Method in charging a battery powered device and apparatus for working the method

A method in charging a battery powered device (10) having a built-in charge control circuit (13) controlled by the device, and an apparatus for working the method, including said device. The device including the battery (11) is connected to a battery charger (17), and initially when the battery voltage is too low to initiate normal charging of the battery over the charge control circuit, the battery is connected to the battery charger via a current generator (25) which is switched off when the battery voltage has increased to a level which is high enough to support normal charging of the battery over the charge control circuit.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a method in charging a battery powered device and
 an apparatus for working said method.
 2. Description of the Prior Art
 If the battery in prior art battery powered devices having a built-in
 charge control circuit controlled by the device, and a switch for
 connecting the device including the battery to a battery charger, has been
 discharged to such extent that the voltage thereof is too low to feed the
 device and to activate the charge control circuit thereof in order to
 initiate normal charging of the battery over said circuit when the device
 is connected to the charger, it may be impossible to start the device even
 if it is connected to the charger, and thus to charge the battery.
 When the battery powered device having an almost flat battery is switched
 on the behaviour of the device accordingly is upredictable. Will the
 device be switched on correctly? Will the extra current flow to the device
 caused by decoupling capacitors when the device is switched on lower the
 battery voltage even more?
 Different alternatives are available in order to charge the battery in case
 the device is not able to initiate charging due to lack of enough power
 from the battery when the device including the battery is connected to the
 charger:
 The battery is disconnected from the device and placed in a charger and
 then, when it has been charged, is replaced into the device. This must be
 done by the user himself, which makes the handling of the device more
 tedious;
 The device including the battery is connected to a charger which supplies a
 small trickle charge current which independently of the charge control
 circuit always charges the battery when connected to the charger.
 Eventually, the battery voltage will reach a level which is high enough in
 order to operate the device and initiate normal charging of the battery
 over the charge control circuit. In this case if the trickle charge
 current is supplied by a simple trickle charge resistor the trickle charge
 current has to be very low to prevent damage of the battery should the
 battery be charged already when it is connected to the charger. It follows
 that charging of a flat battery to switch-on level will take long time;
 The device is operated by the charging voltage which is applied directly to
 the device when the battery is connected to the charger. In this case the
 power supplied to the device cannot be controlled satisfactorily as to the
 quality thereof.
 The charging voltage is transformed to a high quality voltage used for
 operating the device when the battery is connected to the charger. This
 solution necessitates extra components to be included into the charger.
 BRIEF SUMMARY OF THE INVENTION
 In order to overcome the drawbacks mentioned above and to make the
 behaviour of the device predictable without any risk of damaging the
 battery by a too high uncontrollable trickle charge current, the invention
 provides a method in charging a battery powered device having a built-in
 charge control circuit controlled by the device, by connecting the device
 including the battery to a battery charger, wherein the device including
 the battery initially when the battery voltage is too low to initiate
 normal charging of the battery over the charge control circuit, is
 connected to the battery charger via a current generator which is switched
 off when the battery voltage has increased to a level which is high enough
 to support normal charging of the battery over the charge control circuit.
 The invention also provides an apparatus for working the method comprising
 a battery powered device having a built-in charge control circuit
 controlled by the device, and a switch for connecting the device including
 the battery to a battery charger under the control of said charge control
 circuit for charging said battery, wherein a current generator is shunted
 over said switch to be switched on when the battery voltage is not high
 enough to initiate, over said charge control circuit, normal charging of
 the battery via said switch.
 When the battery-powered device is connected to the charger and the battery
 voltage of the device is too low for normal operation, the current
 generator charges the battery with a well defined safe charge current
 independently of the normal charging circuit controlled by the device. The
 current generator will be switched off at a predetermined battery voltage
 which may be chosen such that a secure start-up of the device is
 guaranteed. When the predetermined voltage has been reached the device
 will be switched on in normal way and start recharging of the battery over
 the charge control circuit. The charge current supplied by the current
 generator does not have to be low since it is switched off when the
 battery voltage has reached the predetermined value. Thus, there is no
 risk of damage to the battery due to a too high uncontrollable charge
 current being supplied to the battery by the current generator, and a flat
 battery is charged rapidly. The behaviour of the device is predictable.
 The invention allows use of a normal battery. The current generator
 circuit is built into the apparatus and will not be noticed by the user,
 no external components being needed.

DETAILED DESCRIPTION OF THE INVENTION
 A device 10 connected to a battery or chargeable electric cell 11 and
 comprising an electronic circuitry to be powered by the battery is
 included in an apparatus indicated by dot-and-dash lines 12 such as a
 portable wireless telephone set (mobile telephone). A charge control
 circuit 13 known per se in the art forms an integral part of device 10 and
 controls a switch 14 in a conductor 15 extending from a connector 16 to
 device 10 and battery 11 connected in parallel between said conductor and
 ground.
 A battery charger indicated by dot-and-dash lines 17 comprises a
 transformer 18 to be connected to the mains of e.g. 240 AC to transform
 the power to low voltage. The transformer is connected at the low voltage
 side to a rectifier bridge 19 supplying low voltage DC on a conductor
 extending to a connector 21 a smoothing capacitor 22 being connected
 between conductor 20 and ground.
 When apparatus 12 is connected to battery charger 17 at connectors 16 and
 21 in order to charge battery 11 which has still a voltage which is
 sufficiently high to operate device 10 switch 14 will be closed by the
 device over charge control circuit 13 and normal charging of the battery
 from the battery charger will take place. If, however, the voltage of the
 battery is not enough to operate device 10 the switch will be in the open
 position and no charging of the battery from the battery charger can take
 place as explained above.
 According to the invention a supplementary circuit is included into
 apparatus 12, which is indicated by dotted lines 23. This circuit
 comprises a comparator 24 the input of which is connected to conductor 15
 between device 10 and switch 14 to detect the voltage of battery 11, and
 also to a reference voltage Vref supplied e.g. by a band-gap or diode
 powered from the battery charger (circuit not shown). Circuit 23 also
 comprises a current generator 25 (transistor) which is shunted over switch
 14 and the operation of which is controlled by the output of comparator
 24. The reference voltage Vref is given a predetermined value, and this
 value is chosen to represent the lowest battery voltage at which the
 battery is able to operate device 10 when the apparatus is connected to
 the battery charger, and thus to initiate normal charging of the battery
 under the control of the device over the charge control circuit. If the
 detected battery voltage when compared with Vref in comparator 24 is found
 to be lower than Vref the device will not be switched on for normal
 charging of the battery. Instead, the comparator will switch on current
 generator 25 so as to charge the battery by a well-defined charge current
 supplied by the current generator. When the battery has been charged to
 such extent that the battery voltage equals Vref device 10 will be
 switched on and normal charging of the battery under the control of the
 charge control circuit will be initiated and maintained the current
 generator being switched off by the comparator.