GB2031168A

GB2031168A - A battery testing arrangement

Info

Publication number: GB2031168A
Application number: GB7933935A
Authority: GB
Original assignee: Siemens AG
Current assignee: Siemens AG
Priority date: 1978-09-30
Filing date: 1979-10-01
Publication date: 1980-04-16
Also published as: GB2031168B; DE2842817B1; JPS5548664A; NO146792B; NO793084L; NO146792C

Abstract

This operates on the principle that the values of the battery terminal voltage and of the voltage drop at a load-current carrying shunt 6 are converted 8, 9 into digital values and added 11. The total value is multiplied 14 by a digital value dependent 24 on the battery electrolyte temperature and by a digital value dependent 23 on the magnitude of the load current J, to provide an indication 17 of the battery state of charge. Each individual cell 3 of the battery 1 may be separately monitored via multiplexer 26. <IMAGE>

Description

SPECIFICATION A battery testing arrangement This invention relates to a battery testing arrangement for monitoring and indicating the state of charge of a battery. The basic principle of operation of the arrangement is that the e.m.f of the battery and consequently the capacity are determined by the addition andlor comparison of battery terminal voltage and voltage drop at a shunt through which flows battery load current.

Various electrical measuring devices are already known for the determination of the state of charge and discharge of a battery. The known measuring methods are mainly based on the fact that the e.m.f.

of a battery is a measurement of the capacity. A discharge recorder is known from the book "Lead and Steel Accumulators" by Erich Witte, Krauskopf Publishers, 3rd ed., 1967, pp. 61-62, in the case of which a terminal voltage measurement of a battery is corrected in a device by an additional current measurement so that the indicated value reproduces approximately the internal e.m.f. An adjustable shunt is used for the current measurement.

Another measuring device, known from German Auslegeschrift 2 321 108, adjusts the terminal voltage with respect to the load current by means of a sum-and-difference amplifier with adjustable amplification factor being connected at the output side to a fixed shunt without a defined measuring instrument lead. To protect a battery from damage the discharge must be terminated in the event of a minimum voltage - th so-called final discharge voltage. The final discharge voltage amounts to approximately 80% of the battery discharge. This final discharge voltage is detected by the known device and registered accordingly.

It is also known to monitor the capacity of a battery by means of a forward - and backward-running ampere-hour meter (Erich Witte - "Lead and Steel Accumuiators", Krauskopf Publ. 3rd ed. 1967, pp.

64-65).

Since, in practice, the load current of a battery does not correspond to the discharge current belonging to the discharge period of the capacitive data, only an approximate capacitive determination is possible using the known devices. In the case of batteries which are subject to fluctuations in temperature the inaccuracies are increased.

According to the present invention there is provided a battery testing arrangement for monitoring and indicating the state of charge of a battery, comprising first voltage sensing means for sensing terminal voltage value of the battery, a shunt for receiving load current produced by the battery, second voltage sensing means for sensing voltage drop value across the shunt, analog to digital converters for converting into digital form the values sensed by the first and second voltage sensing means, adding means for forming the digital sum value of digital values from the analog to digital converters, temperature sensing means for sensing battery electrolyte temperature value, multiplying means for multiplying said digital sum value by a digital amount depending upon the load current value and the battery electrolyte temperature value, and indicating means for indicating the output value of the multiplying means.

The analog-to-digital converter for conversion of the battery terminal voltage value may be connected to individual cells of the battery via a multiplexer.

Monitoring of the individual battery cells can thereby take place in the case of multi-cell batteries. Thus it becomes possible to apply measures through which is avoided the danger that a defective cell might jeopardise the overall function of a battery.

For a better understanding of the invention and to show how it may be put into effect reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is an illustration which clarifies the basic construction of a battery testing arrangement for monitoring and indicating the state of charge of a battery; and Figure2 is an actual circuit diagram of a battery testing arrangement employing a microprocessor.

In Figure 1 a battery is indicated by the numeral 1, in which battery several cells 3 are arranged in series connection in a space 2. A load or charger 7 is connected to terminals 4 and 5 of the battery in series with a shunt 6.

In order to determine the e.m.f. and thus the fundamental capacity of the battery 1, the terminal voltage U of the battery 1 and the respective load or charge current J are determined, the current J being determined with the aid of the current measuring shunt having resistance Rs. Since the internal resistance Ri of an accumulator battery is virtually constant up to approximately an 80% discharge, the fundamental capacity can be described according to the formula: E +J.Ri i.e. E = U + J . Rs . C where C =~s .

The values of the terminal voltage U and of the voltage drop J. Rs at the shunt 6 are converted by means of analog-to-digital converters 8 and 9 into digital values and added in an adder 11. Before addition, the digital value for the shunt voltage drop is multiplied in a multiplier 10 by the constant value C, provided by a module 25, which corresponds to the ratio of battery internal resistance to shunt resistance.

The total value from adder 11 is multiplied by a digital value which is dependent on the battery acid temperature. This latter value is determined by means of a thermometer probe 12, and is supplied via an analog-to-digital converter 13 to a first input of a multiplier 14 whose second input is supplied from the adder 11. The value obtained in this way is further multiplied by a digital value dependent on the magnitude of the charge current J, which value is supplied to the first input of another multiplier 15, whose second input has been supplied by the output value of the multiplier 14.

A signal value for the battery capacity, which signal value is relevant for the instantaneous operation and can be supplied to an indicating device 17 via a selectodrswitch 16 provided with a contact 18, is present at the output of the multiplier 15.

If, under certain operating conditions, an indication of the capacity is desired which relates only to temperature, then a connection of the indicating device 17 to the output of the multiplier 14 can be made in a simple manner by using the selector switch 16 to close the contact 19. Alternatively the selector switch 16 can be connected to the contact 20 which is joined to the output of the adder 11. An indicating device 22 is also connected to this output via a critical value recorder 21, which indicating device 22 is provided, in the case under consideration, by a lamp which lights up whenever the final discharge voltage of the battery is reached.

The digital value dependent on the charge current is corrected according to a function dependent on the type of battery. For this purpose a function generator 23 is connected in series to the first input of the multiplier 15, which function generator 23 takes into account the dependency of the battery capacity on the charge current. Furthermore, a function generator 24 is connected at its input side to the analog-to-digital converter 13, which function generator takes into account the dependency of the capacity on the acid temperature.

For the monitoring and indicating of the state of charge of a battery formed from several cells 3, it is advantageous to connect the analog-to-digital converter 8 to the individual cells 3 of the battery via a multiplexer 26. The multiplexer 26 connects the voltages of the individual cells 3 one after the other to the analog-to-digital converter 8 so that the capacity of the individual cells is indicated one after the other on the recording device 17 also. A control device 28 serves to control the multiplexer 26 and an indicating instrument 27 for the cell numbers.

In the case of large battery sets it is expedient to connect in series to the analog-to-digital converter 13, for converting the acid temperature into digital values, a multiplexer 29 whose inputs are each connected to a respective temperature probe 12.

This multiplexer 29 is also controlled by the control device 28.

The device illustrated Figure 2 for the monitoring and indicating of the state of charge functions using a a microprocessor system. The principle item is a microprocessor 34, e.g. and 8-bit microprocessor. A read-write memory 31 is used to store variable data.

The programme flow and the required functions are stored in erasable read-only memories 32. Functions which are carried out by discrete blocks in Figure 1 are carried out by the microprocessor 34 of Figure 2.

To avoid unnecessary repetition, not all these functions are described specifically with reference to Figure 2.

To determine battery cell voltage, a component part 33 is used which contains an analog-to-digital converter with multiplexer. This component part 33 is controlled by the microprocessor 34 in such a way that the battery cell voltages are determined in sequence.

In order to increase the degree of precision, the terminal voltage of the battery is measured directly by a measuring transducer 30.

A further analog-to-digital converter 9 is used in determining the voltage drop at the shunt 6.

The acid temperature is determined via a suitable thermometer probe 12 by using another component part 35 containing an analog-to-digital converter with multiplexer. The number of thermometer probes is dependent upon the size of the battery, the physical arrangement of the battery cells and the degree of precision required.

A power supply device 36 supplies the electrical voltage required to supply the entire monitoring system. The battery capacity and the cell number is indicated on a numerical digital indicating instrument. The obtaining of the final discharge voltage is indicated by means of a lamp indicating device 22.

The battery capacity can also be indicated on an analog instrument 38 using a digital-analog converter.

The individual component parts 9 and 31 to 37 are electrically connected to one another via a plug assembly 39.

The described embodiments make it possible to determine the stored capacity in an accumulator battery as precisely as possible and to produce a capacity determination relating to the instantaneous discharge current simply by taking into account corresponding parameters. Great precision is obtained in the determination of the state of charge of the battery and the inclusion of several parameters is made possible using simple equipment which is not susceptible to breakdown.

Claims

1. A battery testing arrangement for monitoring and indicating the state of charge of a battery, comprising first voltage sensing means for sensing terminal voltage value of the battery, a shunt for receiving load current produced by the battery, second voltage sensing means for sensing voltage drop value across the shunt, analog to digital converters for converting into digital form the values sensed by the first and second voltage sensing means, adding the means for forming the digital sum value of digital values from the analog to digital converters temperature sensing means for sensing battery electrolyte temperature value, multiplying means for multiplying said digital sum value by a digital amount depending upon the load current value and the battery electrolyte temperature value, and indicating means for indicating the output value of the multiplying means.

2. An arrangement according to Claim 1, wherein said digital amount comprises a portion dependent upon the load current value and fed to the multiplying means via a correcting function generator.

3. An arrangement according to Claim 1 or 2, wherein said digital amount comprises a portion dependent upon the battery electrolyte temperature value and fed to the multiplying means via a correcting function generator.

4. An arrangement according to any one of the preceding claims, which is operable to monitor and indicate the state of charge of a battery having a plurality of cells, there being a multiplexerfor coupling the individual cells of the battery to the analog to digital converter which is associated with the first voltage sensing means.

5. An arrangement according to any one of the preceding claims, which is operable to monitor and indicate the state of charge of a battery having a plurality of cells, there being a multiplexer for coupling the individual cells of the battery to an analog to digital converter associated with the temperature sensing means.

6. An arrangement according to any one of the preceding claims, and comprising means for multiplying the digital form of the voltage drop value across the shunt by a constant value which corresponds to the ratio of internal resistance of the battery to resistance of the shunt.

7. A battery testing arrangement for monitoring and indicating the state of charge of a battery, the battery testing arrangement being substantially as hereinbefore described with reference to Figure 1 or 2 of the accompanying drawing.