GB1569618A - Automatic electric battery charging apparatus - Google Patents

Description

(54) AUTOMATIC ELECTRIC BATTERY CHARGING APPARATUS (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 thata 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 electric battery charging apparatus and is concerned particularly with such apparatus in which charger terminals are connected through leads to the battery for example by a plug, and it is desirable that if the leads are disconnected from the battery, there should be no possibility of their remaining live.

According to the present invention an electric battery charger includes means responsive to charging current for switching off the supply when the charging current falls below a predetermined value, inhibiting means immediately responsive to battery voltage applied to the charger terminals for inhibiting such switching off, and means responsive, after a delay, to battery voltage applied to the charger terminals, for cancel ling the effect of the inhibiting means.

Thus a current-responsive means is a very convenient way of switching off the supply when a charger plug is withdrawn or the battery is otherwise disconnected from the charger. Some means must however be provided to enable the charger to be switched on again when charging is to be resumed, and the invention provides a means whereby this occurs automatically as soon as the charger is connected to the same battery or to another battery.

The means for switching off the supply may include a comparator comparing a signal representing charging current with a ref erence signal, and a programmable unijunction transistor having the output of the comparator applied to its anode, both these components being fed from the part of the charger supply that is not switched off. The inhibitor means may be arranged to by-pass the comparator output so as to inhibit con duction of the p.u.t. and may comprise an inverter to which a signal dependent on battery voltage is applied, and a transistor to which the inverted output is applied. The battery voltage may be applied to the inverter through an RC network providing the delay.

Conveniently the output of the comparator is also applied to the p.u.t. through an RC network.

Further features and details of the invention will be apparent from the following description of one specific embodiment, taken in conjunction with the accompanying drawing, which is a diagram of part of the circuit of an automatic battery charger.

The embodiment to be described may be employed in association with a wide variety of automatic chargers. It is particularly applicable to a charger such as that described in the present applicants' co-pending British patent specification No. 9393/76, (Case EPS 181). In that arrangement the charging circuit incorporates thyristors which serve not only to control the magnitude of the charging current during the charge, but also serve to switch off the charge when the battery is fully charged. The circuit in accordance with the present invention switches off the supply by preventing firing of the thyristors by delivering a low digital signal to an input of the charger. The present invention, and indeed the specific embodiment to be described, may also be applied to other forms of charger including those employing electromagnetic contactors to switch off the charging current.

The circuit in accordance with the invention includes a comparator in the form of an operational amplifier 10 which compares a current signal voltage, which appears across a small resistance in series with the battery being charged, with a low value reference voltage, in this case about 60 millivolts. This is equivalent to about 32 amperes charging current in the case of a charger rated at 100 amperes. When the charging current falls to 3t amperes or less the output of the comparatar 10 changes from a low state 0 to a high state 1.

The comparator output is connected through a diode 11 and a resistor 12 to the anode of a programmable unijunctibn transistor 13 which is also connected through a series pair of resistors 14 and 15 to a positive supply terminal and through a capacitor 16 to the negative supply terminal.

The cathode of the p.u.t. 13 is also connected through a small resistor 17 to the negative supply terminal whilst the gate is connected by a potential divider 18 to the respective supply terminals and is also connected to the input of a transistor amplifier 19 the output of which provides the signal to the charger for switching off the charging current.

The supply referred to is derived from a point in the charging supply that remains live when the charging current is switched off by the main thyristors. A further supply derived from the battery through the charger terminals is applied to a series resistorcapacitor network 20, and forms the supply for a logic inverter 21 having its input connected to the tapping of the resistor-capacitor network 20 and its output connected to the base of a transistor 22 connected between the common negative supply terminal and the junction of the resistors 14 and 15 connecting the anode of the p.u.t.

13 to the positive terminal of the mainsderived supply.

Accordingly in operation it will be appreciated that when the p.u.t. 13 conducts the output signal to the charger will be low and the main thyristors will stop the charging current. During normal charging the charging current will be very much greater than that required to balance the reference signal, and the current through the resistors 14 and 15 will be by-passed through the diode 11 and the comparator 10 so that the p.u.t. 13 will remain non-conducting, The output signal to the charger will then be in a high state and will not disable the thyristor firing circuits.

As soon as the battery is disconnected the charging current will cease, the output of the comparator 10 will go high and the capacitor 16 will charge up to the trigger voltage and switch the p.u.t. on. The gate voltage will accordingly fall and the output signal to the charger will be in a low state, stopping the main thyristor gate pulses and switching the charger off.

Certain further conditions must however also be considered. If a battery is still connected and a failure of the mains supply should occur, the supply to the circuit from the mains supply will go to zero and if the p.u.t. 13 was conducting it will cease to do so.

If and when the mains supply is restored, with a battery still connected, the gate of the p.u.t. 13 will immediately receive potential but the anode voltage will rise only gradually due to its RC network 14, 15 and 16. In these circumstances the output to the charger goes high and allows a period of about 2 seconds for charging current to be established. When the charging current exceeds 3t amperes the comparator output goes low so as to reduce and/or keep down the potential of the p.u.t. 13 anode and its capacitor 16 so that charging continues normally.

If on the other hand at this time there is no battery connected, no charging current flows so that after a period of some two seconds the main thyristor firing pulses are stopped and the charger output terminals become electrically dead.

The inverter 21 fed from the battery, through a resistor capacitor network, is provided to cater for the circumstance that the mains supply is switched on before a battery is plugged in. In these circumstances the power supply to the p.u.t. circuit is established immediately and because no battery charging current can flow the p.u.t. 13 switches on after some two seconds delay and stops the thyristor firing pulses, so that the battery connection terminals become dead. The transistor across the RC network 14 and 16 is not conducting under these conditions since its base drive is supplied via the inverter circuit 21 from the battery which is as yet disconnected.

When the battery is reconnected, power is applied to the inverter 21 and its RC network 20 giving an initial 'low' signal to the inverter input and a 'high' state at its output. This causes the transistor 22 to conduct temporarily so as to drain current from the tapping of the p.u.t. RC network 14, 15 and 16 thereby causing the p.u.t. 13 to switch off and start up the thyristor firing pulses. Charge current is thus initiated and normal charging continues. The potential of the input to the inverter 21 and its capacitor rises within a couple of seconds so that the output of the inverter goes low, the transistor 22 cuts off and thereafter its circuit performs no function.

It will accordingly be appreciated that a charger equipped with a circuit as described eliminates any danger of the charging plug or leads being disconnected from the battery and short-circuited while live, whether or not users are encouraged to switch off the supply before disconnecting the battery and only switch it on again after a battery has been connected.

WHAT WE CLAIM IS:- 1. An electric battery charger including means responsive to charging current for switching off the supply when the charging current falls below a predetermined value, inhibiting means immediately responsive to battery voltage applied to the charger ter

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

Claims (6)

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

   The comparator output is connected through a diode 11 and a resistor 12 to the anode of a programmable unijunctibn transistor 13 which is also connected through a series pair of resistors 14 and 15 to a positive supply terminal and through a capacitor 16 to the negative supply terminal.

   The cathode of the p.u.t. 13 is also connected through a small resistor 17 to the negative supply terminal whilst the gate is connected by a potential divider 18 to the respective supply terminals and is also connected to the input of a transistor amplifier 19 the output of which provides the signal to the charger for switching off the charging current.

   The supply referred to is derived from a point in the charging supply that remains live when the charging current is switched off by the main thyristors. A further supply derived from the battery through the charger terminals is applied to a series resistorcapacitor network 20, and forms the supply for a logic inverter 21 having its input connected to the tapping of the resistor-capacitor network 20 and its output connected to the base of a transistor 22 connected between the common negative supply terminal and the junction of the resistors 14 and 15 connecting the anode of the p.u.t.

   13 to the positive terminal of the mainsderived supply.

   Accordingly in operation it will be appreciated that when the p.u.t. 13 conducts the output signal to the charger will be low and the main thyristors will stop the charging current. During normal charging the charging current will be very much greater than that required to balance the reference signal, and the current through the resistors 14 and 15 will be by-passed through the diode 11 and the comparator 10 so that the p.u.t. 13 will remain non-conducting, The output signal to the charger will then be in a high state and will not disable the thyristor firing circuits.

   As soon as the battery is disconnected the charging current will cease, the output of the comparator 10 will go high and the capacitor 16 will charge up to the trigger voltage and switch the p.u.t. on. The gate voltage will accordingly fall and the output signal to the charger will be in a low state, stopping the main thyristor gate pulses and switching the charger off.

   Certain further conditions must however also be considered. If a battery is still connected and a failure of the mains supply should occur, the supply to the circuit from the mains supply will go to zero and if the p.u.t. 13 was conducting it will cease to do so.

   If and when the mains supply is restored, with a battery still connected, the gate of the p.u.t. 13 will immediately receive potential but the anode voltage will rise only gradually due to its RC network 14, 15 and 16. In these circumstances the output to the charger goes high and allows a period of about 2 seconds for charging current to be established. When the charging current exceeds 3t amperes the comparator output goes low so as to reduce and/or keep down the potential of the p.u.t. 13 anode and its capacitor 16 so that charging continues normally.

   If on the other hand at this time there is no battery connected, no charging current flows so that after a period of some two seconds the main thyristor firing pulses are stopped and the charger output terminals become electrically dead.

   The inverter 21 fed from the battery, through a resistor capacitor network, is provided to cater for the circumstance that the mains supply is switched on before a battery is plugged in. In these circumstances the power supply to the p.u.t. circuit is established immediately and because no battery charging current can flow the p.u.t. 13 switches on after some two seconds delay and stops the thyristor firing pulses, so that the battery connection terminals become dead. The transistor across the RC network 14 and 16 is not conducting under these conditions since its base drive is supplied via the inverter circuit 21 from the battery which is as yet disconnected.

   When the battery is reconnected, power is applied to the inverter 21 and its RC network 20 giving an initial 'low' signal to the inverter input and a 'high' state at its output. This causes the transistor 22 to conduct temporarily so as to drain current from the tapping of the p.u.t. RC network 14, 15 and 16 thereby causing the p.u.t. 13 to switch off and start up the thyristor firing pulses. Charge current is thus initiated and normal charging continues. The potential of the input to the inverter 21 and its capacitor rises within a couple of seconds so that the output of the inverter goes low, the transistor 22 cuts off and thereafter its circuit performs no function.

   It will accordingly be appreciated that a charger equipped with a circuit as described eliminates any danger of the charging plug or leads being disconnected from the battery and short-circuited while live, whether or not users are encouraged to switch off the supply before disconnecting the battery and only switch it on again after a battery has been connected.

   WHAT WE CLAIM IS:- 1. An electric battery charger including means responsive to charging current for switching off the supply when the charging current falls below a predetermined value, inhibiting means immediately responsive to battery voltage applied to the charger ter

   minals for inhibiting such switching off, and means responsive, after a delay, to battery voltage applied to the charger terminals, for cancelling the effect of the inhibiting means.
2. 2. Apparatus as claimed in Claim 1 in which the means for switching off the supply includes a comparator comparing a signal representing charging current with a reference signal, and a programmable unijunction transistor having the output of the comparator applied to its anode, both these components being fed from the part of the charger supply that is not switched off.
3. 3. Apparatus as claimed in Claim 2 in which the inhibitor means are arranged to by-pass the comparator output so as to inhibit conduction of the p.u.t. and comprise an inverter to which a signal dependent on battery voltage is applied, and a transistor to which the inverted output is applied.
4. 4. Apparatus as claimed in Claim 3 in which the battery voltage is applied to the inverter through an RC network providing the delay.
5. 5. Apparatus as claimed in any one of Claims 2 to 4 in which the output of the comparator is also applied to the p.u.t.

   through an RC network.
6. 6. An electric battery charger including means for controlling the switching on and off of charging current, the controlling means being substantially as herein described, with reference to the accompanying drawing.