to some kind of a monitoring device so that the condition of the battery is determined automatically by comparing, e.g., the cell or battery voltages and the discharging current with the cell- or battery-specified discharge values during the discharging. However, current automatic monitoring devices require a lot of programming and accurate information on the battery to be monitored and, therefore, are generally fairly expensive. The automatic monitoring method using a monitoring device can be applied in all types of batteries.

The automatic monitoring device should be able to localize and indicate a faulty unit which can comprise one cell, a few or several cells connected in series when the capacity of the unit decreases under a certain level. The monitoring device should operate without disturbing the load and it should be possible to carry out the monitoring without detaching the battery from the load.

The solution of US-patent 3 925 771 is known, as the prior art, for checking the voltage in a circuit comprising two voltage sources connected in series. In the solution disclosed in the patent, the voltage of each voltage source is monitored by dividing the voltage across each voltage source by a resistance divider and by monitoring the divided voltages by transistors and detectors. In the device according to the patent the monitoring is thus implemented by arranging respective components for each voltage source to monitor the voltage across each voltage source.

US-patent 4 424 4912 discloses an automatic detector of voltage unbalance in which a defective one of the cells connected in series is localized by measuring the difference voltages between the cells. One solution of ti patent is that first, the voltage across each cell is measured and after that the voltages of adjacent cells are compared with one another to form the difference voltage of the two adjacent cells. Another solution disclosed in the patent is that the sum voltage across each of the two adjacent cells and the voltage in the centre between them are measured and these voltages are compared for forming the difference voltage between the voltages. Thus the solution disclosed in US-patent 4 424 491 also carries out at least one measurement for each cell and, consequently, the monitoring device comprises at least as many measuring component units as there are cells to be monitored.

Publication DE-3146 141 discloses a battery monitoring device in which the difference voltage between each two adjacent voltage units (cells) is measured and this monitoring device thus also comprises as many measuring units as there are voltage units. Publication WO-86/02738 relates to a battery monitoring arrangement which measures the difference voltages of cell branches connected in parallel. Each branch comprises at least two cells connected in series and the publication discloses an example of 56 cells connected in series. The monitoring device presented in the publication relates to the measurement of difference voltages of parallel voltage units (branches) , and the publication does not handle the monitoring of units connected in series. In order to monitor the parallel units, the difference voltage between each parallel unit is measured in the solution of the publication so that it results in as many difference voltages as there are parallel voltage units in the battery. Thus it requires four difference voltage measurements to be able to localize a faulty branch of the four units (branches) connected in parallel. The monitoring device according to the publication detects the faulty parallel branch but it is not able to distinguish the faulty one of the 56 cells connected in series in the branch.

Consequently, in all prior art solutions the voltage across each unit has been measured to detect a faulty voltage unit, i.e., a faulty battery cell or a faulty voltage unit consisting of a few or several cells connected in series, and the voltages have been compared with one another, or difference voltages between all the adjacent cells or partial blocks have been measured, and these have been compared with one another. A disadvantage of the solutions of the prior art is that a lot of calculating must be done and the monitoring device contains a lot of electronics because voltage measurements are carried out across each unit.

The purpose of the present invention is to provide a monitoring device and a method for quick detection of a fault in a battery, and to localize the damaged battery cell and carry out the monitoring by using a smaller amount of calcula¬ tion and electronics than earlier. The monitoring device according to the invention is implemented to monitor four voltage units connected in series. The monitoring device thus detects the damaged battery cell among four battery cells connected in series. The method according to the invention and the monitoring device according to the invention can be used to detect a fault and to localize the faulty unit by using three measurements. Each measurement is an difference voltage measurement. First, the difference voltage between two adjacent units in measured in the invention and, in addition, the difference voltage between the other two adjacent units neither of which was measured during the first measurement. Third, a measurement is carried out in which the difference voltage between the sum voltage of the units related to the first measuring and the sum voltage of the units related to the second measuring is measured.

The invention is characterized in that, for monitoring four voltage units connected sequentially in series, the monitoring device comprises: - a first difference voltage measuring member for measuring the difference voltage of the voltages across two adjacent voltage units, a second difference voltage measuring member for measuring the difference voltage of the voltages across• the other two adjacent voltage units, a third difference voltage measuring member for measuring the difference voltage between the sum voltage across the two first-mentioned adjacent voltage units and the sum voltage across the other two said adjacent voltage units, and comparing means for comparing said difference voltages with a preset alarm limit value and for giving an alarm signal when at least one difference voltage exceeds the alarm limit value.

The invention is considered in detail in the following with reference to the appended drawings in which: Fig. 1 presents a block diagram of the measuring arrangement according to the invention,

Fig. 2A presents by a block diagram an example of the first measurement carried out by the invention, Fig. 2B presents by a block diagram an example of the second measurement carried out by the invention, and Fig. 2C presents by a block diagram an example of the third measurement carried out by the invention.

Fig. 1 presents the basic principle of the monitoring carried out according to the invention. Referring to Fig. 1, monitoring device according to the invention is used to monitor four voltage units Bl - B4. connected in series which can consist either of one battery cell or several battery cells connected in series and/or in parallel. Line J communicating with the monitoring device is connected to both terminals S (the higher and the lower potential terminal) of each voltage unit, i.e., to electrodes S. The solution measures the three difference voltages so that

the first measurement gives the difference voltage of the voltages across two adjacent units, e.g., difference voltage ΔU_1>2 of the voltages across units Bl and B2, the second measurement provides the difference voltage of the voltages across the other two adjacent units which were not included in the first measurement, i.e., difference voltage ΔU₃₎₄ of the voltages across units B3 and B4, and the third measurement provides the difference voltage between the sum voltage of the units related to the first measurement and that of the sum voltage related to the second measurement, i.e., difference voltage ΔU_1+2ι3+4 of the voltages across blocks B1+B2 and blocks B3+B4.

Fig. 2A presents an exemplary coupling for carrying out the first measuring. In Fig. 2A, two adjacent units are chosen for the first measuring: units Bl and B2. The measuring of the difference voltage can be effected in any known method. In Fig. 2A the difference voltage is formed in the corresponding manner as presented in US-patent 4 424 491, i.e., voltages V_N.j and V_N+1 are taken to the inverting input of operation amplifier OP1 via resistances R_u and R₁₂. Voltage V_N between units Bl and B2 is taken directly to the non-inverting input of operational amplifier 0P1, and this middle voltage acts as a reference voltage which voltages V_N__! and V_N+J are compared to. The output of the operational amplifier is a value of the comparison between the voltages brought to the inverting and non-inverting inputs of the operational amplifier, whereby any difference voltage ΔU_1>2 is obtained as an output. Thereafter, absolute value |ΔU_1;2| is taken from difference voltage ΔU_lj2 obtained as the output of the operational amplifier and it is compared to, i.e., alarm limit value ΔU_H preset by comparator CO Pl, whereby an alarm is obtained from a faulty cell if the alarm limit is exceeded. The output signal provided by comparator COMPl thus serves as the alarm signal, whereby alarm limit ΔU_H is exceeded when the output signal of comparator COMPl is of a certain size.

Figs. 2B and 2C present exemplary couplings for effecting the second and the third measuring which are implemented in a manner corresponding to the first measuring, as can be seen in the figures. Thus in Fig. 2B, voltages V_N.j and V_N+, are taken, correspondingly, to the inverting input of operational amplifier OP2 via resistances R₂₁ and R₂₂. Voltage V_N between units B3 and B4 is taken directly to the non-inverting input of operational amplifier OP2 and this middle voltage serves as a reference voltage which voltages V_N.j and V_N+1 are compared with. The output of the operational amplifier is a value of the comparison between the voltages brought to the inverting and non-inverting inputs of the operational amplifier, whereby any difference voltage ΔU_3>4 is obtained as an output. Thereafter, absolute value |ΔU₃₄| is taken from difference voltage ΔU_3;4 obtained as the output of the operational amplifier and it is compared with alarm limit value ΔU_H preset by comparator COMP2, for example, whereby an alarm is obtained of a faulty cell if the alarm limit is exceeded. Thus the output signal provided by comparator COMP2 serves as the alarm signal, whereby alarm limit ΔU_H is exceeded when the output signal of comparator C0MP2 is of a certain size. Corre¬ spondingly, in Fig. 2C voltages V_N__! and V_N+ι are taken to the inverting input of operational amplifier OP3 via resistances R₃₁ and R₃₂. Voltage V_N between units B2 and B3 is taken directly to the non-inverting input of operational amplifier 0P3 and this middle voltage serves as a reference voltage, which voltages V_N.j and V_N+1 are compared with. The value of the operational amplifier is a value of the comparison between the voltages brought to the inverting and non-inverting inputs of the operational amplifier, whereby any difference voltage

ΔU_1+2;3+4 is obtained as the output. Thereafter, absolute value

|ΔUι_+2>3+4| is taken from difference voltage ΔUι_+2j3+4 obtained as the output of the operational amplifier and it is compared with alarm limit value ΔU_H preset by comparator C0MP3, for example, whereby an alarm of a faulty cell is obtained if the alarm limit is exceeded. Thus the output signal provided by comparator C0MP3 serves as the alarm signal, whereby alarm limit ΔU_H is exceeded when the output signal of comparator C0MP3 is of a certain size.

Consequently, if even one of the absolute values |ΔU₁₂|, |ΔU₃₄| or |ΔU_1+2)3+4| of the three measured difference voltages exceeds the set alarm limit value, an alarm is obtained of a faulty cell, which can be localized on the basis of the difference voltage values. The sensitivity of the measuring can be adjusted by making the amplifications of operational amplifiers 0P1, 0P2, and 0P3 adjustable. Generally, the amplification of operational amplifiers 0P1, 0P2, and 0P3 is 2, whereby real difference voltages are obtained as the outputs of the operational amplifiers. If the amplification is 1, the output voltage of the operational amplifier is only half of the real difference voltage when using the offset measuring shown in Figs. 2A, 2B, and 2C. This is due to the coupling arrangement presented in the figures for measuring the difference voltage, which is described in US-patent 4 424 491. However, the invention is not limited to this forming method of difference voltages but other methods for forming the difference voltage between two voltages formed in series can be used within the appended claims. Correspondingly, the sensitivity to alarms can be adjusted by changing alarm limit value ΔU_H, whereby when the value is decreased, alarms are obtained more frequently. The user of the monitoring device may adjust alarm limit value ΔU_H as he monitors the given alarms and notices that part of the alarms are not necessary, whereby when adjusting alarm limit value ΔU_H higher, alarms are not given as easily and those unnecessary alarms can be avoided which are perhaps only caused by momentary voltage variations.

The present invention can be used to quickly detect a fault in a battery and to localize the faulty battery cell and effect the monitoring of the battery by using a smaller amount of calculation and electronics. Furthermore, the sensitivity to alarms of the device is easy to adjust and, consequently, it can be adapted to suit each user in accordance with the requirements and situation.

Claims

1. A monitoring system for a battery, comprising a battery and a monitoring device (L) , the battery of the system comprising at least four voltage units (B1-B4) connected in series, each of the units comprising a terminal with a higher potential and a terminal with a lower potential, an electric coupling (S) being connected to the terminals, and electric conductors (J) being connected to the electric couplings (S) of each voltage unit, the conductors being connected in electric contact with the monitoring device (L) to monitor the voltages provided by the voltage units (B1-B4) , characterized in that the monitoring device (L) comprises, in order to monitor four voltage units (B1-B4) connected sequentially in series: - a first difference voltage measuring member (0P1, R_π, R₁₂) for measuring the difference voltage (ΔU_1>2) of the voltages across two adjacent voltage units (Bl, B2) ,

- a second difference voltage measuring member (0P2, R₂,, R₂₂) for measuring the difference voltage (ΔU_3;4) of the voltages across the other two adjacent voltage units (B3, B4) ,

- a third difference voltage measuring device (OP3, R₃₁, R₃₂) for measuring the difference voltage (ΔUι_+2>3+4) between the sum voltage across the two first-mentioned, adjacent voltage units

(Bl, B2) and the sum voltage across the said other two adjacent voltage units (B3, B4) , and

- comparing means (COMPl, COMP2, COMP3) for comparing said difference voltages (ΔUι₂, ΔU₃₄, ΔUι₊₂₃₊₄) with a preset alarm limit value (ΔU_H) and for providing an alarm signal when at least one difference voltage (ΔU₁₂, ΔU₃₄ or ΔU,_+2>3+4) exceeds the alarm limit value (ΔU_H) .

2. A monitoring system according to Claim 1, characterized in that the monitoring device (L) comprises means (ABS1, ABS2, ABS3) for forming the absolute values of said difference voltages (ΔU₁₂, ΔU₃₄, ΔU₁₊₂₃₊₄) and for feeding the formed absolute values to said comparing means (COMPl, C0MP2, C0MP3) for effecting said comparison.

3. A monitoring system according to Claim 2, characterized in that the monitoring device (L) comprises, for effecting each of said measurements, a respective monitoring unit (LI, L2, L3) , whereby each of said difference voltage measuring members (0P1, R_n, R₁₂; 0P2, R₂₁, R₂₂; OP3, R₃₁, R₃₂) are included in the respective monitoring units (LI, L2, L3) each of which further comprises respective comparing means (COMPl, COMP2, COMP3) and respective means for forming the respective absolute values (ABS1, ABS2, ABS3) .

4. A monitoring device according to Claim 1, characterized in that each difference voltage measuring member (0P1, R_π, R₁₂; 0P2, R₂₁, R₂₂; 0P3, R_3l , R₃₂) comprises, for measuring the difference voltage of two voltages formed in series: - means (R_π, R₁₂; R₂ι, R₂₂; R₃₁, R₃₂) for summing the two voltages formed in series into a higher (V_N+1) and a lower (V_N.j) voltage point for forming a sum voltage, and

- means (0P1; OP2; OP3) for comparing said sum voltage with a middle voltage obtained from between said two voltages formed in series, for forming the difference voltage between the sum voltage and the middle voltage.

5. A monitoring device according to Claim 1, characterized in that said means (OP1; OP2; OP3) for forming the difference voltage comprise an operational amplifier (OP1; OP2; OP3) whose amplification is adjustable.

6. A monitoring system according to Claim 1, characterized in that each voltage unit (B1-B4) comprises at least one electro- chemical cell.