Constant current and constant voltage battery charger

Details Constant current and constant voltage battery charger Constant current and constant voltage battery charger

1998-10-05

2000-07-11

Riley, Shawn

Electricity: battery or capacitor charging or discharging

Battery or cell charging

Pulsed

320160, 320162, H02J 700

Patent

active

06087810&

DESCRIPTION:

BRIEF SUMMARY
This application is a U.S. National Phase Application of PCT International Application PCT/JP96/03091.


FIELD OF THE INVENTION

The present invention relates to a charger, and more particularly to a constant-current/constant-voltage battery charger for charging a battery with a constant current and constant voltage.


BACKGROUND OF THE INVENTION

Recently, in an apparatus powered by a battery, primary cells that cannot be recharged, such as manganese primary cells, are being gradually replaced by secondary cells that may be recharged. These cells, such as nickel cadmium batteries and small sealed lead batteries may be used repeatedly. However, the secondary cell may have lower energy density than the primary cell, and hence to obtain the same cell capacity as the primary cell, a larger secondary cell may be required Accordingly, the secondary cell has been developed with increased capacity. Lately, the lithium ion battery capable of a much higher capacity than that of the nickel cadmium battery or small sealed lead battery has been introduced. This cell uses cobaltin lithium in the positive electrode and various compositions of carbon in the negative electrode. The resulting cell has a capacity 250 to 300% greater than the conventional nickel cadmium battery.
The lithium ion battery is generally charged by the constant current and constant voltage method similar to that of the lead battery. That is, it is charged at a constant current rate until the cell voltage reaches a set voltage value, and then charged at constant voltage thereafter. However, if the voltage charged is a little lower than an optimum value, then the lithium ion battery is under charged, and if the voltage charged is a little higher than an optimum value then the battery is overcharged. More specifically, when charged at a voltage exceeding 4.1 V in the lithium ion battery of which negative electrode is natural graphite material, or when charged at a voltage exceeding 4.2 V in the lithium ion battery of which negative electrode is coke material, these cells may be overcharged. The performance of an overcharged lithium ion battery deteriorates more quickly than a nickel cadmium battery. In a worst case, the lithium ions in the battery are formed in an acicular crystal structure called dendrite, and precipitate as metal lithium. This may penetrate through the separator, used as a partition between the positive electrode and negative electrode of the battery, possibly leading to a short-circuit in the battery, smoke or fire.
On the other hand, the constant-current/constant-voltage battery charger may be realized by using a resistance or series regulator, but it is often realized using a chopper circuit to satisfy efficiency and heat generation concerns.
A conventional constant-current/constant-voltage battery charger is described below by referring to the drawings in FIG. 9, FIG. 10A, and FIG. 10B.
FIG. 9 is a circuit diagram showing a constant-current/constant-voltage battery charger of the prior art. In FIG. 9, power source 1 supplies electric power for a chopper circuit 2. Chopper circuit 2 controls the charging current and charging voltage. Cell voltage detection circuit 5 detects the voltage of battery 6, such as a lithium ion battery, to be charged. Voltage controlled oscillator (VCO) has a variable oscillation frequency and depends on the applied voltage. Amplifier 11 amplifies the detected cell voltage from the cell voltage detection circuit 5 which is delivered to the voltage controlled oscillator 9 as a control voltage. Current limiting resistor 12 limits the charging current during constant current charging.
FIG. 10A is a graph showing characteristics of cell voltage v and charging current i as a function of charging time t of a lithium ion battery using a constant-current/constant-voltage battery charger of the prior art. FIG. 10B is a magnified view of part A of FIG. 10A. In FIG. 10A and FIG. 10B, V1 is a set constant voltage charging value, i1 is a fixed constant current charging value, tc is a changeover point from constant

REFERENCES:
patent: 5442274 (1995-08-01), Tamai
patent: 5576608 (1996-11-01), Nagai et al.
patent: 5637979 (1997-06-01), Tamai et al.
patent: 5777457 (1998-07-01), Lee
Japanese language search report for Int 'l Appln. No. PCT/JP96/03091 dated Feb. 4, 1997.
English translation of Japanese language search report.

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