WO1993010466A1

WO1993010466A1 - Apparatus for monitoring the voltage of a dc supply

Info

Publication number: WO1993010466A1
Application number: PCT/GB1992/002145
Authority: WO
Inventors: Desmond Clarke; John Molyneux; Robert Anthony Schofield
Original assignee: Silent Power Gmbh Fur Energiespeichertechnik
Priority date: 1991-11-21
Filing date: 1992-11-20
Publication date: 1993-05-27
Also published as: GB9124721D0; GB2261958A

Abstract

An apparatus for monitoring the voltage of a DC supply includes electronic means for generating a signal having a characteristic representing the voltage of the supply. The signal is transmitted to a receiver. The electronic generating means is arranged to receive its electrical power from the DC supply being monitored. The DC supply may have a plurality of series-connected DC supply units. Apparatus for monitoring the voltage of such a DC supply comprises a plurality of voltage monitoring apparatus arranged for connection across the series-connected units. The electronic generating means of each voltage monitoring apparatus is powered from a respective one or series-connected group of the units and generates a respective signal having a characteristic representing the voltage of the unit or group of units across which it is connected.

Description

APPARATUS FOR MONITORING THE VOLTAGE OF A DC SUPPLY

The present invention relates to an apparatus for monitoring the voltage of a DC supply.

One aspect of the present invention is particularly applicable to an apparatus and a method for monitoring a battery having a plurality of series-connected cells or groups of cells. O89/00344 (Chloride Silent Power Limited), for example, discloses various constructions of batteries of sodium sulphur cells, each comprising one or more arrays of single cells or series arrangements of cells arranged between bus plates. To provide a battery capable of powering a vehicle, it may be necessary for about three thousand sodium sulphur cells to be assembled together in blocks of series-connected arrangements of cells, the series arrangements in each block group being connected in parallel and the blocks of arrangements being connected in series. Each block of cells will have a voltage determined by the number of cells in each of the parallel-connected series arrangements of cells, of, say, 6, 8 or 10 volts. A number of blocks of cells are connected in series to provide a battery of the required voltage, eg. 600V battery for operation of a hybrid trolley bus. For ease of mechanical handling, the battery may be divided into a number of sections. In the following description, references to a 'battery' should be construed to cover battery sections formed of one or more blocks unless the context requires otherwise.

A first aspect of the present invention provides an apparatus for monitoring the voltage of a DC supply comprising: electronic means for generating a signal having a characteristic representing said voltage and for transmitting said signal to a receiver wherein said electronic generating means is arranged to receive its electrical power from said DC supply.

The first aspect of the present invention enables the voltage of a DC supply, such as a group of cells, to be monitored and a signal representing said voltage to be transmitted to a receiver, such as a central controller for the battery. The first aspect of the present invention makes use of the appreciation that a DC supply has a relatively low impedance. Supply of electrical power to said electronic generating means from the DC supply being monitored will therefore not affect the voltage of the DC supply significantly.

Said electronic means may comprise a voltage-to-frequency converter for producing a signal having a frequency representing said voltage. In this way, a signal may be generated which represents the instant voltage of the DC supply. Said electronic means may further include a divider arranged for proportionately reducing the frequency of the signal produced by the converter. The combination of a voltage to frequency converter and a divider enables the conversion factor between the input voltage and the output frequency to be, in part, independent of the frequency response of the transmitter and receiver, providing greater flexibility in the choice of capacitor and resistor components which determine said conversion factor. Also the divider acts as an integrating digital filter to reduce sensitivity to short term variations in the voltage being monitored. The above apparatus may advantageously include isolating means to provide said signal electrically isolated from said electronic generating means. A potential divider may be incorporated for providing an input signal to said electronic means which is a predetermined fraction of the value of said voltage.

A second aspect of the present invention provides apparatus for monitoring a battery having a plurality of series-connected cells or blocks of cells, the apparatus comprising: a plurality of electronic voltage monitoring circuits arranged for connection across the series-connected cells or blocks of cells so that each voltage monitoring circuit is powered from a respective one or series-connected group of the cells or blocks of cells, each said voltage monitoring circuit providing a signal having a characteristic representing the voltage of said one or series-connected group of the cells or blocks of cells; ^• and respective isolating means for each said voltage monitoring circuit to provide said signal . electrically isolated from said voltage monitoring circuit.

This arrangement allows the voltage of each series-connected cell or block of cells to be monitored locally to provide, to a central controller, a respective signal representing the voltage of each cell or block of cells and isolated from the voltage monitoring circuits. The apparatus can therefore be arranged so that the number of isolating means required is determined by the number of cells or blocks of cells to be monitored individually. Although a respective said monitoring circuit may be provided for each series-connected cell or block, in certain arrangements a single said circuit may be connected across one or more series-connected cells or blocks.

This aspect of the present invention is contrasted with other proposals for monitoring a battery comprising a plurality of series-connected cells or blocks of cells. An isolated data logger has been proposed to enable monitoring of the voltage of a battery. The data logger would scan all the series-connected cells or blocks of cells in the battery being monitored and produce, for each cell or group of cells, an analogue signal which is converted to a digital word representing the value of the voltage. To provide the accuracy required to distinguish, say, a voltage change of 0.05V in a battery section of 64V would require a twelve bit word, and hence twelve isolating means.

In contrast, the number of isolating means required by the described apparatus is determined by the number of cells or blocks of cells which are being individually monitored. For a battery or battery section of voltage 64V produced by 8 blocks of cells each having a voltage of 8V only 8 isolating means are required. The apparatus also obviates the need to use expensive equipment such as data loggers which might otherwise have been required. Said voltage monitoring circuit may comprise a voltage to frequency converter for producing a signal having a frequency representing said voltage.

Embodiments of the present invention will now be described, by way of example only, and with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of an apparatus embodying the present invention for providing both overvoltage protection and for producing a signal representative of the instant state of the DC supply;

Figure 2 shows, in greater detail, a circuit for generating said signal representative of the voltage of the DC supply; and Figure 3 shows, in greater detail, a circuit for providing overvoltage protection. As shown in Figure 1, a DC supply 2 to be monitored has a positive terminal 4 and a negative terminal 6. Electronic generating means, including active components, are connected to the DC supply 2 to generate a signal having a characteristic representing the voltage of the DC supply. The electronic generating means shown consist of a voltage to frequency converter 8. The voltage to frequency converter 8 produces a signal whose frequency depends on the value of the voltage of the DC supply 2 at the input of the converter 8. The frequency of the output signal from the converter 8 determines the frequency of light emission of a light emitting diode 12a which forms part of an opto-isolator 12. Light emission from the light emitting diode 12a is transmitted to and received by a phototransistor 12b forming the other part of the opto-isolator 12. The opto-isolator 12 thus provides electrical isolation of the output signal from the converter 8.

The DC supply 2 is also connected to the input of a comparator 10. The output of the comparator operates an opto-isolator 16, comprising a light emitting diode 16a and a photo-transistor 16b, when the voltage at the input of the comparator 10 exceeds a predetermined value. A signal is thereby produced on line 18 at the output of the photo-transistor 16b to indicate whether or not the voltage of the DC supply 2 exceeds a predetermined value. The comparator circuit thereby provides overvoltage protection.

The circuit of Figure 1 provides an apparatus which allows local monitoring of the voltage of a DC supply 2 and transmission of an output signal dependent on the monitored voltage to a receiver, which may be remote. The output signals of the circuit are isolated from the electronic generating means. The circuit of Figure 1 also makes use of the appreciation that the voltage being monitored is a DC supply voltage which, by its nature, has a low impedance. The active components of the electronic generating means can therefore be electrically powered by the DC supply voltage being monitored without any significant effect on the value of the DC supply voltage. As shown in Figure 1, the power input terminals of the coverter 8 and comparator 10 are connected to the positive and negative terminals 4, 6 of the DC supply 2. Reference voltages required for operation of the converter 8 and comparator 10 are generated from the DC supply voltage using voltage regulators (not shown) . The inputs of the converter 8 and comparator 10 are not connected directly to the DC supply 2 but taken from potential dividers 20a, 20b, 22a, 22b which produce input signals to the converter 8 and comparator 10 which are fractions of the value of the DC supply voltage.

As mentioned in the introduction, the DC supply to be monitored may comprise a plurality of series-connected DC supply units, such as the blocks of cells in a battery or battery section. With such a DC supply, a voltage monitoring apparatus may be provided locally for each block of cells with the output signals transmitted to a central controller via leads connected to the opto-isolators.

A plurality of voltage monitoring circuits for monitoring the instant voltage of a plurality of series-connected blocks of cells is shown in Figure 2. For ease of illustration, only 3 voltage monitoring circuits have been shown, although, as indicated by the captions, the circuit of Figure 2 is intended for use with a battery or battery section comprising eight blocks. A voltage monitoring circuit 30 and an opto-isolator 32 are provided for each block 34. A voltage to frequency converter 36 and a counter 38 are provided in each voltage monitoring circuit 30. As shown, the converter 36 and counter 38 are powered by the output of the respective block 34. The input signal to the converter 36 may be a fraction of the supply voltage, the fraction being determined by the resistors 40a, 40b, 40c in a potential divider which may also provide off-set compensation. For ease of construction, one of the resistors, eg. 40c, in the potential divider may be "set on test" to obviate the need for an adjustable resistor.

The conversion factor between the input voltage and output frequency of the converter 36 is determined by a resistor 42 and a capacitor 42b. In the specific embodiment disclosed, the conversion factor was chosen to be IV to 1kHz, with a full scale frequency of 10kHz representing 10V to enable a physically small stable capacitor to be used for the capacitor 42b.

The output of the converter 36 is connected to the input of the counter 38 which is effective as a divider for proportionately reducing the frequency of the signal produced by the converter 36. In the specific embodiment shown, the counter 38 is a divide by 1,024 counter, producing a signal of maximum frequency 10Hz for transmission across the opto-isolator 32 to a central controller. The minimum period in the output signal to be measured by the central controller is therefore 0.Is.

As shown, the converter 36, the counter 38 and the light emitting diode 32a (of the opto-isolator 32) all operate from the supply voltage being monitored. The component types and values are therefore chosen to minimise current consumption, in a manner known to those skilled in the art. For example, the light emitting diode 32a operates with a forward current of 0.5mA. Conveniently, the voltage monitoring circuit 30 and opto-isolating means 32 may be provided on a single printed circuit board. Leads 36 from the blocks 34 are connected to the printed circuit board via screw connectors as are leads for a 5V power supply (to power the photo-transistor 32b of the opto-isolator 32 which is electrically isolated from the voltage monitoring circuit 30). Frequency outputs from the converters 30, via the opto-isolators 32, are connected to a central controller board via a sixteen way connector and ribbon cable. Every second core of the ribbon cable is connected to 0V at the controller end to provide screening.

As indicated in Figure 2, the output terminals of the blocks 34 are also connected to overvoltage protection circuits shown in greater detail in Figure 3. Conveniently, the overvoltage protection are provided on the same printed circuit board as the voltage monitoring circuits 30. A voltage protection circuit 50 and opto-isolator 52 are provided for each block 34 being monitored. The overprotection circuit 50 includes a micropower voltage comparator 54 which operates the opto-isolator 52 when the block voltage limit is exceeded. The value of the block voltage limit is set by resistors 56a, 56b. Further resistors 56c and 56d may be connected in parallel with the resistor 56b if required. A resistor 58 connected to the inverting input of the comparator 54 provides hysteresis to the comparator 54 to prevent any spurious output signals as the voltage of the block being monitored approaches the block voltage limit. As previously indicated, each comparator 54 in the overvoltage protection circuitry is powered by the respective block being monitored.

Claims

1. Apparatus for monitoring the voltage of a DC supply comprising: electronic means for generating a signal having a characteristic representing said voltage and for transmitting said signal to a receiver wherein said electronic generating means is arranged to -receive its electrical power from said DC supply.

2. Apparatus according to Claim 1 and including isolating means to provide said signal electrically isolated from said electronic generating means.

3. Apparatus according to Claim 1 or 2 further comprising a potential divider for providing an input signal to said electronic means which is a predetermined fraction of the value of said voltage.

4. Apparatus according to any one of the preceding claims wherein said electronic means comprises a voltage-to-frequency converter for producing a signal having a frequency representing said voltage.

5. Apparatus according to Claim 4 wherein said electronic means further includes a divider arranged for proportionately reducing the frequency of the signal produced by the converter.

6. Apparatus for monitoring the voltage of a DC supply having a plurality of series-connected DC supply units, the apparatus comprising a plurality of voltage monitoring apparatus according to any one of Claims 1 to 6 arranged for connection across the series-connected units so that the electronic generating means of each said voltage monitoring apparatus is powered from a respective one or series-connected group of the units, and generates a respective said signal having a characteristic representing the voltage of said one or series-connected group of the units.

7. Apparatus for monitoring a battery having a plurality of series-connected cells or blocks of cells, the apparatus comprising: a plurality of electronic voltage monitoring circuits arranged for connection across the series-connected cells or blocks of cells so that each voltage monitoring circuit is powered from a respective one or series-connected group of the cells or blocks of cells, each said voltage monitoring circuit providing a signal having a characteristic representing the voltage of said one or series-connected group of the cells or blocks of cells; and respective isolating means for each said voltage monitoring circuit to provide said signal electrically isolated from said voltage monitoring circuit.

8. Apparatus according to Claim 7 wherein each said voltage monitoring circuit includes a respective potential divider for providing an input signal to the voltage monitoring circuit which is a predetermined fraction of the value of said voltage.

9. Apparatus according to Claims 7 or 8 wherein said voltage monitoring circuit comprises a voltage-to-frequency converter for producing a signal having a frequency representing said voltage.

10. Apparatus according to Claim 9 wherein said voltage monitoring circuit further includes a divider arranged for proportionately reducing the frequency of the signal produced by the converter.