Battery Charging System With Battery Pack of Different Charging Voltages Using Common a Battery Charger

1. Field of the Invention

The present invention generally relates to a battery charging system, and in particular to a system for charging a rechargeable battery such as a lithium-ion battery and a rechargeable battery pack for use in the system.

2. Description of the Related Art

Recently, a rechargeable battery or a secondary battery has been widely used as a power supply for portable or hand-held electronic equipment in consideration of running costs. Such a rechargeable battery has predetermined charging conditions including charging current and time. Especially in the case of a rechargeable lithium-ion battery, the details about charging conditions have been defined.

In general, a constant-voltage and constant-current charging method is used to charge a rechargeable lithium-ion battery. More specifically, the rechargeable lithium-ion battery is charged with a constant current until the battery voltage has risen to a predetermined charge control voltage and thereafter with a constant voltage. Such a charging method has been disclosed in U.S. Pat. No. 5,237,259, for example.

There has been commercially available rechargeable lithium-ion batteries having different prescribed voltages. Therefore, it is necessary for a single battery charger to provide a plurality of charge control voltages each corresponding to the prescribed voltage of a rechargeable lithium-ion battery. To cope with this, the battery charger may be provided with a plurality of constant-voltage circuits which are selectively used depending on the type of a rechargeable lithium-ion battery pack. The battery charger is further provided with a battery type identifying means. And the rechargeable lithium-ion battery pack is provided with a battery type indicating means. When the battery pack is electrically connected to the battery charger, the battery type identifying means of the battery charger identifies the type of the lithium-ion battery by checking the battery type indicating means of the battery pack. According to the battery type, the battery charger provides the corresponding constant voltage to the battery pack after the battery voltage has risen to the predetermined charge control voltage.

However, the rechargeable lithium-ion battery pack needs the battery type indicating means and the charger needs the battery type identifying means. Therefore, it is difficult to miniaturizing the size and weight of the batter charging system as well as the battery pack.

An object of the present invention is to provide a battery charging system which can cope with a plurality of battery types with reduced amount of hardware.

Another object of the present invention is to provide a rechargeable battery pack which can be charged without the need of changing a battery charger depending on its battery type.

According to the present invention, a battery charging system includes a battery charger and a plurality of rechargeable battery packs each having a different predetermined charge voltage. Each of the rechargeable battery packs includes a rechargeable battery having a predetermined charge voltage, and a circuit for generating a control voltage depending on a battery voltage of the rechargeable battery so that the control voltage reaches a predetermined control voltage when the battery voltage reaches a predetermined charge voltage of the rechargeable battery. The battery charger charges the rechargeable battery depending on the control voltage received from a rechargeable battery connected thereto.

Since the circuit of each rechargeable battery pack generates the control voltage such that the control voltage reaches the predetermined control voltage when the battery voltage reaches the predetermined charge voltage of the rechargeable battery, the battery charger can cope with the plurality of rechargeable battery packs each having a different predetermined charge voltage in other words, since the predetermined control voltage is set to a single common voltage among the rechargeable battery packs, the battery charger can charge any one of the different rechargeable battery packs.

FIG. 1 is a block diagram showing a battery charging system according to a first embodiment of the present invention;

FIG. 2A is a graph illustrating time-varying charging voltage and current in the case where a 4.1 V Li-ion battery pack is connected to a battery charger of the battery charging system of FIG. 1;

FIG. 2B is a graph illustrating time-varying charging voltage and current in the case where a 4.2 V Li-ion battery pack is connected to the battery charger of FIG. 1;

FIG. 3 is a block diagram showing a battery charging system according to a second embodiment of the present invention; and

FIG. 4 is a block diagram showing a battery charging system according to a third embodiment of the present invention.

Referring to FIG. 1, a battery charging system according to a first embodiment of the present invention is comprised of a battery charger 10 which is provided with a connection surface having power supply terminals T.sub.+ and T.sub.- and a control voltage detection terminal T.sub.E-. The battery charger 10 can be connected to one of a plurality of rechargeable battery packs including rechargeable lithium-ion batteries of different prescribed charge voltages, respectively. Each of rechargeable battery packs is provided with a connection surface having power receive terminals T.sub.B + and T.sub.B- and a control voltage supply terminal T.sub.BE which correspond to the power supply terminals T.sub.+ and T.sub.- and the voltage detection terminal T.sub.E, respectively.

The battery charger 10 may use a widely-used lithium-ion battery charger which can provide a constant current until the control voltage reaches a predetermined voltage and thereafter keep the output voltage at a voltage appearing on the power supply terminal T.sub.+ at that time.

For example, as shown in FIG. 1. the battery charger 10 includes a DC power source 101 which inputs a DC power applied on power input terminals as indicated by `++` and `-` and generates an output voltage V.sub.OUT with limited current I.sub.OUT. The output current I.sub.OUT is detected by a current detector 102. The output voltage V.sub.OUT with limited current I.sub.OUT is output to the power supply terminals T.sub.+ and T.sub.- through an on-off switch 103.

The control voltage detection terminal T.sub.E is connected to a control voltage detector 104 which detects a control voltage V.sub.DEC of a connected rechargeable battery pack. A charge controller 105 performs the charge control of the connected rechargeable battery pack using the detected output current I.sub.OUT the detected control voltage V.sub.DEC and a reference voltage input from a reference voltage generator 106. As will be described later, the charge controller is set to a single predetermined control voltage V.sub.VC, for example, V.sub.CV =4.1 V which may be the lowest prescribed battery voltage among the lithium-ion batteries.

More specifically, when a rechargeable battery pack is connected, the charge controller 105 performs constant-current charging control of the DC power source 101 based on the detected output current I.sub.OUT until the detected control voltage V.sub.DEC reaches the predetermined control voltage V.sub.CV. When the detected control voltage V.sub.DEC has reached the predetermined control voltage V.sub.CV, the charge controller 105 changes the charging control of the DC power source 101 from constant-current to constant-voltage. When the constant-voltage charging control is started, the DC power source 101 keeps the current output voltage V.sub.OUT constant.

In this manner, the battery charger 10 provide a constant current until the control voltage V.sub.DEC reaches the predetermined control voltage V.sub.CV, and thereafter performs the constant-voltage charging control with keeping the output voltage V.sub.OUT constant and with the output current I.sub.OUT gradually decreasing.

It should be noted that the charge controller 105 changes the charging control from constant-current to constant-voltage by checking to see only whether the detected control voltage V.sub.DEC reaches the predetermined control voltage V.sub.CV.

For simplicity, two rechargeable battery packs are shown in FIG. 1: 4.2 V-rechargeable battery pack 20 and 4.1 V-rechargeable battery pack 30.

The 4.2 V-rechargeable battery pack 20 includes a rechargeable lithium-ion battery 201 of the prescribed charge voltage of 4.2 V. The rechargeable lithium-ion battery 201 is connected to the power receive terminals T.sub.B+ and T.sub.B- through a protector 202.

Further, the power receive terminals T.sub.B+ and T.sub.B- are electrically connected through a voltage divider comprising two resistors R1 and R2 connected in series. The control voltage supply terminal T.sub.BE is electrically connected to a tap of the voltage divider Therefore, the tap voltage is represented by V.sub.BC .times.R2/(R1+R2), where V.sub.BC is a voltage across the lithium-ion battery 201 during charging. In other words, the tap voltage is lower than the current battery voltage V.sub.BC by a voltage drop of V.sub.BC .times.R1/(R1+R2). Therefore, by setting the resistance ratio of the voltage divider to R1:R2=1:41, a voltage of 4.1 V appears on the control voltage supply terminal T.sub.BE when the battery voltage V.sub.BC across the rechargeable lithium-ion battery 201 reaches 4.2 V.

Practically, in consideration of tolerance of a resistance, temperature and time-varying characteristics of the resistors R1 and R2, a chip-type thin-film resistor may be used for the resistors R1 and R2. Further, to reduce in the current flowing through the voltage divider, in this embodiment, the following resistors are selected: R1=16.5 k.OMEGA. and R2=680 k.OMEGA..

The 4.1 V-rechargeable battery pack 30 includes a rechargeable lithium-ion battery 301 of the prescribed charge voltage of 4.1 V. The rechargeable lithium-ion battery 301 is connected to the power receive terminals T.sub.B+ and T.sub.B- through a protector 302.

Further, the control voltage supply terminal T.sub.BE is electrically connected to the power receive terminal T.sub.B+ through a resistor R1 of 16.5 k.OMEGA. in this embodiment. Since the battery voltage V.sub.BC across the rechargeable lithium-ion battery 301 appears on the control voltage supply terminal T.sub.BC, a voltage of 4.1 V appears on the control voltage supply terminal T.sub.BE when the battery voltage V.sub.BC across the rechargeable lithium-ion battery 301 reaches 4.1 V.

Therefore, as described above, in the case where the battery voltage V.sub.BC across the rechargeable lithium-ion battery 201 reaches 4.2 V and the battery voltage V.sub.BC across the rechargeable lithium-ion battery 301 reaches 4.1 V, the same voltage of 4.1 V appears on the control voltage supply terminal T.sub.BE of each rechargeable battery pack. As described before, in this embodiment, the charge controller 105 is set to the predetermined control voltage V.sub.CV -4.1 V at which the charging control is changed from constant-current to constant-voltage. The charging control operation will be described hereinafter.

As shown in FIG. 2A, in the case where the 4.1 V Li-ion battery pack 30 is connected to the battery charger 10, the charge controller 105 performs the constant-current charging control of the DC power source 101 by monitoring the detected output current I.sub.OUT. The constant-current charging control causes the DC power source 101 to supply the constant current I.sub.OUT to the 4.1 V Li-ion battery 301 with gradually increasing in the output voltage V.sub.OUT or the charging battery voltage V.sub.BC across the rechargeable lithium-ion battery 301. Since the battery voltage V.sub.BC appears on the control voltage supply terminal T.sub.BE, the detected control voltage V.sub.DEC is also equal to the output voltage V.sub.OUT. Therefore, when the detected control voltage V.sub.DEC reaches the predetermined control voltage V.sub.CV =4.1 V, the charge controller 105 changes the charging control of the DC power source 101 from constant-current to constant-voltage. When the constant-voltage charging control is started, the DC power source 101 keeps the current output voltage V.sub.OUT at the predetermined control voltage V.sub.CV =4.1 V. In this charging state, the output current I.sub.OUT gradually decreases as shown in FIG. 2A.

As shown FIG. 2B, in the case where the 4.2 V Li-ion battery pack 20 is connected to the battery charger 10, the charge controller 105 performs the constant-current charging control of the DC power source 101 by monitoring the detected output current I.sub.OUT. The constant-current charging control causes the DC power source 101 to supply the constant current I.sub.OUT to the 4.2 V Li-ion battery 301 with gradually increasing in the output voltage V.sub.OUT or the charging battery voltage V.sub.BC across the rechargeable lithium-ion battery 201.

Since the respective resistors R1 and R2 of the voltage divider are set to R1=16.5 k.OMEGA. and R2=680 k.OMEGA. and the resistance ratio R1/R2 is approximately 1/41, the detected control voltage V.sub.DEC is lower than the output voltage V.sub.OUT by a voltage drop of V.sub.OUT .times.R1/(R1+R2) as shown in FIG. 2B. Therefore, when the output voltage V.sub.OUT reaches 4.2 V. the detected control voltage V.sub.DEC reaches the predetermined control voltage V.sub.CV =4.1 V. In other words, when the output voltage V.sub.OUT reaches 4.2 V, the charge controller 105 changes the charging control of the DC power source 101 from constant-current to constant-voltage. When the constant-voltage charging control is started, the DC power source 101 keeps the current output voltage V.sub.OUT at 4.2 V and the output current I.sub.OUT gradually decreases as shown in FIG. 2B.

As shown in FIGS. 2A and 2B, in the case where the output voltage V.sub.OUT of the 4.2 V-rechargeable battery pack 20 reaches 4.2 V and the output voltage V.sub.OUT of the 4.1 V-rechargeable battery pack 30 reaches 4.1 V, the same voltage of 4.1 V appears on the control voltage supply terminal T.sub.BE of each rechargeable battery pack. When the detected control voltage V.sub.DEC reaches the predetermined control voltage V.sub.CV =4.1 V, the charge controller 105 changes the charging control from constant-current to constant-voltage. Therefore, the battery charger 10 can automatically provide one of the different constant voltages, 4.1 V and 4.2 V, without the need of a charge control voltage switch between 4.1V and 4.2 V. In other words, the battery charger 10 can cope with a plurality of different types of rechargeable battery packs.

As described before, it is necessary to reduce in the current flowing through the resistors R1 and R2 of the voltage divider of the 4.2 V-rechargeable battery pack 20. In the first embodiment of FIG. 1, the relatively high resistance (R1=16.5 k.OMEGA., R2=680 k.OMEGA.) is employed to prevent the discharge of the Li-ion battery 201. Another means may be used as will be described hereinafter.

Referring to FIG. 3, the 4.2 V-rechargeable battery pack 20 is further provided with a switch SW connected between the resistors R1 and R2. The switch SW is opened when the 4.2 V rechargeable battery pack 20 is removed from the battery charger 10 and is closed when it is connected to the battery charger 10. Therefore, when the 4.2 V-rechargeable battery pack 20 is removed from the battery charger 10, the both electrodes of the 4.2 V Li-ion battery 201 are perfectly opened.

Such a switch mechanism may be easily implemented. For example, the battery charger 10 is provided with a protrusion at a predetermined location of a connector surface having the terminals T.sub.+, T.sub.E and T.sub.-. On the other hand, the 4.2 V-rechargeable battery pack 20 is provided with a normally-open switch SW which can be closed by the protrusion of the battery charger 10 when they are connected.

Referring to FIG. 4, a battery charger 11 has the power supply terminals T.sub.+ and T.sub.- and two control voltage detection terminals T.sub.E1 and T.sub.E2 which are connected to each other therein.

Each of rechargeable battery packs has power receive terminals T.sub.B+, and T.sub.B- and two control voltage supply terminals T.sub.BE1 and T.sub.BE2 which correspond to the power supply terminals T.sub.+ and T.sub.- and the voltage detection terminals T.sub.E1 and T.sub.E2, respectively. A 4.2 V-rechargeable battery pack 21 includes a rechargeable lithium-ion battery 201 of the prescribed charge voltage of 4.2 V. The rechargeable lithium-ion battery 201 is connected to the power receive terminals T.sub.B+ and T.sub.B- through a protector 202. Further, the power receive terminal T.sub.B+ is connected to the control voltage supply terminal T.sub.BE1 through a resistor R1 and the power receive terminal T.sub.B- is electrically connected to the control voltage supply terminal T.sub.BE2 through a resistor R2. A 4.1 V-rechargeable battery pack (not shown) is not provided with the resistor R2.

When the 4.2 V-rechargeable battery pack 21 is removed from the battery charger 11, the terminals T.sub.BE1 and T.sub.BE2 of the resistors R1 and R2 are opened. When the 4.2 V-rechargeable battery pack 21 is connected to the battery charger 11, the terminals T.sub.BE1 and T.sub.BE2 are closed to form the voltage divider as in the case of the 4.2 V-rechargeable battery pack 20 of FIG. 1. Therefore, when the 4.2 V-rechargeable battery pack 21 is removed from the battery charger 10, the both electrodes of the 4.2 V Li-ion battery 201 are perfectly opened.