GB2147163A

GB2147163A - Method and device for discharging a nickel-cadmium battery

Info

Publication number: GB2147163A
Application number: GB08422516A
Authority: GB
Inventors: Wolfgang Weinlechner
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-09-06
Filing date: 1984-09-06
Publication date: 1985-05-01
Also published as: FR2551585A1; DE3332163C1; IL72832A0; GB8422516D0; JPS6072173A

Abstract

In discharging a nickel- cadmium battery 1 with serially interconnected cells 9, all of the cells are bridged over almost simultaneously by respective discharge resistors (40), (Fig. 2), each having a resistance value preventing the exceeding of the maximum discharge current. Contact elements (31), (32) are mounted in bores in a carrier plate 22 so as to be aligned with a corresponding pole (6), (7) of a cell when the plate is positioned over the battery. A respective discharge resistor (40) is connected between each pair of elements (31), (32) associated with a corresponding cell 9. Contacts (34), (35) of the elements are spring biased towards the cell poles, and incandescent lamps 47 or LEDs connected between corresponding pairs of elements (31), (32) can be used to indicate that the contacts (34), (35) are engaged with the cell poles. <IMAGE>

Description

SPECIFICATION Method and device for discharging a nickelcadmium battery The present invention relates to a method of and a device for discharging a nickel-cadmium battery comprising a plurality of serially interconnected cells.

Nickel-cadmium batteries are used to increasing degree because of a number of advantages and find extensive employment particularly in aircraft. They require intensive maintenance and care, wherein different kinds of discharge processes play an important part. In these discharge processes, a slide resistor is connected between the positive and the negative pole of the battery and adjusted so that the permissible maximum discharge current is not exceeded. By means of this single resistor, the series-connected cells of the battery are discharged together, such as in the following maintenance operations prescribed by a maintenance and repair manual for nickel-cadmium aircraft batteries of the firm VARTA.The designations 15 and l,o employed for the magnitudes of charging and discharging currents signify currents having intensities which, measured in amperes, are numerically equal to one fifth and one tenth, respectively, of the nominal capacity of the battery measured in ampere-hours.

One of the maintenance operations, in which a nickel-cadmium battery must be partially discharged, is the so-called capacity check, in which the battery is initially discharged by a current 15 to the final discharge voltage of, for example, 1.0 volts. In that case, the voltage of the cells must be monitored continuously so that the voltage of individual cells, which at the start of the discharge have a low state of charge, does not fall into a critical range in which pole reversal of the cell concerned, and thereby damage of the battery, might occur. After this discharge operation, the battery is subjected to a socalled normal charge and, one hour after completion of the charging, the battery is again discharged for five hours by a current 15. After this, each of the cells must still possess at least a voltage of 1.0 volts.If this is the case, then the battery has nominal capacity and is capable of use after re-charging. Any cells having a voltage below 1.0 volts are removed and replaced by good cells.

A further such maintenance operation is the basic overhaul of a battery, in which the cells are taken out of the battery casing for cleaning purposes. Before this can occur, the battery must be deep-discharged. This again takes place with the use of a slide resistor which is applied to the battery from outside and set so that a current 11o flows. The discharge is not interrupted on the attainment of 1.0 volts per cell, but is continued. Each cell, the voltage of which attains 0.5 volts in the further course, must be short-circuited at once, or else a pole reversal of the cell can occur. Since the cells reach this critical value at different points in time in dependence on the original state of charge, a constant monitoring of all cells not yet short-circuited is required in order to avoid damage to the battery.All cells must remain in the shortcircuited state for about 12 hours. Only then can their disassembly be started.

In the case of so-called "normalisation" of the cells, a deep discharge of the described kind is also performed. This normalisation is required every three to four months for a nickel-cadmium battery in order to ensure reliable operation. Moreover, it must be performed each time the final charging voltage of even only one cell of the battery at the end of a recharging process does not lie between 1.55 and 1.7 volts.

As already mentioned, the voltages of the individual cells must be monitored during all these discharge processes in order to avoid pole reversal. This is inordinately time-consuming and demands a very carefully working, trained personnel, whereby the costs of the maintenance are increased appreciably.

Even more of a problem is the short-circuiting, required in the case of the deep discharge, of cells which have reached the critical voltage of 0.5 volts. If a cell is short-circuited too early, i.e. at too high a voltage, due to a measuring error which can easily occur at this low voltage, then spark-overs and burning of the contacts and/or a thermal overload and destruction of the cell can occur. Moreover, a high degree of care is required on the application of the short-circuit brackets in order that unintentional short-circuiting of neighbouring cells, which still have too high a voltage, does not occur.

It would thus be desirable to make the discharging of nickel-cadmium batteries possible with a substantially smaller amount of working time and in that case exclude the risk of a pole reversal of individual cells.

According to a first aspect of the present invention there is provided a method of discharging a nickel-cadmium battery comprising a plurality of serially connected cells, the method comprising the step of bridging all of the cells substantially simultaneously by means of a corresponding plurality of discharge resistors which each electrically interconnect the poles of a respective cell and the resistance value of which is such as to maintain the discharge current below a predetermined maximum permissible value.

According to a second aspect of the present invention there is provided a device for discharging a nickel-cadmium battery comprising a plurality of serially interconnected cells arranged with the poles thereof substantially in a single plane at one side of the battery, the device comprising a carrier plate, a plurality of contact elements so arranged at one major face of the plate that on disposition of the plate against the battery at said one side thereof each of the contact elements electrically contacts a respective one of the cell poles, and a respective discharge resistor electrically interconnecting each two contact elements for contact with the two poles of an associated one of the cells.

Due to the fact that each individual cell can be discharged by its own discharge resistor to the desired extent, it is no longer required to connect an overall discharge resistor, as it was hitherto used in the form of a slide resistor, to the output terminals of the battery. In spite of the fact that the short-circuiting of individual cells is superfluous, it is prevented that any cell is uncoupled from the discharge process.

Because of the individual loading of each cell, the maintenance of a flow through the entire battery is no longer required. Pole reversal of cells is excluded even when the voltage at individual cells falls more rapidly into the critical range than in the case of the neighbouring colls.

A discharge process by a device embodying the invention may be carried out in such a manner that, for example, the lid of the battery is removed and the carrier plate is placed on the battery in place of the lid and clamped fast by tightening fasteners provided for fastening the lid to the battery casing. As a result, the individual discharge process starts at about the same time for all cells. With the aid of the incandescent lamps, which serve as an indicating device and in place of which other suitable devices, for example luminescent diodes or the like, can be employed, it can be checked in the simplest manner whether an electrically conductive contact exists between the poles of each cell and the respective discharge resistor.

In this state, the arrangement can then be allowed to stand until the desired discharge is performed, for example eleven hours plus twelve hours rest time for a deep discharge.

In this entire time interval, monitoring of the voltage across the individual cells is not required. Since there is no danger of pole reversal, short-circuiting is also superfluous. If desired, the battery with the placed-on discharge device can be stored for an unlimited time interval.

If a capacity check is to be undertaken, the required measurements can be carried out at, for example, upwardly protruding portions of the contact elements with the discharge device in place.

Altogether, for example in the case of a deep discharge, a reduction in the required working time from about ten hours to about thirty minutes may be able to be achieved.

Because of the simple and certain operation, less highly-qualified personnel can be employed.

An example of the method and an embodiment of the device of the present invention will now be more particularly described with reference to the accompanying drawings, in which: Fig. 1 is a perspective view of a nickelcadmium battery, which comprises serially interconnected cells, and of a discharge device embodying the invention, the device being arranged above the battery and ready for placing thereon: and Fig. 2 is a partial sectional view along the line Il-Il of Fig. 1, with the device placed on the battery.

Referring now to the drawings, there is shown in Fig. 1 a battery 1 which comprises an approximately parallelepipedonal casing 2, which can be closed at the top by a lid (not shown). This lid is normally held fast by tightening fasteners 4, of which only two are to be seen in Fig. 1 and which are shown mounted on one side of the casing 2. Two further tightening fasteners of that kind are disposed on the opposite side.

With the lid removed, as illustrated in Fig.

1, positive and negative poles 6 and 7, lying at the top, of cells 9 of the battery are freely accessible.

In the illustrated embodiment, the battery 1 comprises twenty cells 9 of that kind, of which only some are illustrated while the others are indicated symbolically. The cells 9 of the battery 1 are all connected in series such that the positive pole 6 of each cell 9 is connected through a connector 10 with the negative pole 7 of a neighbouring cell 9. The positive pole 6 of the first cell 9 in this series is connected through a line 11 with a corresponding terminal pin 12 of a plug connection 13 mounted at the outside of the battery, and the negative pole 7 of the last cell 9 in the series is connected through a corresponding line 14 with a further terminal pin 15 of the plug connection 13, so that current can be taken off at this plug connection 13.

In Fig. 1 the individual cells 9 of the battery 1 are, for the sake of clarity, represented as spaced from each other. In fact, however, they are disposed in direct contact at their vertical walls and are so tightly packed in the casing 2 that they mutually support each other against movement.

Illustrated above the battery in Fig. 1 is a discharge device 20, which is adapted to the shape of the battery and comprises a carrier plate 22, the dimensions of which are chosen to approximately correspond to those of the battery lid. The carrier plate 22 can be placed in direction of the arrows F onto the upper side of the casing 2. At two opposite side edges, the carrier plate 22 has two hooks 23 (of which only two are illustrated in Fig. 1) which are so arranged that they engage the fasteners 4 when the plate 22 is placed on the casing 2, whereby the plate 22 can be firmly pressed against the upper side of the casing.

The carrier plate 22 comprises a respective discharge arrangement 25, of which only six are illustrated and the remainder are indicated symbolically in Fig. 1, for each cell 9 of the battery 1.

The construction of such a discharge arrangement 25, which is the same for all discharge arrangements, will now be described with reference to Fig. 2: Illustrated schematically in the lower region of Fig. 2 is the upper part of a cell 9, which at its upper side has two pole bolts, one of which forms the positive pole 6 and the other the negative pole 7 of the cell. Screwed onto the thread of each of these pole bolts are two hexagonal nuts 27 and 28, of which the lower nut 27 serves to connect the pole bolt, passing into the interior of the cell, tightly with the cell housing. The upper nut 28 serves to fasten the connector 10, leading to the respective neighbouring cell, to the pole bolt.

In the illustration of Fig. 2, the discharge device 20 is placed on the upper side of the battery for the performance of the discharge process exemplifying the invention. As is apparent from Fig. 2, each discharge arrangement 25 comprises two contact elements which are constructed as pins 31 and 32 and extend through corresponding bores 29 and 30 in the carrier plate. The upper parts of the pins 31 and 32 are firmly connected with the carrier plate 22, for example through gluing.

At their lower ends, the pins 31 and 32 have movable lower parts 34 and 35, which are displaceable in the longitudinal direction of the pins 31 and 32 and biassed through springs 36 and 37 into lower end positions.

The bores 29 and 30 are so positioned in the carrier plate 22 that they are exactly aligned with the pole bolts, lying thereunder, of the associated cell 9 on the proper position- ing and tightening of the carrier plato 22. On tightening of the carrier plate 22, the lower parts 34 and 35 of the pins 31 and 32 are thus pressed slightly upwardly against the force of the springs 36 and 37 and thereby effect a good contact with the poles 6 and 7 of the cell 9.

The pins 31 and 32 of each discharge arrangement 25 protrude, by their upper parts, upwardly beyond the upper side of the carrier plate 22 to provide space for mounting of a discharge resistor 40 between the pins.

Connecting wires 41 and 42 of the resistor 40 are electrically conductively connected with the pins 31 and 32 through welding or soldering or in other known manner, so that a current discharging the cell 9 can flow through the resistor 40 when the carrier plate 22 is placed on the battery 1.

The resistance value of the discharge resistor 40 can be, for example, 0.5 ohms. With a typical initial voltage of the cell of 1.25 volts, an initial discharge current of 2.5 amperes thus flows. Advantageously, resistors with a ceramic housing and a load capacity of 5 amperes are used. As a result, excessive heating of the resistors 40 and thereby of the discharge arrangements 25 is avoided.

A lamp holder 44 is so fastened directly on the upper end of each pin 32 that one of its terminals is disposed in conducting connection with the pin. The second terminal of the lamp holder 44 is electrically connected by a wire 45 with the other pin 31 of the discharge arrangement 25. It can thus be checked through an incandescent bulb 47 screwed into the lamp holder 44 whether, when the carrier plate 22 is in place, a conducting contact exists between the lower parts 34 and 35 of the pins 31 and 32 and the respectively associated pole bolts so that a discharge takes place by way of the resistor 40. Commercially available small bulbs for a voltage of 1.5 volts and a nominal current of 0.1 amperes, which thus possess a resistance value of 12.5 ohms, can be used as the bulbs 47.Since the resistance provided by each bulb 47 is parallel to the associated resistor 40, the resistance value of which amounts to only 0.5 ohms, virtually the entire discharge current flows by way of the resistor 40.

As indicated in Fig. 1, a discharge arrangement 25 is provided at the carrier plate 22 for each cell 9 of the battery 1. The disposition of the discharge arrangements 25 on the carrier plate 22 is matched to the disposition of the cells 9 in the battery casing 2. An individual discharge device 20 must therefore be provided for each type of battery.

As a result of the illustrated arrangement, all discharge arrangements 25 come into contact with the pole bolts of the associated cells 9 virtually at the same time by means of the lower parts 34 and 35 of their pins 31 and 32 when the carrier plate 22 is placed on the battery upper side, so that the discharge process starts practically at the same time for all cells 9.

As already mentioned, the production of the required electrical contact can be checked with the aid of the bulbs 47, which light up only when the pins 31 and 32 touch the pole bolts. This indicates with certainty that a discharge current is flowing through the discharge resistor 40.

The discharge device 20 remains on the battery for the purpose of the deep discharge at least until all cells 9 have reached the state of deep discharge. For a current of 110, a voltage of about 0.5 volts is attained after ten hours. If discharging is continued after attainment of this voltage, then the voltage of the cell falls very rapidly within about one hour to 0 volts. Thereafter, a minimum rest time of twelve hours is generally prescribed by the manufacturers, during which time batteries discharged by conventional methods must be further discharged with the cells short-circuited. In the case of a method exemplifying the present invention, the discharge device 20 remains on the battery 1 so that a minimum discharge time of about 23 hours results. If the time during which the discharge device 20 is placed on the battery 1 is extended to about 30 to 40 hours, then it can be presumed with certainty that each of the cells 9 has been completely discharged according to the instructions of the manufacturers and thereby attained the desired state of deep discharge.

If such a long wait is not desired, the attainment of the deep discharge can be determined by accurately measuring the voltage drop across the pins 31 and 32 associated with each cell.

It is possible to store nickel-cadmium batteries 1 with the discharge device 20 placed thereon for as long as desired in the deeply discharged state, there being no upper limit in time.

Claims

1. A method of discharging a nickel-cadmium battery comprising a plurality of serially connected cells, the method comprising the step of bridging all of the cells substantially simultaneously by means of a corresponding plurality of discharge resistors which each electrically interconnect the poles of a respective cell and the resistance value of which is such as to maintain the discharge current below a predetermined maximum permissible value.

2. A method as claimed in claim 1, wherein the resistive interconnection of the poles of the cells is maintained at least until all the cells have reached the state of deep discharge.

3. A method as claimed in either claim 1 or claim 2, comprising the step of monitoring the presence of an electrically conductive connection betweon the poles of each cell and the associated resistor, the monitoring step being carried out with use of an indicating device.

4. A method as claimed in any one of the preceding claims, wherein each of the resistors is a fixed value resistor arranged to limit the discharge current to a value which in amperes is numerically equal to substantially one tenth of the nominal ampere-hours capacity of the battery.

5. A method as claimed in any one of the preceding claims, wherein the resistive interconnection of the poles of the cells is maintained for substantially 30 to 40 hours.

6. A method substantially as hereinbefore described with reference to the accompanying drawings.

7. A device for discharging a nickel-cadmium battery comprising a plurality of serially interconnected cells arranged with the poles thereof substantially in a single plane at one side of the battery, the device comprising a carrier plate, a plurality of contact elements so arranged at one major face of the plate that on disposition of the plate against the battery at said one side thereof each of the contact elements electrically contacts a respective one of the cell poles, and a respective discharge resistor electrically interconnecting each two contact elements for contact with the two poles of an associated one of the cells.

8. A device as claimed in claim 7, wherein each of the contact elements comprises an elongate member mounted in a respective bore in the plate and having an end portion projecting from the plate at each major face thereof, the end portion projecting from said one major face serving to contact the respective cell pole and the other end portion being connected to the associated resistor by electrically conductive means.

9. A device as claimed in either claim 7 or claim 8, comprising a respective incandescent lamp electrically connected between the contact elements of each said two elements, the lamp being directly connected to one of the elements and connected by a wire to the other element.

10. A device as claimed in claim 9, wherein the resistance value of each of the lamps is substantially higher than that of the associated one of the resistors.

11. A device as claimed in any one of claims 7 to 10, wherein the carrier plate is provided with means to enable pressing of the plate against the battery.

12. A device as claimed in claim 8 or any one of claims 9 to 11 when appended to claim 8, wherein the first-mentioned end portion of each elongate member is movable relative to the plate and resiliently biassed in a direction away from the plate.

13. A device substantially as hereinbefore described with reference to the accompanying drawings.