GB2086674A - Battery charging apparatus - Google Patents

Abstract

The Battery charging apparatus includes a charger 11 consisting of a transformer 12 and a rectifier 13. A controller 16 includes a microprocessor providing a timer and a cell voltage level detector. The battery is charged for a predetermined length of time or until a predetermined voltage is reached in a first phase, followed by a second phase, the duration of which is adjusted by the timer in dependence upon the length of the first phase. Alternatively, or additionally, the controller may initiate a preliminary charge if the predetermined voltage in the first phase is reached within a set, short, time before returning the charging cycle to the first phase. <IMAGE>

Description

SPECIFICATION Battery charging apparatus The present invention relates to a battery charging apparatus and seeks to provide an improved system of battery charging which automatically adjusts the charging cycle in dependence upon the initial state of the battery to be charged. The present invention is particularly, but not exclusively, applicable to the charging of lead-acid batteries for traction purposes.

Battery charging systems are known having a first charging phase in which a discharged battery is charged until a predetermined battery cell voltage is attained and then continuing the charge in a second phase which lasts for a predetermined length of time to ensure that the battery is fully charged and that the charge in all the cells is equalised.

The predetermined cell voltage referred to above in lead-acid batteries is typically 2.37 volts, the voltage at which gassing, i.e. the emission of hydrogen, occurs, but if charging is terminated immediately this voltage is reached, the battery is not completely charged and hence the second charging phase is provided. It is also known to provide a further charging phase, after the second phase of the main charge, which consists of time spaced equalising pulses of charging current which ensure that the battery and the individual cells are all fully charged. Such a charging system is disclosed in our earlier Patent No.

1456226. The above systems work extremely well for charging batteries having a uniform state of discharge such as traction batteries of milk floats which are used on a regular run each day and charged overnight.

The state of dicharge of the battery after a regular run is fairly constant and therefore the battery charging apparatus can be set up to charge them satisfactorily. However, the known systems have the disadvantage that they may overcharge a battery which is only partially discharged, with the consequent risk of damage. If a partially discharged lead-acid battery is charged by the automatic system described above, the gassing voltage of 2.37 volts is reached quite quickly but the battery is then subjected to the full length of the second charging phase. If a battery is only partially discharged the length of time required in this second phase is very much shorter than for a fully discharged battery and therefore there is the risk that overcharging and a consequent risk of damage to the battery may be caused.

The present invention seeks to provide battery charging apparatus which will overcome, or least minimise, this known disadvantage.

According to one aspect of the present invention there is provided battery charging apparatus including a battery charger connectable to a battery to be charged, and control means having a timing means and a battery cell voltage level detector, the control means, on initiation of a charging cycle, connecting the charger to the battery to be charged for the first charging phase lasting for a predetermined length of time or until a first predetermined battery voltage is reached, and then for a second phase of charging lasting for a predetermined set period of time, the timing means measuring the length of time of the first charging phase and adjusting the length of time of the second charging phase in dependence upon the length of time of the first phase.

Preferably, if the first predetermined voltage is not reached within said predetermined length of time, the charging cycle is terminated.

In a preferred embodiment of the invention, the maximum time of charging during the first phase is 11 hours, but if the first phase lasts for less than 5 hours, the second phase of charging is reduced in steps of 1/12 of the said set period, to a minimum of 4/1 2 of the set period. Preferably,the minimum period of 4/1 2 of the set period obtains if the first phase of charging lasts for less than 1 hour.

If a battery is highly discharged, having for example an initial voltage of less than 1.7 volts per cell, the battery has an unusally high impedance at the start of charging and this can cause the first predetermined voltage to be reached almost immediately. in these circumstances the charging apparatus would switch to the second phase of charging immediately with the result that at the end of the charging cycle the battery would still be only partially charged. The present invention alsoseeks to provide means to obviate this disadvantage.

According to the second aspect of the present invention there is provided battery charging apparatus including a battery charger connectable to a battery to be charged, and control means having a timing means and a battery cell voltage level detector, the control means, on initiation of a charging cycle, connecting the charger to the battery to be charged for a first charging phase lasting for a predetermined length of time and then for a second phase of charging, the control means being such that if the voltage level detector detects an initial battery cell voltage of less than a predetermined lower valve and the first charging phase lasts for less than a certain period of time, a precharge cycle is initiated in which a prliminary charge is applied to the battery for a certain length of time before the first phase of charging is re-initiated.

Apparatus constructed in accordance with the second aspect of the invention may be incorporated in apparatus constructed in accordance with the first aspect described above but may also be incorporated in the known charging systems such as that described in our earlier Patent No. 1456226.

A preferred embodiment of the present invention will now be described by way of example with refernce to the accompanying informal drawings in which: Figure 1 shows a schematic circuit of charging apparatus in accordance with the present invention and, Figure 2 shows a diagram illustrating the principle of operation of the apparatus shown in Fig. 1.

Figure 3 shows a wiring diagram of the block circuit shown in Fig. 1 and.

Figure 4 shows a hardware block diagram of the controller shown in Figs. 1 and 3.

Referring now to Figs. 1 and 3 a battery 10 to be charged is connected to a charger 11 which consists of a transformer 1 2 and a rectifier 1 3. The rectifier 1 3 typically consists of a fullwave rectification rectifier bridge circuit. The charger is connected to a main power (a.c.) supply by way of a switch assembly 1 4. A current shunt 1 5 is connected between the charger 11 and the battery 10.

The switch 14 may, as shown in Fig. 3, consist of two contactor assemblies 1 4a, 1 4b arranged, in a known battery charging process, to give, selectively, two charging rates.

A controller 1 6 includes a micro-processor (Fig. 4) providing a timer and a cell voltage level detector circuit. The controller is connected to the switch assembly 14, the charger 11, the current shunt 1 5 and the battery 10.

The apparatus also includes a security fuse 1 7 which is connected between the main a.c.

power supply and the controller 16, a resistance 18 and a transformer 1 9 being in circuit between the fuse and the controller.

The security fuse 1 7 is arranged to rupture to disconnect the apparatus from the main supply in the event of failure in either the controller 1 6 or in the charging current supply.

Controller failure is detected by a watch dog timer 20 (Fig. 4). If the watch dog timer 20 is not reset by the micro-processor at intervals not exceeding twenty milliseconds, the watch dog timer activates a relay 20a causing the security fuse 1 7 to rupture. Similarly, the controller 16 monitors the charging current flowing through the current shunt 1 5. If current flows when no charging should be taking place the security fuse 1 7 is ruptured. To prevent current overload, if a current greater than 150% of full rated load flows through the current shunt 15, the charger is switched off.

The controller 1 6 includes a bank of four binary weighted switches by which the desired time for the second phase of charging may be set in steps of 2 hour over the range 0-73 hours.

Referring now to Fig. 4, which shows in block diagram form the main components in the controller 1 6. The main functions of the controller 1 6 are controlled by a microprocessor 21. The controller also includes a selector 22 to which the battery is connected and which is capable of being set to accommodate the particular size of battery e.g. 1 2 volt or 24 volt being charged. The battery cell voltage, and the reference voltages are compared in a comparator 23. Similarly, the charging current is sensed by an anologue to digital convertor 24 connected to the micro-processor 21.

Referring now to Fig. 2 there is shown a diagram of a charging cycle of a lead-acid traction battery in which the battery voltage is plotted against the length of time of charge.

The voltage of a normally discharged traction battery at commencement of charge is typically 2.1 volts per cell and during a first phase of charging, extending from point A to point B on the time scale, the cell voltage rises up to the voltage at which gassing occurs, typically 2.37 volts, at which point the first phase of charging is terminated, typically after a maximum of nine hours. The length of this first phase is monitored so that it cannot exceed a predetermined maximum number of hours, typically eleven. If the cell voltage does not reach 2.37 volts within this time, this is indicative of a damaged or failed battery and the charging cycle is terminated and the charger is switched off. When the cell voltage reaches 2.37 volts at which gassing of the cells starts, the charging cycle switches to the second phase of charging which lasts from point B to point C.The time taken for the battery to reach 2.37 volts is measured by the timing device in the controller 1 6. If the time is greater than 5 hours, the full second phase of charging is carried out. Typically the second phase lasts for three hours but is set to a desired time in steps of > hour over the range of 0-7 hours by the binary switches in the controller 1 6. If the time taken in the first phase for the cell voltage to reach 2.37 volts lasts for less than 5 hours, the second phase or gassing charge, period is reduced in steps of 1/12 of the set period (e.g. 3 hours) to a minimum of 4/12 of the period. This minimum period (which in the example given is 4/1 2 of 3 hours) is used when the first phase of charge lasts for less than 1 hour.

At the end of the second phase of charging the charge cycle is switched to an equalisation charging phase in which a 2 minute charge is applied to the battery at 1 5 minute intervals if the battery voltage has fallen below a preset threshold voltage of 2.17 volts at the end of the 1 5 minute time interval. The cell voltage at the end of the second phase of charging is typically between 2.4 and 2.8 volts dependent upon the age and condition of the cell.

The apparatus also includes means far detecting a highly discharged battery, which has ;In unusually high impedance at the start of c:l;ii!liri!l. The diagram of Fig. 2 shows in dotted outline the voltage pattern when a highly discharged battery is charged. In such batteries the initial battery cell voltage is less than 1.7 volts and because of the high impedance, the gassing voltage, which normally occurs at the end of the first phase, of 2.37 volts is reached almost immediately, typically in less than 5 minutes. The controller is therefore provided with means to detect when the initial battery cell voltage is less than 1.7 volts and if this is detected and a cell voltage of 2.37 volts is reached within 5 minutes of the start of charging, an additional preliminary charge is inserted in the charging cycle. In the preliminary charge a precharge is applied at the full rate for half an hour before the normal first phase of charging reinstituted, The initiation of charging starts automatically when the apparatus is switched on when the battery voltage is between nominal values of 1.7 and 3.0 volts per cell. If the initial battery voltage is between 1.3 and 1.7 volts per cell, charging starts only if an override switch is operated manually. The use of the term voltage level has been used with reference to the voltage of individual cells but equally may refer to the voltage of a battery consisting of a plurality of cells which provide e.g. a 1 2 volt or 24 volt battery.

Although described with reference to leadacid batteries it will be appreciated that the apparatus could be readily adapted to charge other types of battery such as Nickel-Cadmium batteries.

Claims (12)

1. Battery charging apparatus including a battery charger connectable to a battery to the charged, and control means having a timing means and a battery cell voltage level detector, the control means, on initiation of a charging cycle, connecting the charger to the battery to be charged for a first charging phase lasting for a predetermined length of time or until a first predetermined battery voltage is reached, and then for a second phase of charging lasting for a predetermined set period of time, the timing means measuring the length of time of the first charging phase and adjusting the length of time of the second charging phase in dependence upon the length of time of the first phase.

2. Battery charging apparatus including a battery charger connectable to a battery to be charged and control means having a timing means and a battery cell voltage level detector, the control means, on initiation of a charging cycle, connecting the charger to the battery to be charged for a first charging phase lasting for a predetermined length of time and then for a second phase of charging, the control means being such that if the voltage level detector detects an initial battery cell voltage of less than a predetermined lower value and the first charging phase lasts for less than a certain period of time, a precharge cycle is initiated in which a preliminary charge is applied to the battery for a certain length of time before the first phase of charging is re-initiated.

3. Battery charging apparatus as claimed in claim 2 wherein on re-initiation of the first charging phase, the control means is such that the first charging phase lasts for a prederermined length of time or until a first predetermined battery voltage is reached, the timing means measuring the length of time of the first charging phase and adjusting the length of time of the second charging phase in dependence upon the length of time of the first charging phase.

4. A method of charging a battery including the steps of charging the battery in a first charging phase lasting for a predetermined length of time or until a first predetermined battery voltage is reached, measuring the length of time of the first charging phase then charging the battery in a second charging phase lasting for a set period of time and adjusting the length of said set period of time in dependence upon the length of time of said first charging phase.

5. A method of charging a battery in a charging cycle including the steps of detecting the initial battery voltage, charging the battery in a first charging phase lasting for a predetermined length of time or until a predetermined battery voltage is reached, measuring the length of time of the first charging phase, and if the initial voltage is less than a reference voltage and the length of time in the first, charging phase to reach said predetermined battery voltage is less than a reference time, terminating the charging cycle and initiating a precharge cycle in which a preliminary charge is applied to the battery for a certain length of time before re-initiating the first charging phase, the charging cycle including a second charging phase carried out after completion of the first charging phase.

6. A method as claimed in claim 5, wherein the length of time for completion of the first charging phase, after re-initiation thereof, is- determined and the length of time of the second charging phase is adjusted in dependence upon the length of time of the first charging phase.

7. A method as claimed in claim 4, 5 or 6 wherein the charging cycle is terminated if said predetermined battery voltage is not reached within said predetermined length of time.

8. A method as claimed in any one of claims 4 to 7 wherein the maximum time of charging during the first charging phase is eleven hours.

9. A method as claimed in any one of claims 4 to 8 wherein the maximum time of charging during the second charging phase is set at three hours.

10. A method as claimed in claim 8 or 9 wherein the time of the second phase of charging is reduced in steps of 1/12 of the set period, if the time of the first phase lasts for less than five hours.

11. A method as claimed in claim 10 wherein a mimimum period of charging for the second phase of 4/12 of the set period obtains if the first phase lasts for less than one hour.

12. A method of charging a battery substantially as described herein with reference to, and as illustrated in Figs. 1 to 4 of the accompanying drawings.

1 3. Battery charging apparatus substantially as described herein with reference to, and as illustrated in, Figs. 1 to 4 of the accompanying drawings.