GB2089592A - Battery arrangement - Google Patents

Abstract

A battery arrangement comprises a plurality of cells, (e.g. NiCad) the cells being divided into two parts (11, 12) and switching means (13) for selectively connecting the two parts (11, 12) in series for use as a source and in parallel for charging purposes. <IMAGE>

Description

SPECIFICATION Battery arrangement This invention relates to a battery arrangement comprising a plurality of individual cells.

At the present time, there are two main types of battery arrangement, one which is known as the lead-acid type and the other which is known as the nickel cadmium type.

Lead-Acid batteries have a voltage range of their individual cells of between 1.6 to 2.2 volts.

Thus a battery of a rated 12 volts (6 cells) might have a voltage range of between 9.6 to 13.2 volts on load.

These batteries are normally charged by battery chargers of the constant voltage type i.e. the voltage applied to the battery remains constant at a chosen level between 2.3 and 2.5 volts per cell (13.8 volts to 15.0 volts for a 12 volt battery). The battery current falls during charging until it reaches a low steady value when the battery is fully charged.

Nickel-cadmium batteries have a voltage range between 1.00 and 1.25 volts per cell. Thus a 12 volt battery (10 cells) would have a voltage range of 10.0 to 12.5 volts on load. Battery chargers for nickel-cadmium cells are, in contrast to those for lead-acid cell charging, normally of the constant current type i.e. the current passed through the battery remains at a constant rate which allows full charging of the battery, typically, for example, in 8 to 1 6 hours. Under these conditions, the cell voltage ranges between 1.3 volts and 1.45 volts per cell (13.0 to 14.5 volts for 12 volt battery) where the temperature is 200C.Because the nickel-cadmium cell has a negative temperature coefficient of for example 4mV per OC, the above figures would become 13.8 volts to 15.3 volts at 00C and 12.2 volts to 13.7 volts at 40 C.

In many circumstances, equipment of various sorts is supplied complete with lead-acid cells which have a relatively short life as compared to the nickel-cadmium cells but are very much cheaper. In order to overcome the difficulty which is experienced with lead-acid cells as to life, many purchasers of equipment decide to replace the lead-acid cells with nickel-cadmium cells, particularly at a time when the lead-acid cells fail.

There is however the difficultly that the owner will often have charging apparatus for the lead-acid cells or such charging apparatus may well be built into the equipment itself. Unfortunately it is not possible safely to charge nickel-cadmium batteries using the chargers normally used for lead-acid batteries. If a 12 volt nickel-cadmium battery is connected to a constant voltage charger intended for lead-acid batteries, at low temperature the nickel-cadmium battery will take an excessively long time to charge. For example, if the charger voltage is 14.5 volts and the nickel-cadmium battery is at 00C it is necessary for the voltage to rise to 1 5.3 volts at the end of the charging period.

Since however the maximum voltage supplied is limited to 14.5 volts by the charger, the charging current is reduced to a trickle when the battery is -only partially charged.

On the contrary however, if the battery is at 400C, there is an excessive current through the battery as the voltage of the battery at this temperature rises to only 13.7 volts at the end of the charge at a safe charging current. Unless careful monitoring is undertaken, the charging current is allowed to continue and will damage the battery.

In a third circumstance, if the battery is at 200 Centigrade the voltage at the end of the charge is 14.5 volts so that in this case apparently only a safe overcharge current flows. However, the effect of this overcharging current is to raise the temperature of the battery. This causes the current to increase, further raising the temperature and resulting in thermal runway which inevitably damages the battery.

Thus up to this time an attempt to replace leadacid cells by nickel-cadmium cells without the necessity of either having the equipment redesigned to include a different form of charger or the necessity of buying a completely new charging apparatus has not been successful.

The present invention seeks to provide a battery arrangement in which batteries which normally require a constant current type of charger for charging can be charged on a constant voltage type of charger safely and successfully.

According to the invention, a battery arrangement comprises a plurality of cells wherein the cells are divided into two parts and switching means are provided for selectively connecting the two parts in series for use as a source and in parallel for charging purposes.

Preferably the battery arrangement comprises nickel-cadmium cells.

In addition to the switching arrangement of the battery, it is desirable if the number of cells provided in the arrangement exceeds the number of cells called for by the rated voltage by at least one cell.

A current limiting device may be provided for each part of the arrangement which is operable only when the parts are connected in parallel.

The invention will now be described in greater detail, by way of example, with reference to the drawings, the single figure of which shows schematically one embodiment of a circuit arrangement for a battery arrangement in accordance with the invention.

As shown in the drawing, the battery 10 which is intended to represent a 12 volt nickel-cadmium battery, normally having 10 cells, is provided with 11 cells in all. The provision of the additional cell is a safety factor which will avoid the possibility of thermal runaway even at 400 C. At the end of the charge the battery voltage is of 15.1 volts which is higher than the proposed charger voltage of 14.5 volts and in this way the heating up of the battery is avoided. The additional cell of course has the advantage of allowing 10% more energy storage in the battery. As can be seen, the cells of the battery are divided into two parts 11 and 12 which are as nearly as possible equal, in this case 5 cells in the part 11 and 6 cells in the other part 12.

A two part change-over switch 13 is provided which enables the cell parts 11 and 12 to be connected either in series or parallel. Thus when the cells are connected in series (as shown) they provide an output of 14.6 volts (00 Centigrade) at the terminals 14 and 15 which is within the range normally specified for loads for this type of battery.

In the position of the switch 13 which is opposite to that shown in the drawing, the two parts 11 and 12 of the battery arrangement 10 are connected in parallel and furthermore each of the parts is provided with a limiting resistor 16.

The voltage ranges over the part arrangements 11 and 12 are 6.1 volts to 7.7 volts and 7.3 volts to 9.2 volts respectively for all states of charge between 0 Centigrade and 400 Centigrade. With the charger voltage typically at 14.5 volts this gives sufficient head voltage to allow the current limiting resistor 16 to be placed in series with each battery part. Ideally there would be a true constant current circuit instead of the resistor 1 6 so that the charging current for each battery part 11 or 12 would be independent of the part battery voltage and also of the charger voltage. However the type of current limiting element necessary to achieve this is expensive and in practice a simple resistance 16 as shown can be used to give a reasonably constant current and to avoid thermal runaway and damage due to overcharging.

Also provided in the charging circuit are diodes 17 in series with the battery parts 11 and 12 and with current limiting resistors 16. These diodes 17 are used to prevent the battery parts 11 and 12 from discharging into each other or into a load if the unit is not connected to a charger.

Furthermore a current flow indicator can be provided in at least one of the two branches of the battery parts and this may suitably be formed by a light emitting diode 18.

It will be seen that the arrangement in accordance with the invention as disclosed in the above embodiment provides not only for adequating charging but provides for a complete protection of the cells in the event that the switch is placed in the wrong position for the particular function required. Thus in the case where the switch is in the charging position and the batteries are connected to a load, the diodes (17) will prevent the battery parts from discharging into the load and no damage will be done to the battery. In the case of an attempt to charge the batteries when the switch is in the load position, the additional cell will prevent thermal runaway and while the battery will not charge up properly in this state, nevertheless no harm will be done to the battery itself.

It is to be understood that various modifications may be made to the above described arrangement without departing from the scope of the invention.

For example, other forms of current limiting arrangement may be provided and more complicated forms may be used such that a true constant current circuit is formed. While the embodiment has described the arrangement with cells forming a 12 volt battery, it will be appreciated that the same arrangements could be used for providing other battery voltages, e.g. 6 volts. The manually operable change-over switch could be replaced by an automatically operated switch in which change-over occurs on connection of the charger either as a result of the applied current or voltage or combination of current and voltage or as a mechanical effect produced by the physical connection of the charger. In certain circumstances, electronic switching, electromagnetic switching or other types of switching could be used. The configuration of the series/parallel switching circuit could also be different from that shown.

If automatic switching is used, the extra cell may no longer be necessary to prevent overcharging, because the action of the automatic switch would be such that the battery cannot be charged in the series configuration.

Claims (10)

1. A battery arrangement comprising a plurality of cells wherein the cells are divided into two parts and switching means are provided for selectively connecting the two parts in series for use as a source and in parallel for charging purposes.

2. An arrangement as claimed in claim 1, wherein the battery arrangement comprises nickel-cadmium cells.

3. An arrangement as claimed in claim 1 or 2, wherein the number of cells exceeds the rated voltage of the arrangement by at least one cell.

4. An arrangement as claimed in claim 1,2 or 3, wherein each part of the arrangement is provided with a current limiting device operable only when the parts are connected in parallel.

5. An arrangement as claimed in claim 4 wherein a diode is provided in series with each current limiting device.

6. An arrangement as claimed in claim 4 or 5, wherein a current flow indicator is provided in series with at least one of the current limiting devices.

7. An arrangement as claimed in claim 6, wherein the current flow indicator comprises a light emitting diode.

8. A method of charging a battery arrangement comprising dividing the cells of the battery arrangement into two parts and connecting the parts in series for use as a source and in parallel for charging.

9. A battery arrangement substantially as described herein with reference to the drawing.

10. A method of charging a battery arrangement substantially as described herein with reference to the drawing.