Abstract:
A battery charger having a plurality of series connected sections for serially charging a plurality of rechargeable batteries, for example rechargeable batteries of AA or AAA size. Each charging section includes a charging path for a battery and a parallel bypass path for bypassing a battery when it is fully charged. The charging path and the bypass path of each charging section each include an electrically operable switching device, which devices are preferably MOSFETs. Control circuitry is included to ensure one switching device is off when the other is on. MOSFET switching devices are connected into the circuit in directions to ensure they are not burnt out by the charging currents. A discharge circuit may be included for the batteries to discharge briefly between pulses of charging current thereby providing for “negative pulse charging” of the batteries. The charger provides for improved efficiency of charging in that very little power is consumed by the switching devices in the charging paths.

Description:
BACKGROUND  
         [0001]    The present invention relates to a battery charger having a plurality of series connected charging sections for charging a plurality of rechargeable batteries.  
           [0002]    Series charging is typically used for simultaneously charging a plurality of rechargeable batteries of small voltage, for example batteries of AA or AAA size, typically of 1.2 to 2 volts terminal voltage. This is because it allows for fast charging and requires a power supply of lesser current rating than would be required for a charger that is arranged to charge the batteries in parallel. However series charging presents problems in that if a battery is removed from the charging circuit, the series circuit will be broken and charging will cease, or if a battery is fully charged before others in the series, it may be damaged or destroyed by continued passage of the charging current through it. Thus battery chargers for series charging of a plurality of batteries need to provide for individual batteries in the series circuit to be by-passed by the charging current.  
           [0003]    Hong Kong Short-Term Patent No. 1045076, entitled “An Intelligent Serial Battery Charger and Charging Block”, discloses a serial battery charger including a number of serially connected battery charging sections in which each battery charging section is characterised by a first and a second parallelly connected branch. The first branch includes terminals for connecting to the battery to be charged and a current blocking device, and the second branch includes a by-passing switch which shunts across the terminals of the first branch when activated. The blocking device in the first branch prevents adverse reverse current flow from the battery to the charger when there is no power supply and also functions as a current block to prevent adverse flow of current from the battery into the shunting by-passing switch when the power supply to the charging section is in operation. In this disclosure the current blocking device (claimed as “a one-way electronic device”) is a diode and more specifically, in practical embodiments of the development, a Schottky-barrier diode, and the by-passing switch is a FET, more specifically a MOSFET. This patent specifically states that a MOSFET is not suitable for the current blocking “one-way electronic device”. Thus the charging circuit of this Hong Kong patent is limited to the combined use of a diode as the “one-way electronic device” for current blocking in its first (charging) parallel branch of the circuit and a MOSFET (an “electronically controllable by-passing switch”) in the second (by-passing) parallel connected branch. Limitations of this disclosed charging circuit are that when charging a battery, the diode consumes a relatively large amount of the available power thereby slowing the charging rate compared to what might otherwise be possible. Furthermore, the diode, being a one-way device, does not readily provide for a circuit configuration allowing for a discharge current to flow from a battery, as in for example a charger providing for negative pulse charging of a battery.  
           [0004]    An object of the present invention is to provide a battery charger having a plurality of series connected charging sections for charging a plurality of rechargeable batteries which is improved compared to the above identified Hong Kong patent. There are two main improvements which may be separately realised in different embodiments of the invention. The first is that components may be used in the charging sections of the battery charger circuit that consume less power than a diode. The second is that such components also facilitate the provision of an embodiment that provides for negative pulse charging.  
         SUMMARY OF THE INVENTION  
         [0005]    The present invention provides a battery charger having a plurality of series connected charging sections for charging a plurality of rechargeable batteries, wherein each charging section comprises a charging path for a charging current to flow through a battery connected into the charging path, and  
           [0006]    a by-pass path for the charging current to by-pass the charging path when a battery connected therein is fully charged. The charging path and the by-pass path each include in series therewith an electrically operable switching device, which is preferably a solid state device, for example each device may be a FET or preferably a MOSFET. The charger furthermore includes control circuitry for operating the two electrically operable switching devices of each charging section. The switching devices of each charging section are operated such that when one is conductive the other is non-conductive. Generally the switching device in the charging path of a charging section will be conductive whilst the switching device in the by-pass path of that charging section is non-conductive for passage of the charging current through a battery in the charging path and not through the by-pass path, and to prevent any discharge current from the battery from passing through the by-pass path upon cessation of the charging current. For by-passing a battery that is fully charged in a charging section, the switching device in the charging path of that charging section will be non-conductive whilst the switching device in the by-pass path of that charging section will be conductive for the charging current to by-pass the charging path and thus the battery.  
           [0007]    Preferably the control circuitry includes a micro-processor for providing control signals for effecting operation of the switching devices of each charging section to render them either conductive or non-conductive. More preferably, with solid state switching devices, a single control signal is provided for each charging section, and this signal is effective to cause one of the switching devices of that charging section to switch on such that it is conductive and the other switching device to switch off such that it is non-conductive.  
           [0008]    Preferably the charger further comprises a discharge circuit which can be opened or closed via the control circuitry, whereby when the switching device in the charging path of a charging section is conductive and the switching device in the by-pass path of that charging section is non-conductive, cessation of the charging current together with closure of the discharge circuit provides for a discharge current to flow from the battery through the switching device of that charging section and through the discharge circuit. For preceding charging sections in the series connected charging sections, the discharge current may flow through the switching device in the by-pass path of such preceding sections.  
           [0009]    Generally the charger will include a constant current source which is switchable on and off via the control circuitry. Preferably the charger is operable for the constant current source to supply the charging current to a charging section in pulses having a long duty cycle and for the battery in that charging section to discharge between the charging pulses, the discharge periods having a short duration, thereby providing negative pulse charging of the battery.  
           [0010]    The invention according to a preferred embodiment thereof provides for a two-way electrically controllable solid state switching device, most readily realised in a MOSFET, to be used instead of a one-way diode, that is a non-electronically controllable switching device as in the above mentioned Hong Kong patent. Contrary to the findings in that Hong Kong patent, it has been discovered that a MOSFET can be used to provide a blocking function in the charging path without burning out, as will be described in detail hereinbelow.  
           [0011]    For a better understanding of the invention and to show how it may be carried into effect, preferred embodiments thereof will now be described, by way of non-limiting example only, with reference to the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0012]    [0012]FIG. 1 is an idealised current waveform illustrating negative pulse charging.  
         [0013]    [0013]FIG. 2 is a battery charger circuit according to a preferred embodiment of the invention that employs N-channel MOSFETs as switching devices.  
         [0014]    [0014]FIG. 3 is another embodiment of the invention which employs P-channel MOSFETs as switching devices; and  
         [0015]    [0015]FIG. 4 is a further embodiment of the invention which employs relay-switches as switching devices. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENT  
       [0016]    Negative pulse charging of a rechargeable battery facilitates fast and efficient charging of the battery. It involves a cyclic charging regime wherein a charging current I C  (see FIG. 1) is supplied to the battery for a specified time period ‘a’, following which the battery is allowed to discharge for a specified time period ‘b’, and this cycle is repeated until the battery is fully charged. Generally the discharge time period is short compared to the charging time period, for example, for a total one hour charging period, a cycle can consist of one second of charging followed by 0.1 second of discharging.  
         [0017]    With reference to FIG. 2, a battery charger circuit according to an embodiment of the invention comprises a DC power source  10 , a constant current source  12 , a Microprocessor Control Unit  14  and other control circuitry including transistors  16   a,    16   b  . . .  16   n  and  18   a,    18   b  . . .  18   n,  and a plurality of charging sections generally referenced  20   a,    20   b  . . .  20   n  (where ‘a’ signifies a first described integer and ‘n’ signifies a number n th  such integer). The charging sections  20   a  to  20   n  are connected in series, as described in more detail below. The constant current source  12  is connected to the positive of the main power source  10  and supplies charging current to the first of the series connected charging sections  20   a  along a line  22  under control of the Microprocessor Control Unit  14  via a signal on a control line  26 . The negative of the power source  10  (and the current return paths) are illustrated as grounded, see reference  24 .  
         [0018]    Each charging section  20   a,    20   b  . . .  20   n  comprises a charging path  28  (the first of which is connected to line  22 ), that includes contacts  30  and  32  for contacting the terminals of a battery  34  (respectively  34   a,    34   b  . . .  34   n ) connectable into each charging section  20   a,    20   b,  . . .  20   n,  and a bypass path  36  (the first of which is also connected to line  22 ). The charging path  28  of each charging section  20   a,    20   b  . . .  20   n  includes in series therewith an electrically operable solid state switching device, namely an N-channel MOSFET, respectively  38   a,    38   b  . . .  38   n,  connected such that charging current from line  22  flows through the N-channel MOSFETs respectively  38   a,    38   b  . . .  38   n  in the source terminal to drain terminal direction (that is, in the forward direction of its internal diode) and through a battery, respectively  34   a,    34   b  . . .  34   n  via respective pairs of contacts  30  and  32 . Thus the source terminal S of the first MOSFET  38   a  is connected to line  22  and its drain terminal D is connected to contact  30  for contacting the positive terminal of the battery  34   a.  The source terminal S of the next MOSFET  38   b  is connected to the contact  32  for contacting the negative terminal of the battery  34   a  and its drain terminal D is connected to the contact  30  for contacting the positive terminal of the next battery  34   b,  and so on.  
         [0019]    The by-pass path  36  of each charging section  20   a,    20   b  . . .  20   n  also includes, in series therewith, an electrically operable solid state switching device, namely an N-channel MOSFET respectively  40   a,    40   b  . . .  40   n,  connected such that a charging current from line  22  when bypassing a charging path  28  flows through the respective N-channel MOSFETs  40   a,    40   b  . . .  40   n  in the drain terminal D to source terminal S direction (that is, in the reverse direction of its internal diode). The charging path  28  and by-pass path  36  of each charging section  20   a,    20   b,    20   n,  are connected in parallel by a line  41  connected between the negative battery contact  32  and source terminal S of the by-pass path MOSFET  40  of that charging section  20 . Thus the charging sections  20   a,    20   b  . . .  20   n  are series connected and each charging section comprises parallely connected charging and by-pass paths  28  and  36 .  
         [0020]    Control circuitry comprising the Microprocessor Control Unit  14  and, for each charging section  20   a,    20   b  . . .  20   n,  a pair of switching transistors respectively  16   a  and  18   a,    16   b  and  18   b,  . . .  16   n  and  18   n,  operates the N-channel MOSFETs  38  and  40  of each charging section by providing signals to influence the voltage levels at their gate terminals to either switch a MOSFET on, that is render it conductive, or switch the MOSFET off, that is render it non-conductive. The Microprocessor Control Unit  14  has a number of control line outputs  42   a,    42   b  . . .  42   n,  one for each respective charging section  20   a,    20   b,  . . .  20   n.  Each control line output  42  is connected to the base of the first switching transistor  16  for a charging section  20 . The collector of the transistor  16  is connected to the gate terminal of the MOSFET  40  of the by-pass path  36  of that charging section  20 , that is, at a circuit point referenced  44 , (respectively  44   a,    44   b  . . .  44   n  for the charging sections  20   a,    20   b  . . .  20   n ) and the emitter of the transistor is grounded at  24 . The collector circuit point  44  of the transistor  16  is also connected to the base of the switching transistor  18  via a line  46 . The collector of the switching transistor  18  is connected to the gate terminal of the MOSFET  38  of the charging path  28  of that charging section  20 , that is, at a circuit point referenced  48  (respectively  48   a,    48   b  . . .  48   n ) and the emitter of the transistor  18  is grounded at  24 . The gate terminals of the MOSFETs  38  and  40  are also connected to a circuit control or reference voltage Vcc via lines referenced  50  and  52 . As is known, appropriate resistors are included in the base circuits of the transistors  16  and  18  and gate circuits of the MOSFETs  38  and  40 .  
         [0021]    The charger circuit also includes a discharge circuit which is a continuation of line  22  to a switch  54  which is closable and openable under a control signal from Microprocessor Control Unit  14  supplied via a line  56  to, respectively, connect and disconnect a resistive load  58  into and out of the discharge circuit. The discharge circuit is completed by connection of the other side of the resistive load  58  to ground at  24 .  
         [0022]    Before describing the operation of the overall charging circuit, it will be convenient to describe the operation of a switching transistor pair  16 - 18  for switching the N-channel MOSFETs  40  and  38  on and off. With reference to the first charging section  20   a,  a high signal on control line  42   a  will switch on transistor  16   a  which will cause a low voltage at circuit point  44   a  and thereby switch off the MOSFET  40   a  because the voltage at its gate terminal is low. Thus MOSFET  40   a  is rendered non-conductive. Simultaneously the low voltage at circuit point  44   a  will switch off the transistor  18   a  thereby causing a high voltage at circuit point  48   a  which will switch on the MOSFET  38   a  because the voltage at its gate terminal is high. Thus the MOSFET  38   a  will be rendered conductive. Conversely, a low voltage signal on control line  42   a  will switch off the transistor  16   a,  thereby causing a high voltage at circuit point  44   a  and switching on the MOSFET  40   a,  and simultaneously switching on the transistor  18   a,  which will cause a low voltage at circuit point  48   a  and thus switching off of the MOSFET  38   a.  When the MOSFET  38   a  is on, charging current from line  22  will flow in path  28  (note that switch  54  of the discharge circuit will be open) through the MOSFET  38   a  and battery  34   a  to charge the battery, whilst MOSFET  40   a  which is off and thus non-conductive, will prevent the charging current from by-passing the charging path  28 . When the battery  34   a  is fully charged, MOSFET  38   a  is switched off and MOSFET  40   a  is switched on such that the charging current then flows through by-pass path  36  and through either the following MOSFET  40   b  or MOSFET  38   b  depending on which one is conductive and which is non-conductive.  
         [0023]    Operation of the charging circuit when charging all batteries  34   a,    34   b  . . .  34   n  will now be described. During a charging period ‘a’ (see FIG. 1) a high signal on control line  26  of Microprocessor Control Unit  14  switches on the constant current source  12  such that a charging current Ic can flow in line  22 . The Microprocessor Control Unit  14  also outputs a high signal on lines  42   a,    42   b  . . .  42   n,  which (as described hereinabove) switches by-pass path MOSFETs  40   a ,  40   b  . . .  40   n  off and charging path MOSFETs  38   a,    38   b  . . .  38   n  on. The Microprocessor Control Unit  14  also outputs a low signal on line  56  which opens the switch  54  such that no current can flow through the discharge circuit. Thus the charging current Ic flows from constant current source  11  through line  22  and through the charging paths  28  of each charging section, that is, through MOSFET  38   a,  battery  34   a,  MOSFET  38   b,  battery  34   b  . . . MOSFET  38   n,  battery  34   n,  thereby charging the batteries. During a discharging period ‘b’ (see FIG. 1), Microprocessor Control Unit  14  outputs a low signal on control line  26  which switches off the constant current source such that no charging current Ic can flow. High signals are maintained on control lines  42   a,    42   b  . . .  42   n  such that the by-pass path MOSFETs  40   a,    40   b  . . .  40   n  remain off and the charging path MOSFETs  38   a,    38   b  . . .  38   n  remain on. The Microprocessor Control Unit  14  also outputs a high signal on control line  56  which closes switch  54  to complete the discharge circuit. Because the charging path MOSFETs  38   a,    38   b  . . .  38   n  remain on, there is a low impedance path across each from the drain to the source terminals whereby a discharge current can flow from the positive terminal contacts  30  of the batteries  34   a - 34   n  through charging paths  28  including MOSFETs  38   n  . . .  38   b,    38   a  (that is, in reverse direction to the charging current flow) to line  22  through switch  54  and load  58 .  
         [0024]    If one of the batteries  34   a,    34   b  . . .  34   n  becomes fully charged before the others, the charging circuit operates to by-pass that battery and continue charging the others. The fully charged status of a battery may be detected by appropriate circuitry (not shown) for detecting when a battery reaches a predetermined temperature, as is known. Assuming battery  34   b  is detected as fully charged, during a charging period ‘a’, the Microprocessor Control Unit  14  outputs a low signal on control line  42   b  which (as described hereinabove) switches by-pass path MOSFET  40   b  on and charging path MOSFET  38   b  off. This causes the charging current Ic to flow from battery  34   a,  through paralleling connection  41 , through MOSFET  40   b  (from its drain to its source terminals&#39; direction), through the next paralleling connection  41  to the charging path  28  of the next charging section  20   n,  that is through MOSFET  38   n  and battery  34   n . Thus the by-pass path  36  of charging section  20   b  acts to by-pass or shunt the charging current Ic due to the low impedance in by-pass path  36  provided by the MOSFET  40   b  and the high impedance blocking provided in charging path  28  by MOSFET  38   b.  During a discharge period ‘b’, constant current source  12  is turned off and switch  54  closed by Microprocessor Control Unit  14  as before, however the discharge path now comprises battery  34   n,  MOSFET  38   n , connection  41 , MOSFET  40   b  (in the source to drain direction) connection  41 , battery  34   a,  MOSFET  38   a,  line  22 , switch  54  and load  58 . It will be evident from the above explanation how the charging circuit operates if any other battery or more than one of the batteries become fully charged whilst others are still being charged until they all become fully charged, at which stage a cut-out (not shown) can operate to maintain the constant current source  12  off.  
         [0025]    The N-channel MOSFETs  38   a,    38   b  . . .  38   n  of the charging paths  28  are connected such that the charging current passes through them when they are switched on in the direction of their source terminal to drain terminal. It has been found that the MOSFETs  38   a,    38   b  . . .  38   n  when so connected do not burn out. For example, if the MOSFET  38   a  is connected with its drain to line  22  and its source to contact  30  (i.e. the other way around), then when the by-pass MOSFET  40   a  is on, the MOSFET  38   a  will be off but its internal diode will also be in a forward biased condition such that a current path can be established from the positive terminal of battery  34   a  through the internal diode of the MOSFET  38   a  and the switched-on MOSFET  40   a  to the negative terminal of battery  34   a.  Since MOSFET  40   a  has a low impedance when switched on, and the nominal voltage of battery  34   a  upon fully charged is around 1.2 V, but the nominal voltage drop of a forward biased diode is only about 0.7 V, a large current will be generated through the said current path. Such a current will cause the N-channel MOSFET  38   a  if connected the other way around to that illustrated in FIG. 2 to burn out. The configuration of the two N-channel MOSFETS  38  and  40  of each charging section  20   a,    20   b  . . .  20   n  prevents such burn outs.  
         [0026]    An embodiment of the invention according to FIGS. 1 and 2 offers greater efficiency when charging compared to the prior art circuit of the Hong Kong Patent. For example, for a one hour charger with a charging current of 2 Amps, when using a MOSFET with internal resistance R DS-ON  of 0.015 ohms in the charging path as in FIG. 2, the power loss on one MOSFET device as given by its forward impedance times the current squared is: 0.015 ohms×2 Amps×2 Amps=0.06 watts. In contrast the power loss on one device when that device is a Schottky barrier diode as in the Hong Kong Patent, the power loss as given by the voltage drop across that device times the current is 0.5 v.×2 Amp=1.0 watts. Thus there is only a 6% power loss per device in the charging path in the circuit of FIG. 2 compared to the one-way diode device in the charging paths of the circuit of the Hong Kong Patent.  
         [0027]    A further advantage is that the circuit of FIG. 2 offers the possibility of providing for negative pulse charging because of the possible two-way current flow through the MOSFETs, with the addition of minimal further components. That is, fundamentally only two extra components namely switch  54  and load  58  need be provided.  
         [0028]    If a negative pulse charging regime is not required, the discharge circuit  22 - 54 - 58  may be omitted. Thus the provision of a discharge circuit is an optional feature of the invention.  
         [0029]    N-channel MOSFETs instead of P-channel MOSFETs are preferably used because they are generally less expensive. However P-channel MOSFETS may be used if desired. FIG. 3 illustrates a circuit in which P-channel MOSFETS have been used to provide the electrically operable switching devices in the charging and bypass paths. The FIG. 3 circuit is generally equivalent to that of FIG. 2 and thus the same reference numerals are used to indicate corresponding components. Persons skilled in the art will, in light of the description provided above of the functioning of the FIG. 2 circuit, readily understand the functioning of the FIG. 3 circuit and thus further description thereof is unnecessary. As in the FIG. 2 circuit, charging current in the charging paths  28  flows through the P-channel MOSFETs  38  in a direction that corresponds to the forward direction of their internal diodes, and the charging current when flowing in the bypass paths  36  flows through the P-channel MOSFETs  40  in a direction that corresponds to the reverse direction of their internal diodes (as is known, the current does not actually flow through the internal diode of a MOSFET).  
         [0030]    Also, electrically operable switching devices other than the MOSFETs  38  and  40  may be provided. For example, non-solid state switching devices such as relay switches may be used. Persons skilled in the art will readily be able to provide appropriate control circuitry to operate the coils of the relays. For example, FIG. 4 illustrates a circuit which is generally equivalent to the FIG. 2 and  3  circuits, but in which the MOSFETS are replaced by relay switches. Reference numerals the same as used in FIG. 2 have again been used for corresponding componentry to illustrate the equivalency of the circuits. Thus in the FIG. 4 circuit, relay switches  38   a,    38   b  . . .  38   n  and  40   a,    40   b  . . .  40   n  are used. Each such relay switch comprises (as referenced for relay switch  38   a ) an operating coil  60  bridged by a diode  62  and connected to the switching transistors  16  and  18  for operation thereby. As is known, when current flows through a coil  60  it causes the relay contacts  64  to close.  
         [0031]    The invention described herein is susceptible to variations, modifications and/or additions other than those specifically described and it is to be understood that the invention includes all such variations, modifications and/or additions which fall within the scope of the following claims.