GB1564691A

GB1564691A - Dual voltage battery powered electric systems

Info

Publication number: GB1564691A
Application number: GB10132/78A
Authority: GB
Original assignee: Chloride Group Ltd
Current assignee: Chloride Group Ltd
Priority date: 1978-03-14
Filing date: 1978-03-14
Publication date: 1980-04-10

Description

(54) DUAL VOLTAGE BATTERY POWERED ELECTRIC SYSTEMS (71) We, CHLORIDE GROUP LIMITED, a Company registered under the laws of England, of 52 Grosvenor Gardens, London SW1W OAU, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to dual-voltage battery-powered electric systems.

For many purposes it is desirable to provide a battery, for example of 48 or 72 volts, to supply, for example, motive power for d vehicle, but to employ a lower voltage such as 12 volts for auxiliary purposes. Thus the high voltage circuit is commonly desirable for motive power of delivery vehicles whilst the auxiliaries, such as lights, horn, turn indicators, wipers etc., are commercially available from mass-production sources requiring 12 volt operation.

In such circumstances it is common practice to tap the battery at 12 volts to power these auxiliaries, but this results in the tapped portion discharging to a greater depth than the rest of the battery; on subsequent recharge, either the 12 volt portion is still not fully recharged when the remainder of the battery is properly recharged, or the remainder has ,to be overcharged to bring the tapped portion up to full charge. Either practice can be harmful to the battery.

According to the present invention a dualvoltage battery powered electric system includes a high-vokage circuit powered directly by a multi-cell electric storage battery, means for charging the battery, a low-voltage circuit powered directly by selected cells of the battery, and means powered by the battery for charging the said selected cells.

Preferably the means for charging the said selected cells will be powered by the remaining cells of the battery.

The means for charging the selected cells may include a DC/DC converter, conveniently of semi-conductor type.

It has previously been proposed to circulate the tapped portion of the battery from one group of cells to another to even up the overload/overcharge. This reduces but does not eliminate the harmful effects.

Another solution is to power the auxiliaries from a DC/DC converter, which changes the (say) 72 volt supply to a 12 volt supply. This removes the problem, but with the increasing load of auxiliaries demands a unit capable of handling a high peak load, and this is not cheap.

Many of the auxiliary loads are intermittentturn indicators, horn, for instance, and so represent a load of very much lower average power than peak power. However, because of the very short thermal time constant of the semi-conductor devices used in the converter, the devices have to be rated for peak power. Also, if the converter fails, the auxiliaries are put out of action.

In accordance with the invention the high loads and high in-rush currents are supplied by the selected cells and the charge withdrawn is restored more slowly by the converter. Consequently, the converter need only be rated for the lower, average power drawn by the auxiliaries, which makes it a cheaper unit; furthermore, failure of the converter does not leave the auxiliaries powerless.

The means for charging the selected cells may be responsive to the ratio of the voltage of those cells to that of the remaining cells (or of the battery as a whole), so as to effect charging when the ratio falls below a given value. For example a potential divider may be connected across the whole battery, the charge of the selected cells being controlled by a voltage comparator comparing the voltage of the tapping of the potential divider with that of the junction between the selected cells and the remaining cells. These points may be connected respectively to the inputs of a differential amplifier controlling an inverter charging the selected cells through a transformer and a rectifier and powered by the remaining cells.

It is well-known that the voltage of a battery is in general an unsatisfactory criterion for terminating a charge, since it varies with many conditions other than rhe state of charge, such as the age of the battery, its condition and temperature, and, of course, the current put into it or taken out of it. In the present case, however, all these factors affect the selected cells and the remaining cells equally and hence the ratio of voltage should normally provide a reliable criterion of their relative state of charge. Any current in the high voltage circuit, whether discharging charging, will flow equally through the selected cells and the remaining cells. If any discharge current flows in the low-voltage circuit it will be discharging the selected cells and hence it will be appropriate to charge those cells.

In a typical embodiment of this invention, a stepdown DC/DC converter is arranged to charge the 12 volt section of the battery from the 72 volt whole. Current limit is provided as a protection measure to keep the currents flowing under overload or fault conditions within safe bounds. The amount of charge put into the tapped portion is controlled, to keep the voltage per cell of the tapped portion the same as that of the remainder. In this way, the state of charge of the tapped portion remains the same as that of the remainder, and all cells discharge equally (ignoring transient outof-balances caused by the intermittent loads).

The invention may be put in practice in various ways but certain specific embodiments will be briefly described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a diagram illustrating the circuit of a dual-voltage battery powered electric system in accordance with the present invention incorporating a fly-back inverter; Figure 2 is a modified construction of the part of the circuit shown in Figure 1 within the chain dotted square marked M in which the converter is of the type known as a forward inverter.

Figure 3 is a diagram similar to Figure 2 in which the flyback converter incorporates two thyristors; Figure 4 is a diagram similar to Figure 3 in which the forward converter incorporates two thyristors; Figure 5 is a further diagram similar to Figure 2 illustrating a push-pull converter; and Figure 6 is similar to Figure 5 and shows a push-pull converter incorporating two thyristors.

In each case the inverter shown in Figure 1 in the chain dotted box marked 2 has its output connected to a rectifier supplying charging current to the selected cells 20, nominally of 12 volts. The power supply to the inverter is taken from the complete battery 10 nominally of 72 volts. Alternatively the capacity of the inverter and rectifier may be reduced by supplying it from only the remaining cells so that the net charging current is the whole output from the inverter rather than being reduced by the current supplied to the inverter input.

Various known types of inverter can be employed.

In one arrangement shown in Figure 1 the inverter is of the fly-back type. This comprises a transformer 4 having a primary winding 6 connected in series with a transistor 8 across the supply i.e. the terminals of the complete battery 10. The transistor 8 is periodically turned on and off under the control of an oscillator 12 and driver circuit 14. The transformer 4 has a secondary winding 16 connected in series with a diode 18 across the selected cells 20. When the transistor 8 is switched on, current flows through the primary winding 6 and when it is switched off the fall of current induces an e.m.f. in the secondary winding 16 which is connected to pass a charging current through the diode 18 to charge the selected cells 20.

A further arrangement employing an inverter of forward type, of which the relevant portion is illustrated in Figure 2 is similar to that described above except that the secondary winding 16 is connected in the opposite direction, and a tertiary winding 22 is connected in series with a diode 24 across the supply. In this case when the transistor 8 is switched on the rise of current induces an e.m.f. in the secondary winding passing a charging current to charge the selected cells, and when the transistor is switched off the tertiary winding passes a current through the associated diode 24 to feed energy back into the supply.

In modifications, illustrated in Figures 3 and 4 respectively, of either of the arrangements described above the transistor 8 is replaced by a main thyristor 26 shunted by a capacitor 28 in series with a commutating thyristor 30, the latter being shunted by a choke 32 in series with a diode 34.

In a further arrangement illustrated in Figure 5 employing a self oscillating push-pull inverter, a transformer 32 has a primary winding 34 with a centre tapping connected to one supply terminal and each end connected to the other supply terminal through one of a pair of transistors 36. The transformer has a secondary winding 38 supplying charging current to the selected cells through a full wave rectifier 40, and a tertiary or feed-back winding 42 connected to the primary winding 44 of a saturable transformer 45. The latter has a secondary winding 46 connected between the bases of the transistors 36, each of which has a diode 48 connected between its base and emitter.

The transistors 36 switch on alternately, so that the battery voltage is applied first in one direction across one half of the primary winding 34, and then in the other direction across the other half, producing a symmetrical square wave output. The change in flux induces a base current into the base of the turned-on transistor 36 until the saturable transformer 45 saturates; at this time the transistor 36 turns off, so that the feedback transformer 45 comes out of saturation again, and turns on the other transistor 36. The diodes 48 between base and emitter allow the base drive currents to by-pass the baseemitter junction of the 'off' transistor.

In a modification of the push-pull arrangement illustrated in Figure 6 employing thyristors the tertiary winding 42 is omitted and the primary winding 34 has two tappings near opposite ends each connected to the anode of one of a pair of thyristors 50 whose cathodes are connected through a common choke 52 to the negative supply terminal. The anodes are also connected together through a capacitor 54. Each end of the primary winding 34 is connected through a diode 56 to the negative supply terminal.

An oscillator trigger circuit is connected to trigger the two thyristors alternately.

The operation of this circuit is well known and it is thought unnecessary to describe it.

Many other known types of inverter may be employed, for example, push-pull types using either transistors or thyristors, or Morgan Circuits. Moreover the inverter may be self oscillating or may incorporate a separate triggering oscillator. In each arrangement the supply to the inverter is controlled by a switching circuit controlled by a corn- parator 58 in the form of a differential amplifier. One input of the latter is connected to the tapping of a potential divider 60 connected across the supply, that is, the whole battery 10, while the other input is connected to a terminal of the selected cell 20. The latter connection includes a resistor 62 and capacitor 64 to filter out any switching voltages that could give rise to malfunctioning.

Thus when the voltage per cell of the selected cells is less than that of the battery as a whole (or less than a chosen proportion) the inverter will be switched on to charge the selected cells.

The switching circuit 66 may also be controlled to switch off the charge if the charging current exceeds a certain value. For this purpose a small resistor 68 may be inserted either in the charging circuit or in the circuit of the primary winding of the transformer, the voltage across it being employed through an overload circuit 74 as a further control over the switching circuit The use of the ratio of the voltage of the tapped portion to the voltage of the whole battery makes the operation of the recharging of the tapped portion largely independent of the main load fluctuations, or of the tem- perature or condition of the battery Load current taken by the traction motor 70 will pass through all cells, and lower their terminal voltage below the open-circuit level. Assuming the cells in the battery are reasonably well balanced, the cells in the tapped portion will fall by the same proportion as those in the rest of rhe battery, and the ratio of the tapped portion to the whole will not alter. However, the use of some auxiliary item of load 72 will increase the voltage drop of the tapped portion and cause the comparator to detect an out-of-balance voltage.

The duration of the re-charging of the tapped portion can be several hours, and may even extend beyond one day In a typical vehicle, the average load might consist of side, tail and number plate lamps, and windscreen wipers, with a power consumption of about 60 watts, and yet the peak load could be as high as 350 watts. If the converter is rated at about 60 watts, the tapped section will discharge somewhat faster than the rest of the battery during driving periods, but will be restored to a balanced state during nondriving intervals. Even if the battery is not fully balanced at the end of its day's duty, the re-charging of the tapped portion to restore the balance can continue during the recharge of the whole battery, so that the properly balanced state of charge of all cells is achieved by the time the vehicle is ready for use again with a fully-charged battery.

WHAT WE CLAIM IS: 1. A dual-voltage battery powered electric system including a high-voltage circuit powered directly by a multi-cell electric storage battery, means for charging the battery, a low voltage circuit powered directly by selected cells of the battery, and means powered by the battery for charging the said selected cells.

2. A system as claimed in Claim 1 in which the means for charging the said selected cells is powered by the remaining cells of the battery.

3. A system as claimed in Claim 1 or Claim 2 in which the means for charging the selected cells includes a DC/DC converter.

4. A system as claimed in Claim 4 in which the converter is of semi-conductor type.

5. A system as claimed in any one of the preceding Claims in which the means for charging the selected cells is responsive to the ratio of the voltage of those cells to that of the remaining cells, (or of the battery as a whole), so as to effect charging when the ratio falls below a given value.

6. A system as claimed in Claim 5 in which a potential divider is connected across the whole battery, the charge of the selected cells being controlled by a voltage comparator comparing the voltage of the tapping of the potential divider with that of the jlmction between the selected cells and the remaining cells.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. across the other half, producing a symmetrical square wave output. The change in flux induces a base current into the base of the turned-on transistor 36 until the saturable transformer 45 saturates; at this time the transistor 36 turns off, so that the feedback transformer 45 comes out of saturation again, and turns on the other transistor 36. The diodes 48 between base and emitter allow the base drive currents to by-pass the baseemitter junction of the 'off' transistor. In a modification of the push-pull arrangement illustrated in Figure 6 employing thyristors the tertiary winding 42 is omitted and the primary winding 34 has two tappings near opposite ends each connected to the anode of one of a pair of thyristors 50 whose cathodes are connected through a common choke 52 to the negative supply terminal. The anodes are also connected together through a capacitor 54. Each end of the primary winding 34 is connected through a diode 56 to the negative supply terminal. An oscillator trigger circuit is connected to trigger the two thyristors alternately. The operation of this circuit is well known and it is thought unnecessary to describe it. Many other known types of inverter may be employed, for example, push-pull types using either transistors or thyristors, or Morgan Circuits. Moreover the inverter may be self oscillating or may incorporate a separate triggering oscillator. In each arrangement the supply to the inverter is controlled by a switching circuit controlled by a corn- parator 58 in the form of a differential amplifier. One input of the latter is connected to the tapping of a potential divider 60 connected across the supply, that is, the whole battery 10, while the other input is connected to a terminal of the selected cell 20. The latter connection includes a resistor 62 and capacitor 64 to filter out any switching voltages that could give rise to malfunctioning. Thus when the voltage per cell of the selected cells is less than that of the battery as a whole (or less than a chosen proportion) the inverter will be switched on to charge the selected cells. The switching circuit 66 may also be controlled to switch off the charge if the charging current exceeds a certain value. For this purpose a small resistor 68 may be inserted either in the charging circuit or in the circuit of the primary winding of the transformer, the voltage across it being employed through an overload circuit 74 as a further control over the switching circuit The use of the ratio of the voltage of the tapped portion to the voltage of the whole battery makes the operation of the recharging of the tapped portion largely independent of the main load fluctuations, or of the tem- perature or condition of the battery Load current taken by the traction motor 70 will pass through all cells, and lower their terminal voltage below the open-circuit level. Assuming the cells in the battery are reasonably well balanced, the cells in the tapped portion will fall by the same proportion as those in the rest of rhe battery, and the ratio of the tapped portion to the whole will not alter. However, the use of some auxiliary item of load 72 will increase the voltage drop of the tapped portion and cause the comparator to detect an out-of-balance voltage. The duration of the re-charging of the tapped portion can be several hours, and may even extend beyond one day In a typical vehicle, the average load might consist of side, tail and number plate lamps, and windscreen wipers, with a power consumption of about 60 watts, and yet the peak load could be as high as 350 watts. If the converter is rated at about 60 watts, the tapped section will discharge somewhat faster than the rest of the battery during driving periods, but will be restored to a balanced state during nondriving intervals. Even if the battery is not fully balanced at the end of its day's duty, the re-charging of the tapped portion to restore the balance can continue during the recharge of the whole battery, so that the properly balanced state of charge of all cells is achieved by the time the vehicle is ready for use again with a fully-charged battery. WHAT WE CLAIM IS:

1. A dual-voltage battery powered electric system including a high-voltage circuit powered directly by a multi-cell electric storage battery, means for charging the battery, a low voltage circuit powered directly by selected cells of the battery, and means powered by the battery for charging the said selected cells.

7. A system as claimed in Claim 6 in which

the tapping of the potential divider and the junction between the selected cells and the remaining cells are connected respectively to the inputs of a differential amplifier controlling an inverter charging the selected cells through a transformer and a rectifier and powered by the remaining cells.

8. A dual-voltage battery powered electric system substantially as specifically herein described with reference to Figure 1, or Figure 1 as amended in any one of Figures 2 to 6, of the accompanying drawings.