Method of fast-charging of a rechargeable battery

Details Method of fast-charging of a rechargeable battery Method of fast-charging of a rechargeable battery

2000-12-26

2001-12-25

Toatley, Gregory (Department: 2838)

Electricity: battery or capacitor charging or discharging

Battery or cell charging

Multi-rate charging

C320S148000

Reexamination Certificate

active

06333621

ABSTRACT:

This application is based on application No. 11-368792 filed in Japan on Dec. 27, 1999, the content of which incorporated hereinto by reference.
BACKGROUND OF THE INVENTION
The present invention relates mainly to a method of charging a rechargeable battery such as nickel-cadmium (Ni—Cd) battery or nickel-metal hydride (Ni-MHD) battery, which shows a decrease, &Dgr;V, in voltage from the peak voltage after it is fully charged.
In order to charge a rechargeable battery, it is important to detect the full-charge state and to stop supplying charging current on the completion of full capacity charge (100%), because overcharging causes reduced battery quality on one side, and incomplete charge, i.e. less than 100% capacity charge and leads to a shorter service time on the other. But it is not easy to charge a rechargeable battery to the full capacity under various conditions.
A rechargeable battery such as Ni—Cd battery or Ni-MHD battery shows a decrease, &Dgr;V, from the peak voltage when the charging continues after it has benn fully charged. This characteristic is utilized to detect the full-charge state of a rechargeable battery.
Under normal conditions, the full charge state can be detected accurately by detecting the peak voltage or a decrease, &Dgr;V, in voltage. The rechargeable battery with an unduly low output voltage caused by overdischarge shows charge voltage characteristics different from normal characteristics, when it is fast-charged. Some rechargeable batteries with unduly low output voltage are no longer usable because of an internal short circuit. In order to discriminate batteries between usable ones and unusable ones, weak charging current is supplied in the first step of fast-charging until the output voltage reaches a predetermined voltage, for example 1V, then fast-charging begins.
FIG. 1
shows a voltage curve in a fast-charging process of a battery with overdischarge. As shown in the figure, the charge voltage shows a sharp rise immediately after the fast-charge process begins following the preliminary charge then the charge voltage decreases. Thus a characteristic curve shows a decrease, &Dgr;V, in voltage after it reaches a peak value. When the charge voltage changes in the similar curve pattern, the battery charger determines the completion of the charging process and terminates the charging process. Thus charging process is terminated before the battery has been fully charged. The rise to the peak and decrease, &Dgr;V, in the charge voltage is remarkable when the battery temperature is low.
In order to overcome the drawback, a new method has been proposed in which detection of the voltage decrease is nullified for a certain period of time after the quick charge process begins following the preliminary charge. But the method has the difficulty of setting the time period during which the voltage detection is nullified. In the case where an unduly long time period is predetermined, overcharging which tends to reduce battery quality often occurs due to the failure in detecting the full-charge state.
In the case where an unduly short period time is predetermined, charging is no longer possible for the battery which shows the voltage decrease, &Dgr;V, before the full-charge state has been reached.
The present invention has been developed to overcome these drawbacks. The main objective of the present invention is to propose a method of fast-charging of overdischarged rechargeable batteries capable of accurately charging to the full-charge state.
The above and further objectives and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.
SUMMARY OF THE INVENTION
The method of present invention for fast-charging a rechargeable battery resides in that the completion of charging operation is determined by detecting a peak voltage or decrease in battery voltage from the peak voltage. The method of the present invention for fast-charging further includes a step for detecting the initial battery voltage at the beginning of charging process. If the initial voltage is higher than a predetermined level, the battery is fast-charged with a preselected fast-charge current. If the initial voltage is lower than the predetermined level, the battery is fast-charged with a restricted fast charge current which is smaller than the fast-charge current.
The present method of fast-charging has the advantage that it enables to fast-charge a normally discharged battery on one hand, and to charge an overdischarged battery to the full charge state on the other. An overdischarged battery shows a peak voltage followed by a decrease, &Dgr;V, in voltage before the battery is fully charged, when it undergoes a fast-charge operation with normal fast-charge current. In the method of the present invention, an overdischarged battery is fast-charged with restricted fast-charge current which prevents the battery from showing a peak battery voltage or a decrease in &Dgr;V before it is fully charged. The method enables to detect the completion of the fast-charge operation for an overdischarged battery on the basis of peak voltage or a decrease, &Dgr;V, in voltage.
The method of the present invention for fast-charging preferably includes steps for detecting the battery voltage as well as the battery temperature at the beginning of the charging. If the initial battery voltage is higher than the predetermined level and the battery temperature is higher than a predetermined level, the battery is fast-charged with fast-charge current. But, if the initial battery voltage is lower than the predetermined level and the battery temperature is below the predetermined level, the battery is fast-charged with the limited and smaller fast-charge current.
The limited fast-charge current is set, for example, at 50~90% of the fast-charge current. The smaller the limited fast-charge current, the fewer is the incidents of false detection of full-charge state. But smaller restricted fast-charge currents lead to a longer full charge time. The restricted fast-charge current is therefore determined at a larger possible level as long as incidents of false detection of full-charge state can be avoided.
Yet another embodiment of the present invention for fast-charging a rechargeable battery preferably includes a step for preliminary charge with preliminary charge current smaller than restricted fast-charge current preceding the fast-charge in the case where the initial battery voltage is higher than the predetermined level and lower than a level which is predetermined for the preliminary charge.


REFERENCES:
patent: 5497068 (1996-03-01), Shiojima et al.
patent: 5550453 (1996-08-01), Bohne et al.
patent: 0 623 986 (1994-11-01), None
patent: 2 265 056 (1993-09-01), None

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