Battery charging apparatus

This application claims the benefit of priority of U.S. patent application Ser. No. 10/895,221 filed on Jul. 20, 2004 and incorporated here by reference, which in turn claims the benefit of priority of German patent application No. 10335018.7 filed on Jul. 31, 2003.

The present invention relates to an apparatus for charging a battery or a battery charging circuit. This battery charging circuit is especially suitable for charging batteries, whose charging can run under voltage control, like, for example, the case of lithium ion and lead batteries.

It is an object of the present invention to provide a miniaturized and inexpensive charging apparatus for charging a rechargeable battery, such as a lead or lithium ion battery.

The apparatus according to the invention for charging a battery has a first transistor, through which a charging current passes into the battery connected to the charging apparatus. A current source is connected to a control input of this first transistor to supply a control current to it. This current source adjusts or sets the control current for the first transistor so that the first transistor shuts off the charging current to the battery when a predetermined maximum charging or supply voltage is reached at the battery.

The apparatus of the present invention is embodied as a charging circuit, which is made from very small and also low-cost components.

Further advantageous embodiments are characterized by features claimed in the appended dependent claims.

In preferred embodiments a great simplification of the circuitry results when a Zener diode is connected to the control input of the first transistor. As soon as the charging voltage of the battery at the first transistor has reached a maximum value, the Zener voltage of the Zener diode is exceeded so that the control current no longer flows to the control input of the first transistor, but away through the Zener diode.

In further preferred embodiments it is appropriate to connect a second transistor to the first transistor, which keeps the control input of the first transistor at a potential, which is less than the charging voltage of the battery connected to the first transistor after shut off of the first transistor. Because of that in no case will even a small amount of charging current flow into the battery after shut off of the charging current through the first transistor.

A very high charging current can flow when the charging process is initiated, which could close or turn on the second transistor and thus interrupt the charging process. In order to prevent that occurrence the control input of the second transistor is connected to an R/C circuit means for delaying the switching on of the second transistor.

The current source works with reduced circuit work because several diodes are connected in series in a circuit branch through which the charging current is conducted and the current source picks off or taps the voltage drop across the diodes.

The objects, features and advantages of the invention will now be illustrated in more detail with the aid of the following description of the preferred embodiments, with reference to the sole FIGURE, which is a schematic circuit diagram of a circuit arrangement for charging a rechargeable battery according to the invention.

The circuit shown in the drawing for charging a battery has two input terminals 1 and 2 , to which an unshown power supply is connectable and two output terminals 3 and 4 , to which the battery B to be charged is connected. For example, the battery B to be charged is a lithium ion battery. The output terminal 3 is connected to the plus terminal of the battery B and the output terminal 4 is grounded. A plurality of diodes D 1 , D 2 and D 3 and a first transistor T 1 are connected in series between the input terminal 1 and the output terminal 3 . The first transistor T 1 is connected in this series circuit branch so the charging current is supplied through its collector-emitter path to the battery B. The second input terminal 2 and the second output terminal 4 are both connected to ground.

The series circuit branch including diodes D 1 , D 2 and D 3 , to which a current source is connected, causes a voltage drop in the charging current path. Also resistors can be used instead of diodes. Generally diodes have the advantage that the diode voltage is always constant, independent of charging current. This is usually not the case with conventional ohmic resistors. Also resistors heat up with high charging current.

The current source has two transistors T 3 and T 4 . The base of the transistor T 3 is connected with the collector of the transistor T 4 and the base of the collector T 4 is connected with the emitter of the transistor T 3 . The emitter of the transistor T 4 is connected with the control input (base) of the first transistor T 1 . The collector of the transistor T 3 is connected with the input terminal 1 . Furthermore the base of the transistor T 3 is also connected by means of a resistor R 1 with the input terminal 1 . The emitter of the transistor T 2 and the base of the transistor T 4 are connected via a resistor R 2 to the control input of the transistor T 1 . Another diode D 4 is connected between the input terminal 1 and the collector of the transistor T 3 . This diode D 4 acts as a polarity protector in a known manner. The current source comprising the transistors T 3 and T 4 and resistors R 1 and R 2 , as subsequently described, adjusts the control current supplied to the control input of the first transistor T 1 , so that the first transistor disconnects the battery B, when a predetermined maximum charging or supply voltage is reached at the battery, and thus interrupts the charging current from input terminal 1 to the output terminal 3 .

A Zener diode ZD added between the control input of the first transistor T 1 and ground in cooperation with the above-described current source causes the first transistor to become non-conducting when the battery reaches a predetermined maximum supply voltage and thus the charging current is interrupted. As long as the battery voltage at the output terminal 3 is less than the base-emitter voltage UBE 1 of the first transistor T 1 , i.e. the Zener voltage of the Zener diode ZD, the control current supplied by the current source flows nearly completely into the control input (base) of the first transistor and it remains conducting. Only a small portion of the control current flows through a resistor R 3 connected to ground and parallel to the Zener diode ZD. As soon as the battery charging voltage at the output terminal 3 has reached the base-emitter voltage UBE 1 of the first transistor T 1 , i.e. a value corresponding to the Zener voltage ZD, a large portion of the control current supplied by the current source by means of the Zener diode ZD flows to ground. That means that the control current for the first transistor is so small that it is no longer sufficient to keep the first transistor T 1 conducting. Then the first transistor T 1 opens or becomes non-conducting and interrupts the charging current to the battery B.

An R/C circuit part, which comprises a resistor R 4 and capacitor C, connects a base of a second transistor T 2 to the collector of the first transistor T 1 . This guarantees that the first transistor T 1 is no longer conducting when a predetermined maximum supply voltage is reached at the battery B and thus no charging current is supplied to the battery B. The emitter of the second transistor T 2 is connected with the base of the first transistor T 1 and its collector is connected with the current source. If now, as described previously, the maximum supply voltage is reached, the control current at the base of the first transistor T 1 decreases, so that the voltage drop across the collector and base of this first transistor T 1 increases. As a result, this voltage switches on the second transistor T 2 and also the transistor T 3 of the current source. The current source switches off the control current for the first transistor T 1 . Also now the second transistor T 2 , which forms a voltage divider together with the resistors R 1 and R 3 , determines the voltage at the base of the first transistor T 1 . The dimensions of the resistors R 1 , R 3 and the transistor T 2 are selected so that the voltage at the base of the first transistor T 1 is clearly less than the plus potential at the output terminal 3 connected to the battery B. Thus the base-emitter path of the transistor T 1 is reliably blocked. No more current can flow into the charged battery B.

The R/C circuit part connected to the base of the second transistor T 2 causes a delayed switching on of the second transistor T 2 . Because the turn-on of the second transistor T 2 is delayed, the second transistor is blocked and the current source is prevented from being turned off, in cases in which a higher charging current should flow when the charging apparatus is turned on.

When the battery B comprises e.g. a lithium ion cell, a typical base-emitter voltage of the first transistor T 1 of −5 V suffices for the charging process. The shut off voltage for the first transistor T 1 is typically about −4.1 V. With two or more battery cells connected in series either a diode (not shown in the drawing) must be connected between the emitter of the first transistor T 1 and the battery B, or a first transistor T 1 with a higher permissible base-emitter voltage must be used.

It is advantageous to replace the single Zener diode ZD by a plurality of Zener diodes connected in series each with smaller Zener voltage in other embodiments, which have not been shown in the drawing. These Zener diodes have negative temperature coefficients, which compensates for the temperature behavior of the base-emitter voltage UBE 1 of first transistor T 1 and diode D 4 .

Also in other unshown embodiments a reference voltage for switching off the first transistor T 1 can be produced with other circuit means instead of the Zener voltage of a Zener diode.

The disclosure in German Patent Application 103 35 018.7 of Jul. 31, 2003 is incorporated here by reference. This German Patent Application describes the invention described hereinabove and claimed in the claims appended hereinbelow and provides the basis for a claim of priority for the instant invention under 35 U.S.C. 119.

While the invention has been illustrated and described as embodied in a battery charging apparatus, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.

What is claimed is new and is set forth in the following appended claims.