WO2008033064A1

WO2008033064A1 - Method and device for automatic monitoring of battery insulation condition.

Info

Publication number: WO2008033064A1
Application number: PCT/SE2006/050330
Authority: WO
Inventors: Lennart BALGÅRD; Jimmy Kjellsson
Original assignee: Abb Technology Ltd
Priority date: 2006-09-14
Filing date: 2006-09-14
Publication date: 2008-03-20

Abstract

A device and a method for automatic monitoring of the insulation resistance of a battery of a power supply, wherein the battery comprises a plurality of battery cells, a metal enclosure and an electrical insulation between the enclosure and the battery cells. The monitoring device comprises means for measuring 1) a first voltage between the enclosure and one of the battery poles, by means of a first load circuit connected to said battery pole and to the enclosure, 2) a second voltage between the enclosure and one of the battery poles, by means of a second load circuit connected to both poles and to the enclosure, and 3) the voltage between the battery poles. The device further comprises means for switching between the two load circuits. The insulation resistance, calculated- from the measured voltages, may be compared to a preset value representing an insulation failure decision limit.

Description

Method and device for automatic monitoring of battery insulation condition.

TECHNICAL AREA

The present invention relates to an automatic battery cell insulation monitoring system and a device for monitoring the battery cell insulation status.

TECHNICAL BACKGROUND

A battery is a device to store electrical energy. The term battery is generally used to describe a single unit comprised of one or more cells and the cells are the "building blocks" of a battery. A battery can be a single cell but usually a battery is a series combination of individual cells assembled in a pack provided with internal connections, terminations, insulation and casing.

Large battery systems have a number of potential uses. Currently the most common application is backup power or uninterruptible power system (UPS) . The batteries store energy from the electric net and if there is an interruption of power on the electrical net the batteries supply the application with power until the electric power from the net can be brought back. Examples of applications that could use a backup power are; telecom systems, power utilities, computer back-up systems .

One application with increasing importance is the transport sector where large batteries are used to store energy for electric automobiles.

The individual batteries used in these applications are large and contain hundred or more cells in each enclosure. These batteries are then connected in parallel/in series or combinations thereof depending on the application.

In all these applications it is important to measure and monitor the battery voltage, battery insulation and the cell conditions .

PRIOR ART

If a battery has an isolated metal casing, measuring the isolation between the metal casing and the cells internal connection and terminals is a common diagnostic procedure.

For manual isolation measurements there are special instruments (often called "Meggers") that have internal high voltage DC supplies (typically 500V or more) . Using such an instrument on a charged battery will not give a useful result measuring insulation, because both the instrument supply and the internal voltage (which is unknown) between the battery connections and the possible fault location will be part of the measuring circuit.

Another method is to use a voltage meter with known internal resistance and measure the voltages between the casing and each one of the two terminals. From these results the two fault resistances can be calculated. This method is however not suited for an automatic monitoring system.

OBJECT AND SUMMARY OF THE INVENTION

An object of a preferred embodiment of the present invention is to provide a device and system for automatic monitoring of the condition of battery insulation. The battery comprising a plurality of battery cells electrically connected to each other and arranged inside a metal enclosure. The device and system monitors the insulation between the connected battery cells and the metal enclosure by making different voltage measurements between the battery poles and the enclosure.

According to an embodiment of the invention, the device comprises a measuring unit arranged to make a first measurement of a voltage (Ui) between the enclosure and one of the battery poles by means of a first load circuit connected to one battery pole and the enclosure, and a second measurement of a voltage (U₂) between the battery poles and the enclosure by means of a second load circuit connected to both battery poles and the enclosure.

According to an embodiment of the invention, the device comprises a measuring unit arranged to measure the voltage between the battery poles (U_b) .

According to an embodiment of the invention, the device comprises switching means adapted to switch between the load circuits for the first measurement and the second measurement.

According to an embodiment of the invention, the switching means is adapted to minimize the power consumption in the measurement circuit .

According to an embodiment of the invention, the device is adapted to calculate the resistance (R_x) over the insulation between the enclosure and the battery cells.

According to an embodiment of the invention, the device is adapted to monitor the resistance over the insulation (R_x) between the enclosure and the battery cells and generate a warning signal if the resistance over the insulation goes below a predetermined level. According to an embodiment of the invention, the device is adapted to calculate the voltage of a battery cell (U_x) where the resistance over the insulation goes below a predetermined level .

According to an embodiment of the invention, is to provide a power supply system comprising; a battery comprising; a plurality of battery cells electrically connected to each other and to the negative and positive battery poles, a metal enclosure, and an electrical insulation between the enclosure and the battery cells, and a device for automatic monitoring of the battery insulation between said enclosure and battery cells, where the system is adapted to make a first measurement of a voltage (Ui) between the enclosure and one of the battery poles by means of a first load circuit connected to one battery pole and the enclosure, and a second measurement of a voltage (U₂) between the battery poles and the enclosure by means of a second load circuit connected to both battery poles and the enclosure.

According to an embodiment of the invention, a power supply system where the device is adapted to measure the voltage between the battery poles (U_b) .

According to an embodiment of the invention, a power supply system where the means for measuring voltage comprises a switching means adapted to switching between load circuits for measuring the first measurement and the second measurement .

According to an embodiment of the invention, a power supply system where the switching means is adapted to minimize the power consumption in the measurement circuit. According to an embodiment of the invention, the power supply system is adapted to calculating the resistance over the insulation (R_x) between the enclosure and the battery cells.

According to an embodiment of the invention, the power supply system is adapted to monitoring the resistance over the insulation (R_x) between the enclosure and the battery cells and generates a warning signal if the resistance over the insulation goes below a predetermined level.

According to an embodiment of the invention, a method for automatic monitoring of a condition of a battery in a power supply wherein said battery comprises: a plurality of battery cells electrically connected to each other and to the negative and positive battery poles, a metal enclosure, and an electrical insulation between the enclosure and the battery cells, where the method comprises the steps; making a first measurement of a voltage (Ui) between the enclosure and one of the battery poles using a first load circuit connected to one battery pole and the enclosure, and making a second measurement of a voltage (U₂) between the battery poles and the enclosure using a second load circuit connected to both battery poles and the enclosure.

According to an embodiment of the invention, the method further comprises the step; measuring the voltage between the battery poles.

According to an embodiment of the invention, the method comprises switching between load circuits for making said first measurements and said second measurement. According to an embodiment of the invention, the method comprises the step of calculating the resistance over the insulation (R_x) between the enclosure and the battery cells.

According to an embodiment of the invention, the method comprises the step of generating a warning signal if the resistance over the insulation goes below a predetermined level .

According to an embodiment of the invention, the method comprises the step of calculating the voltage (U_x) of a battery cell where the resistance over the insulation goes below a predetermined level.

According to an embodiment of the invention, a computer program product, directly loadable into the internal memory of a digital computer, comprises software code portions for carrying out a method according to any of the claims 14-19 when said product is run on a computer.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings constitute a part of this specification and include exemplary embodiments to the invention, which may be embodied in various forms.

Fig. 1 illustrates an equivalent schematic electric circuit diagram of a battery with an isolation fault.

Fig. 2 illustrates a schematic electric circuit diagram, with open switch 106, according to an embodiment of the invention Fig. 3 illustrates a schematic electric circuit diagram, with closed switch 106, according to an embodiment of the invention Fig. 4 illustrates, in a graph, the changes in measured voltage (Ui, in electric circuit described in Fig 2) as a function of insulation resistance.

Fig. 5 illustrates, in a graph, the changes in measured voltage (U₂, in electric circuit described in Fig 3) as a function of insulation resistance.

Fig. 6 illustrates how two voltage measurements can be used to detect insulation resistance fault.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Detailed descriptions of the preferred embodiment are provided herein. It is to be understood, however, that the present invention may be embodied in various forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching one skilled in the art to employ the present invention in virtually any appropriately detailed system, structure or manner.

Fig. 1 illustrates an equivalent schematic electric circuit diagram of a battery with an isolation fault. A battery 100 comprising a plurality of cells 101 that are electrically connected and arranged inside an isolated metal casing 111. The cells 101 inside the battery are numbered 1, 2, ..., x, ..., n-1, n where cell nr 1 is connected to the negative battery terminal and cell nr n is connected to the positive battery terminal.. There is a possible insulation fault between the metal casing 111 and cell nr x (this cell could be any of the cells in the battery) or the interconnections close to cell nr x. With the insulation fault the battery casing is now connected to a potential U_x (from the negative pole 104) over the resistance R_x 102 (this is the actual insulation fault) . Because there is no current flowing the casing will have the potential U_x.

There are two unknown parameters (U_x and R_x) and to determine them one needs two measurements. The two measurements are described in Fig 2 and Fig 3 where the potential of the battery casing is measured with two different circuits.

Fig. 2 illustrates a schematic electric circuit diagram of a battery with an isolation fault (as in figure 1) connected to an external measuring circuit. Due to the insulation fault, a current will flow in the circuit through R_x, R₁, R₀ and the battery cells 1-x. The switch 106 is open, preventing any current from flowing through R₂. The casing will now have the potential U₁. The potential U₁ will be measured by a measurement device such as a multi channel Analog Digital Converter, ADC 103 using the resistors

R₁ and Ro as a voltage divider.

The multi channel measurement device 103 will also measure the battery voltage U_b between the negative pole 104 and the positive pole 105 using the resistors R4 110 and R₅ 112 as a voltage divider.

Fig. 3 illustrates the same schematic electric circuit diagram as in fig 2 but with the switch 106 closed allowing a current to flow through R₂. The casing will now have the potential U₂. The potential U₂ will be measured by a measurement device such as a multi channel Analog Digital Converter, ADC 103 using the resistors R₁ and Ro as a voltage divider.

Using the two configurations of the measuring circuit to measure the voltages U₁ and U₂ and knowing the values of the resistances R₀, R₁ and R₂ the insulation fault R_x can be determined as well as the potential of the faulty cell U_x with the following equations;

R_x = (U₂-Ui) /( ( (Ub-U₂) /R₂)" (U₂-Ui) /(Ri+Ro) )

U_x = Ui *⁽R_x+Ri+Ro⁾/⁽Ri+Ro⁾

From these equations it is possible to derive a simple decision rule for an isolation resistance fault alarm. If R_x is less than the insulation fault resistance decision limit the battery is defined as needing repairs and is taken offline or is marked for repair at the next service cycle.

If, as an example, R₂ = (Ri+Ro) /2 and the insulation failure decision limit is set to 2*R₂, the following rule can be used to determine if an insulation fault has occurred:

"if U₂ <= (Ui+U_b) /2 there is an isolation fault".

Both the circuit in fig 2 and fig 3 measures the voltage with reference to the negative pole but it is possible to turn the battery and measure the voltage with respect to the positive pole with the same circuit. The resistances (R₂, Ri and Ro) are selected so that the measured voltage (Ui and U₂) are small.

For implementing the logic in the insulation failure decision rule and to make the necessary calculations the circuits are connected to a calculating unit. The calculating unit may further comprise filters for filtering the signals, A/D- converters for converting and sampling the signals and a micro processor. The micro processor comprises a central processing unit CPU performing the following functions: collection of measured values, processing of measured values, calculation of distance to insulation fault, evaluating decision rule and output of result from calculation. The micro processor further comprises a data memory and a program memory. A computer program for carrying out the method according to the present invention is stored in the program memory. It is to be understood that the computer program may also be run on general purpose computer instead of a specially adapted computer.

The software includes computer program code elements or software code portions that make the computer perform the said method using the equations, algorithms, data and calculations previously described. It may also be run in a distributed way over a network. A part of the program may be stored in a processor as above, but also in a RAM, ROM, PROM or EPROM chip or similar. The program in part or in whole may also be stored on, or in, other suitable computer readable medium such as a magnetic disk, CD-ROM or DVD disk, hard disk, magneto-optical memory storage means, in volatile memory, in flash memory, as firmware, or stored on a data server.

Fig. 4 illustrates, in a graph, the changes in measured voltage (Ui, in electric circuit described in Fig 2) as a function of insulation resistance. The x-axis is the insulation resistance where 10GΩ indicates very good insulation, lOkΩ indicates very bad insulation and 10MΩ is in between. The y-axis is the measured voltage. The three curves show how the measured voltage changes with changes in insulation dependent on where in the battery cell stack the insulation fault occurs. Curve 200 shows how voltage (Ui) changes if the insulation resistance fault occurs in close to or at the positive pole (cell n in fig 1) . If the metal enclosure is connected over a small resistance (i.e. the insulation fault) to the battery cell close to the positive pole, the measured voltage between the enclosure and the negative pole will be almost equal to the battery voltage U_b. With perfect insulation the measured voltage will be zero. Curve 202 shows how voltage (Ui) changes if the insulation resistance fault occurs in close or at the negative pole (cell 1 in fig 1) . If the metal enclosure is connected over a small (or large) resistance to the negative pole, the measured voltage between the metal enclosure and the negative pole will be zero. Curve 201 shows how voltage (Ui) changes if the insulation resistance fault occurs in the middle of the connected cells (fig 1) . For a large insulation fault (i.e. low isolation resistance) the metal enclosure will have the voltage of the faulty cell. With good insulation, the voltage of the metal enclosure will be (almost) zero.

Fig. 5 illustrates, in a graph, the changes in measured voltage (U₂, in electric circuit described in Fig 3) as a function of insulation resistance. The x-axis is the insulation resistance where 10GΩ indicates very good insulation, lOkΩ indicates very bad insulation and 10MΩ is in between. The y-axis is the measured voltage. The three curves show how the measured voltage changes with changes in insulation dependent on where in the battery cell stack the insulation fault occurs.

Curve 300 shows how voltage (U₂) changes if the insulation resistance fault occurs in close or at the positive pole (cell n in fig 1) . If the metal enclosure is connected over a small resistance (i.e. the insulation fault) to the battery cell close to the positive pole, the measured voltage between the enclosure and the negative pole will be almost equal to the battery voltage U_b. With perfect insulation the measured potential of the metal enclosure will be U_m which depends on the relative resistance of the resistors R₂ and (Ri+Ro) (in fig 2 and fig 3) .

Curve 302 shows how voltage (U₂) changes if the insulation resistance fault occurs in close or at the negative pole (cell 1 in fig 1) . If the metal enclosure is connected over a small resistance to the negative pole, the measured voltage between the metal enclosure and the negative pole will be close to zero. With good insulation, the measured voltage of the metal enclosure will be close to U_m which depends on the relative resistance of the resistors R₂ and (Ri+R₀) (in fig 2 and fig 3) .

Curve 301 shows how voltage (U₂) changes if the insulation resistance fault occurs in the middle of the connected cells (fig 1) . For a large insulation fault (i.e. low resistance) the metal enclosure will have the voltage of the faulty cell. With good insulation, the measured voltage of the metal enclosure will be close to U_m which depends on the relative resistance of the resistors R₂ and Ri (fig 2 and fig 3) .

Depending on the values of the resistors R₂ and (Ri+Ro) the potential of the metal enclosure can go up or down as the insulation deteriorates.

Fig. 6 illustrates how two voltage measurements can be used to detect insulation resistance fault. The x-axis is the insulation resistance where 10GΩ indicates very good insulation, lOkΩ indicates very bad insulation and 10MΩ is in between. The three scattered curves are voltage measurements of U₂ and the same curves as in fig 5 (i.e. 404 is the same curve as 300, 408 is the same curve as 302 and 407 is the same curve as 301) . The bold curves are the sum (Ui/2 + U_b/2) where Ui is the measurement form the circuit in fig 2 and U_b is the battery voltage.

Curve 403 shows how the voltage sum (Ui/2 + U_b/2) changes if the insulation resistance fault occurs in close or at the positive pole (cell n in fig 1) . If the metal enclosure is connected over a small resistance (i.e. the insulation fault) to the battery cell close to the positive pole, the summed voltage will be equal to the battery voltage U_b (since Ui = U_b) . With perfect insulation the measured voltage will be U_b/2 (since U_x = 0) .

Curve 406 shows how the voltage sum (Ui/2 + U_b/2) changes if the insulation resistance fault occurs in close or at the negative pole (cell 1 in fig 1) . If the metal enclosure is connected over a small (or large) resistance to the negative pole, the summed voltage will be equal to half the battery voltage U_b (since Ui = 0) . Curve 405 shows how the voltage sum (Ui/2 + Ub/2) changes if the insulation resistance fault occurs in the middle of the connected cells (fig 1) . For a large insulation fault the metal enclosure will have the voltage of the faulty cell. With good insulation, the voltage of the metal enclosure will be equal to half the battery voltage U_b (since Ui = 0) .

When the isolation resistance (R_x) goes below a set value, an isolation resistance fault alarm should be indicated. If the decision limit for an isolation fault is R_x <= 2*R₂ and assuming R₂ = (Ri+Ro)/2) the decision rule for an isolation resistance fault alarm is:

"if U₂ <= (Ui+U_b) /2 there is an isolation fault". The dotted line 400 is the isolation resistance (R_x) fault alarm limit. If the battery has an insulation resistance below this level 401 the isolation is assumed to be faulty. If the battery has an insulation resistance above this level 402 the isolation is assumed to be sufficient.

Independent of which cell in the battery stack the isolation fault occurs, the voltage U₂ is equal to (Ui+U_b) /2 when the isolation resistance R_x is equal to isolation resistance fault alarm limit (in this case 2*R₂) . With an insulation resistance below 2*R₂ the voltage U₂ is always less than (Ui+U_b) /2.

The electric losses over the measurement circuit are larger when measuring U₂ (Fig 3, relay 106 closed) than when measuring Ui (Fig 2, relay 106 open) . In order to reduce the electric losses in the measurement device, the time spent on measuring Ui could be much larger (in order of minutes/hours) than the time spent on measuring U₂ (in order of seconds) .

For long measuring times, the switch 106 realized as a relay may determine the frequency of measurements of Ui and U₂. For example, if the relay is guaranteed to function 1 000 000 times and the device is expected to monitor the battery 20 years, the switch is closed (for a few seconds) to measure U₂ once every 21 minutes. The relay should be of a bi-stable (latching) type to minimize the control energy needed.

While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims .

Claims

What is claimed is:

1. A device for automatic monitoring of a condition of a battery of a power supply wherein the battery comprises: a plurality of battery cells electrically connected to each other and to the negative and positive battery poles, a metal enclosure, and an electrical insulation between the enclosure and the battery cells, characterized in that the device comprises a measuring unit arranged to make a first measurement of a voltage (Ui) between the enclosure and one of the battery poles by means of a first load circuit connected to one battery pole and the enclosure, and a second measurement of a voltage (U₂) between the battery poles and the enclosure by means of a second load circuit connected to both battery poles and the enclosure.

2. A device according to claim 1 wherein the device comprises a measuring unit arranged to measure the voltage between the battery poles (U_b) .

3. A device according to any of the claims 1-2 wherein the device comprises switching means adapted to switch between the load circuits for said first measurement and said second measurement.

4. A device according to any of the claims 1-3 wherein said switching means is adapted to minimize the power consumption in the measurement circuit .

5. A device according to any of the claims 1-4, wherein the device is adapted to calculate the resistance (R_x) over the insulation between the enclosure and the battery cells.

6. A device according to any of the claims 1-5, wherein the device is adapted to monitor the resistance over the insulation (R_x) between the enclosure and the battery cells and generate a warning signal if the resistance over the insulation goes below a predetermined level.

7. A device according to any of the claims 1-6, wherein the device is adapted to calculate the voltage of a battery cell (U_x) where the resistance over the insulation goes below a predetermined level.

8. A power supply system including at least one battery comprising a plurality of battery cells electrically connected to each other and to the negative and positive battery poles, a metal enclosure, and an electrical insulation between the enclosure and the battery cells, and a device for automatic monitoring of the insulation between said enclosure and said battery cells of the at least one battery, characterized in that said system is adapted to make a first measurement of a voltage (Ui) between the enclosure and one of the battery poles by means of a first load circuit connected to one battery pole and the enclosure, and a second measurement of a voltage (U₂) between the battery poles and the enclosure by means of a second load circuit connected to both battery poles and the enclosure.

9. A power supply system according to claim 8, characterized in that said device is adapted to measure the voltage between the battery poles (U_b) .

10. A power supply system according to any of the claims 8-9, characterized in that said means for measuring voltage comprises a switching means adapted to switching between load circuits for measuring said first measurement and said second measurement .

11. A power supply system according to claim 10, characterized in that said switching means is adapted to minimize the power consumption in the measurement circuit.

12. A power supply system according to any of the claims 8-11, characterized in that the system is adapted to calculating the resistance over the insulation (R_x) between the enclosure and the battery cells.

13. A power supply system according to any of the claims 8-12, characterized in that said system is adapted to monitoring the resistance over the insulation (R_x) between the enclosure and the battery cells and generate a warning signal if the resistance over the insulation goes below a predetermined level .

14. A method for automatic monitoring of a condition of a battery in a power supply wherein said battery comprises: a plurality of battery cells electrically connected to each other and to the negative and positive battery poles, a metal enclosure, and an electrical insulation between the enclosure and the battery cells, wherein the method comprises the steps; making a first measurement of a voltage (Ui) between the enclosure and one of the battery poles using a first load circuit connected to one battery pole and the enclosure, and making a second measurement of a voltage (U₂) between the battery poles and the enclosure using a second load circuit connected to both battery poles and the enclosure.

15. A method according to claim 14 wherein the method further comprises the step; measuring the voltage between the battery poles .

16. A method according to any of the claims 14-15 wherein the method comprises switching between load circuits for making said first measurements and said second measurement.

17. A method according to any of the claims 14-16 wherein the method comprises the step of calculating the resistance over the insulation (R_x) between the enclosure and the battery cells .

18. A method according to any of the claims 14-17 wherein the method comprises the step of generating a warning signal if the resistance over the insulation goes below a predetermined level .

19. A method according to any of the claims 14-18 wherein the method comprises the step of calculating the voltage (U_x) of a battery cell where the resistance over the insulation goes below a predetermined level.

20 A computer program product, directly loadable into the internal memory of a digital computer, comprising software code portions for carrying out a method according to any of the claims 14-19 when said product is run on a computer.