CROSS-REFERENCE TO RELATED APPLICATION
This application claims priority to and the benefit of Taiwan Application Series Number 102127028 filed on Jul. 29, 2013, which is incorporated by reference in its entirety.
BACKGROUND
The present disclosure relates generally to charging methods for rechargeable batteries.
Rechargeable batteries, capable of being recharged for repeated use, play an essential role in portable electronic devices which become more and more popular nowadays. To extent the time when a portable device is vital and workable, the rechargeable battery in it must be charged as full as possible. A rechargeable battery should not be over charged, nevertheless. An alkaline rechargeable battery, for example, will suffer in permanent damage if over charged with only several micro voltages beyond its full operation voltage.
FIG. 1 demonstrates a charger 10 and a rechargeable battery 20. VBAT denotes the battery voltage across the rechargeable battery 20, and ICHG the charging current from the charger 10 to the rechargeable battery 20. Shown in FIG. 1, rechargeable battery 20 is represented by an equivalent circuit consisting of internal resistor 26, capacitor 24, and main capacitor 22, where internal resistor 26 and capacitor 24 are connected in parallel and main capacitor 22 acts as a reservoir for storing charge. When the rechargeable battery 20 is connected to nothing or an open circuit, the charging current ICHG is zero and the battery voltage VVAT will stabilize eventually at the same level as the voltage across main capacitor 22, which is accordingly denoted by an open-circuit voltage VOCV. In this specification, an open-circuit voltage VOCV could also be the battery voltage VBAT when the charging current ICHG is zero. The open-circuit voltage VOCV, in a way, corresponds to the amount of the charge stored in the main capacitor 22.
FIG. 2 shows signals generated during charging the rechargeable battery 20 in FIG. 1 according a conventional charging method. Shown in FIG. 2 are, from top to bottom, the battery voltage VBAT and the open-circuit voltage VOCV, the charging current ICHG, and the saturation ratio of the rechargeable battery 20 in percentage. The method in FIG. 2 substantially charges the rechargeable battery 20 first in a constant current (CC) mode and then in a constant voltage (CV) mode. For the CC mode, the charging current ICHG is a constant current IMJR continuously charging the rechargeable battery 20, such that the battery voltage VBAT, the open-circuit voltage VOCV, and the saturation ratio all increase linearly. When the battery voltage
VBAT is about a target voltage VTAR, which roughly corresponds to a fully-charged battery, the CC mode ends and the CV mode follows. The charger 10 in the CV mode substantially fixes the battery voltage VBAT at a voltage level of the target voltage VTAR, so the charging current ICHG diminish over time while the open-circuit voltage VOCV approaches to the target voltage VTAR and the saturation ratio steadily gets closer to 100%. The method in FIG. 2 results in the rechargeable battery 20 with an open-circuit voltage VOCV that is very close to, but does not exceed, the target voltage VTAR. The rechargeable battery 20 is almost fully-charged, accordingly.
The method in FIG. 2 has disadvantages, though. For example, in case that the internal resistor 26 has a very large resistance, the duration for the CV mode to fully charge the rechargeable battery will become very long or even impractical. One possible scenario could be that 20% of the overall charging time is spent for the CC mode to have a rechargeable battery reach its 50% charge capacity while 80% of the overall charging time is spent for the CV mode to provide the rest 50% of its total charge capacity.
Accordingly, it is always a demand in the art to shorten the overall charging time.
BRIEF DESCRIPTION OF THE DRAWINGS
Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following drawings. In the drawings, like reference numerals refer to like parts throughout the various figures unless otherwise specified. These drawings are not necessarily drawn to scale. Likewise, the relative sizes of elements illustrated by the drawings may differ from the relative sizes depicted.
The invention can be more fully understood by the subsequent detailed description and examples with references made to the accompanying drawings, wherein:
FIG. 1 demonstrates a charger and a rechargeable battery in the art;
FIG. 2 shows signals generated during charging the rechargeable battery in FIG. 1 according a conventional charging method;
FIG. 3 demonstrates a charger and a rechargeable battery according to embodiments of the invention;
FIG. 4 shows signals generated during charging the rechargeable battery in FIG. 3;
FIG. 5 details some signals during the supplemental time period TSUP; and
FIGS. 6A and 6B show the charging current ICHG in FIG. 4 according to two different embodiments.
DETAILED DESCRIPTION
FIG. 3 demonstrates a charger 60 and a rechargeable battery 20 according to embodiments of the invention. FIG. 4 shows signals generated during charging the rechargeable battery 20 in FIG. 3. Similar to the signals in FIG. 2, FIG. 4 has, from top to bottom, the battery voltage VBAT and the open-circuit voltage VOCV, the charging current ICHG, and the saturation ratio of the rechargeable battery 20 in percentage.
Shown in FIG. 4, the overall charging time TCHG (from tSTART to tEND) is divided into four time periods, including pre-charge time period TPRE, main charge time period TMJR, supplemental time period TSUP, and constant voltage time period TCV, sequentially.
In the beginning, the battery voltage VBAT is less than an under voltage VUV, and pre-charge time period IPRE starts. During the pre-charge time period IPRE, the rechargeable battery 20 is charged in a constant current mode, in which the charging current ICHG is controlled to be a relatively small constant current IPRE, as demonstrated in FIG. 4. The battery voltage VBAT is monitored during the pre-charge time period IPRE. Once the battery voltage VBAT exceeds the under voltage VUV, which is less than the target voltage VTAR, the pre-charge time period IPRE concludes and the main charge time period TMJR begins .
During the main charge time period TMJR, the rechargeable battery 20 is charged in another constant current mode, in which the charging current ICHG is controlled to be a constant current IMJR larger than the constant current IPRE. In one embodiment, the constant current IMJR is 10 times larger than the constant current IPRE. The battery voltage VBAT is also monitored during the main charge time period TMJR, which concludes if the battery voltage VBAT is found to exceed the target voltage VTAR. The supplemental time period TSUP follows the main charge time period TMJR.
During the supplemental time period TSUP, the rechargeable battery 20 is charged in a pulse mode, in which the charger 60 alternatively charges and stops charging the rechargeable battery 20. When the rechargeable battery 20 is charged, the charging current ICHG is a supplemental constant current ISUP, which optionally might become another constant current with a different value after a break of stopping charging. During the supplemental time period TSUP, the charger 60 detects the open-circuit voltage VOCV, which is the battery voltage VBAT when the charging current ICHG is zero. Once the open-circuit voltage VOCV is equal to or exceeds the target voltage VTAR, the supplemental time period TSUP ends and the constant voltage time period ICV follows. The operation during the supplemental time period TSUP will be detailed soon.
During the constant voltage time period ICV, the charger 60 substantially fixes the battery voltage VBAT at a voltage level of the target voltage VTAR, so as to continue charging the rechargeable battery 20. As the open-circuit voltage VOCV has reached the target voltage VTAR in the end of the supplemental time period TSUP, the charging current ICHG drops quickly, and the saturation ratio becomes very close to, if not equal to, 100%. In one embodiment, when the charging current ICHG is less than 10% of the constant current IMJR, as what is happening at time tEND in FIG. 4, the rechargeable battery 20 seems to be fully charged and the constant voltage time period ICV ends. In this final end, the charger 60 is decoupled from the rechargeable battery 20, and the charging current ICHG is kept as about 0.
FIG. 5 details some signals during the supplemental time period TSUP, including the battery voltage VBAT and the open-circuit voltage VOCV, the charging current ICHG, and a sample signal SSAMPLE. As demonstrated in FIG. 5, the main charge time period TMJR ends and the supplemental time period TSUP starts when the battery voltage VBAT exceeds the target voltage VTAR.
The supplemental time period TSUP is composed of a relaxation time period TREL and at least one pulse charge time period TPLS. The supplemental time period TSUP exemplified in FIG. 5 has a relaxation time period TREL and two pulse charge time periods (TPLS-1 and TPLS-2) each pulse charge time period including a coercive charge time period TFRC and a relaxation time period TREL.
During each coercive charge time period TFRC, the charger 60 charges the rechargeable battery 20 in a constant current mode, using a supplemental constant current ISUP. In FIG. 5, both the supplemental constant currents ISUP-1 and ISUP-2 respectively for the coercive charge time periods TFRC-1 and TFRC-2 have the same magnitude with the constant current IMJR, but the invention is not limited to. In other embodiments of the invention, the supplemental constant current ISUP might vary from one coercive charge time period to another. The duration of each coercive charge time period TFRC is the same in FIG. 5, but the invention is not limited to. In one embodiment, for example, the later the coercive charge time period TFRC the shorter the duration of the coercive charge time period TFRC.
During each coercive charge time period TFRC, the rechargeable battery 20 is forced to be charged, regardless the battery voltage VBAT.
A relaxation time period TREL follows the main charge time period TMJR or a coercive charge time period TFRC. During each relaxation time period TREL, the charging current ICHG is zero, the charger 60 presenting an open circuit to the rechargeable battery 20. Due to that the capacitor 24 discharges itself via the internal resistor 26, the open-circuit voltage VOCV and the battery voltage VBAT approach to each other over time. A sample time period TSAMPLE starts a settle time period TSETL after the beginning of a relaxation time period TREL. Demonstrated in FIG. 5, as long as the settle time period TSETL is long enough, the open-circuit voltage VOCV and the battery voltage VBAT are substantially the same. Accordingly, the charger 60 samples and detects the open-circuit voltage VOCV during the sample time period TSAMPLE. In FIG. 5, all settle time periods TSETL have the same duration in length, but the invention is not limited to. In another embodiment, the later the relaxation time period TREL the longer the settle time period TSETL in it. A subsequent settle time period is longer than a previous settle time period for example.
During the sample time period TSAMPLE in the pulse charge time period TPLS-2 in FIG. 5, the open-circuit voltage VOCV, that is the battery voltage VBAT when the charging current ICHG has been zero for a settle time period TSETL, exceeds the target voltage VTAR. Accordingly, the supplemental time period TSUP ends and the constant voltage time period ICV follows.
FIGS. 6A and 6B show the charging current ICHG in FIG. 4 according to two different embodiments, during a supplemental time period TSUP.
In FIG. 6A, the supplemental constant current ISUP is the same for each coercive charge time period TFRC, which nevertheless becomes shorter subsequently. For example, the open-circuit voltage VOCV detected in the end of one relaxation time period TREL is used in one embodiment to determine the duration of a subsequent coercive charge time period TFRC, and the higher the open-circuit voltage VOCV the shorter a subsequent coercive charge time period TFRC. It is expected that the open-circuit voltage VOCV ramps upward over time, so that for a coercive charge time period TFRC, the later the shorter. Through this way, a rechargeable battery can easily avoid overcharge.
In FIG. 6B, each coercive charge time period TFRC has the same duration, but the supplemental constant current ISUP becomes less in a subsequent coercive charge time period. FIG. 6B shows that the supplemental constant current ISUP-1 is larger in magnitude than the constant current IMJR used in the main charge time period TMJR. For example, the open-circuit voltage VOCV detected in the end of one relaxation time period TREL is used in one embodiment to determine the magnitude of the supplemental constant current ISUP in a subsequent coercive charge time period TFRC, and the higher the open-circuit voltage VOCV the less the supplemental constant current ISUP in a following coercive charge time period. This way could also prevent a rechargeable battery from being over charged.
In another embodiment, the open-circuit voltage VOCV detected in the end of one relaxation time period TREL is used to determine both the duration of a subsequent coercive charge time period TFRC and the magnitude of the supplemental constant current ISUP.
In comparison with FIG. 2, FIG. 4 additionally has a supplemental time period TSUP inserted between the main charge time period TMJR and the constant voltage time period TCV. With properly selected coercive charge time period TFRC and supplemental constant current ISUP, a rechargeable battery could be charged to its full capacity soon and avoid any overcharge, resulting in a shorter constant voltage time period TCV in comparison with that in FIG. 2. The overall charge time TCHGmight become shorter in some embodiments of the invention.
While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.
Patent Application
20150028819
