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The 2005 International Power Electronics Conference A Multi-Mode Charging Circuit for Rechargeable Batteries Yao-Ching Hsieh* Chi-Kang Wu** Chin-Sien Moo** To facilitate various battery charging profiles, a multi-mode charging circuit is proposed. This circuit provides not only four fundamental charging functions, such as constant-voltage, constant-current, pulse current and reflex charging; but also the multi-stage charging with hybrid charging modes. The desired charging profile can easily be accomplished by selection of the software control programs. A digital signal processor (DSP) with the associated interface circuits are used as the control kernel. An experimental circuit was built and tested. Experimental results show that the circuit is able to execute the charging functions of various tentative charging strategies with hybrid charging modes. Keyword : multi-stage charging, hybrid charging modes, rechargeable battery listed 1. Introduction (8) « etc; however, pulse or reflex charging has never been above. On the other hand, there are many proposed Many kinds of portable electronic devices are getting multi-stage charging strategies, such as CC-CV, or CV-CV PRUH SRSXODU LQ SHRSOH¶V GDLO\ OLIH VXFK DV QRWHERRNV mobile phones or even (hybrid) electric vehicles. Since WKHVH GHYLFHV FRQWLQXRXVO\ FRQVXPH SOHQW\ RI HQHUJ\ LW¶V reasonable to adopt rechargeable batteries as power source of these applications. However, there are still a lot of obstacles that hinder the applications of rechargeable batteries, such as low capacity, low power density, long FKDUJLQJWLPH«HWF:KLOHWKHEDWWHULHVWKHPVHOYHVFDQQRW be revolutionarily improved, charging technology will be an important topic for research. Besides reducing charging time, the usable capacity after charging and impact on cycle life should also be taken into account. Therefore, there are several aspects of considerations for charging batteries. Although there are several frequently mentioned charging modes, such as constant-current (CC) (1) reflex charging (RX) (5) (6) (7) constant-voltage , pulse charging (PS) (CV), (2) (3) (4) , and , it is not clearly identified which charging mode is the best among the considerations * ** considered into multi-stage charging strategies. In this paper, a DSP-based charging circuit able to implement several charging modes is proposed. This circuit can easily implement four different charging modes, that is, constant-voltage, constant-current, pulse charging, and reflex charging. In addition to carrying out each charging mode alone, the controller can also be programmed to perform multi-stage charging with combinational modes. The charging scenario can be customized by the researchers, such as which charging modes to be adopted, the mode transition points, or the charging intensities. This circuit can serve as a good facility for the subsequent research on charging strategies. A testing circuit was built in laboratory to carry out the basic charging modes and to verify the feasibility of this circuit configuration. Besides, two multi-stage multi-mode charging experiments were performed, and found out that this circuit can exactly follow the commands to execute the desired charging mode at correct charging energy and transit at the pre-defined points. 2. Multi-mode charging circuit Department of Electrical Engineering, Kao-Yuan Institute of Technology, [email protected], 1821, Chung-Shan Rd., Lujhu, Kaohsiung County 821, Taiwan Department of Electrical Engineering, National Sun Yat-sen University, [email protected], 70, Lien-Hai Rd., Kaohsiung 804, Taiwan Fig. 1 shows the circuit configuration of the proposed multi-mode charging circuit. This circuit consists of three parts, that is, energy supplying part, mode-managing part and control unit. Energy supplying part is the unit to 1569 The 2005 International Power Electronics Conference Mode managing part CS2 D2 iCS2 LS D3 Energy supplying part C1 VS Vrec C2 Q A DA D4 T1 QB DB NS C3 NS NP DSP controller Fig. 1 D1 CS1 iC S1 vgs S1 B1 iB1 DS2 RS current feedback Control unit LO DS1 SR voltage feedback Proposed multi-mode charging circuit process the charging energy. The line in AC power is be duty-ratio for the energy-supplying part; and decides the switching states of the switches in mode-managing part. rectified into DC voltage, through a half-bridge inverter to sliced into high-frequency voltage wave to feed the 3. Charging modes step-down transformer, T1. The output of the transformer is rectified and filtered to provide a constant-current or constant-voltage DC source for the mode-managing part at The proposed circuit can perform four different charging modes. The circuit topologies are described in detail in the following for each charging mode. a customized level. Moreover, the charging energy can be regulated by adjusting the duty-ratios of the switches, QA and QB in the half-bridge inverter during the charging 3.1 Constant-current charging mode process to meet the requirement for multi-stage charging. A constant and stable charging current should be provided for constant-current charging mode. Fig. 2 shows the circuit topology at this charging mode. The active switch S1 keeps at off state; therefore, the charging current will flow through diode D1 to feed the battery. The energy supplying part should be gated to provide constant current at this charging mode. The charging current flowing through the battery is sensed via the resistor Rs. The control unit will sense the current and control the duty-ratios of switches QA and QB in the half-bridge inverter to maintain a constant charging current. The mode-managing part is a revised version of the non-dissipative reflex charging circuit (9). By controlling the active switches in the mode-managing part, charging current path can be altered to fulfill the particular charging mode. As switch S1 remains turned off, the circuit will perform constant-current charging. While S1 is continuously switched, and SR remains turned off, pulse charging waveforms can be obtained; otherwise, the circuit executes reflex charging mode. The control unit contains a DSP controller and sensing 3.2 Constant-voltage charging mode interface circuits. The charging current and battery voltage are monitored continuously by the DSP controller. The circuit topology of constant-voltage charging mode is the same as constant-current mode in Fig. 2. The difference is on the energy output of energy supplying part. According to the programmed pre-defined charging scenario, the controller calculates to obtain the required 1570 The 2005 International Power Electronics Conference CS2 CS2 D2 iCS2 D2 iCS2 LS LS D3 D4 IO qpxfs tvqqmzjoh qbsu VS Fig. 2 RS SR D1 vgs S1 D3 CS1 D4 IO iCS1 iB1 qpxfs tvqqmzjoh qbsu VS B1 Circuit topology of constant-current and constant-voltage charging modes Fig. 3 RS SR CS1 iCS1 D1 vgs S1 iB1 B1 Circuit topology of pulse charging mode B1-Cs1-Ls-Cs2-S1. The working behavior of energy supplying In this charging mode, the control unit reads the battery part and active switch S1 are the same as in pulse charging terminal voltage through voltage feedback path. According mode. At the instant of S1 turned on, the battery discharges to the difference between the measured battery voltage and through the resonant path to produce the negative pulse. desired voltage, the control unit will regulate the energy The resonant pulse can only flow unilaterally, since the flowing into the battery by adjusting duty-ratios of the returning path is blocked by diode D3. After the negative half-bridge inverter. While the battery terminal voltage is pulse ceased, the battery is left rested. Until the switch S1 maintained at a fixed level, the battery current will decrease turns off, the charging current flows into the battery; in the gradually. By this charging mode, battery overcharging can meantime, the energy stored in capacitors Cs1 and Cs2 is be avoided; therefore, this is a good choice for the last stage returned back to the battery. Therefore, the energy of of battery charging. negative pulses is not wasted, and the energy efficiency is promoted. 3.3 Pulse charging mode In pulse charging, the charging current is a square-pulse The control unit can be programmed to control the power wave. The idea is to render the battery a short rest period to supplying part and the associated switches to perform a eliminate the undesirable polarization effect. Therefore, a specified charging scenario. The charging profile can be switch is needed to produce an intermittent charging current. one of the four charging modes. Moreover, multiple Fig. 3 shows the circuit topology of pulse charging mode. charging modes can be combined together to charge Control unit will open-circuit the switch SR, and therefore batteries within one charging cycle. If multiple charging disable the resonant path of Cs1, Ls and Cs2. In the same modes are adopted to charge batteries, the control unit will time, switch S1 will be gated at a specified frequency and monitor the terminal voltage of the batteries to decide the duty-ratio to produce the desired pulse current. The transition point for next charging stage. intermittent current will flow through diode D1 to charge CS2 the battery. In this mode, energy supplying part will output D2 iCS2 a constant DC current at a specified level. LS D3 3.4 Reflex charging mode D4 This charging mode also provides the battery with an intermittent charging current. However, for reflex charging, the positive charging pulse is followed by a very narrow negative pulse. Consequently, the circuit will be more complicated in this mode than the other modes discussed previously. Fig. 4 shows the circuit topology. The switch SR is turned on to short-circuit the resonant path of 1571 IO qpxfs tvqqmzjoh qbsu VS Fig. 4 RS D1 vgs S1 SR CS1 iCS1 iB1 Circuit topology of reflex charging mode B1 The 2005 International Power Electronics Conference Table II 4. Circuit implementation On designing the power supplying part, the transformer turn ratio should be carefully selected to supply appropriate voltage output for the next stage. The turn ratio can be obtained from (1). Np D max V rec ««««««... (1) 2V o , max Ns Here, Vo,max is chosen as 30V, in order to provide a wider range of voltage or current output. From the inference of [9], negative impulse intensity, I, and duration, T can be calculated by (2) and (3). I T V B1 Settings Value CC Charging current 1A CV Charging voltage 14V Charging current magnitude 2A Charging pulse frequency 100Hz Charging pulse duty-ratio 50% Table III Experimental results of the four charging modes CC CV PS RX 77 321 56 57 Input capacity(Ah) 1.28 3.38 0.93 0.95 Output capacity(Ah) 1.15 2.46 0.95 0.93 Results Charging time(min) Stabilized battery voltage 12.24 13.08 12.19 12.21 after charging (V) CS ««««««..........(2) 2 LS charging waveforms are shown in Fig. 5(a) and 5(b) respectively. LS C S ««««.(3) 2 The four basic modes of charging experiments were carried out to compare their performances. Except constant-voltage charging, the others are terminated when average battery loaded voltage reached 14V; while constant-voltage would stop at battery current reduced to 100mA. On discharging phases, all experiments discharged with constant 1A current until battery loaded voltages fell to 10V. The charging experiment settings are listed in Table II. S Z0 S The important circuit parameters are listed in Table I. 5. Experimental results A prototype multi-mode charging circuit was built in laboratory, and the controller was programmed according to the charging modes. A 12V Yuasa YB4L-B lead-acid battery with rated capacity of 4Ah is to be charged in the experiments. Before starting a charging experiment, the charging waveforms should be inspected. Pulse and reflex Table I Mode PS, RX where Np and Ns are the turn numbers on primary and secondary windings, Dmax is maximum duty ratio, Vo,max is the maximum required voltage at secondary side of transformer. Experiment settings of the multi-stage charging Circuit parameters Parameter Value Input AC voltage 110V Turn ratio (Np/ Ns) 2.4 Switching frequency of QA, QB 25 kHz Maximal duty ratio of QA, QB 45% Negative pulse magnitude, I 5A Negative pulse duration, T A multi-stage charging experiment with all the charging modes was performed. The charging starts with constant-current mode, and followed by pulse, reflex and finally constant-voltage charging modes. Table IV enumerates the charging parameters and stage transition conditions. Fig. 6 shows the experimental results. The battery voltage and current data were transferred from DSP 11 Ps Inductance, Ls 8.64 PH Capacitance, Cs 3 PF Table III is the experimental results of the charging experiments. Constant-voltage charging spent the longest time, and pulse charging was the shortest. However, by further examining the results, all charging modes but constant-voltage actually fully charging the battery; therefore, the results give no accurate judge to which mode is the best. Furthermore, excluding constant-voltage charging, multi-stage charging is necessary to fully charge a battery. If single-stage charging is adopted, the charging current should be kept at a very low value, and at the expense of longer charging time. 1572 The 2005 International Power Electronics Conference Table IV Experiment settings of CC-PS-RX-CV charging Stage Settings Value Charging current 3A Mode transition voltage 13.75V Charging current magnitude 3A Mode transition voltage 13.75V Charging pulse frequency 100Hz Charging pulse duty-ratio 50% Charging current magnitude 1.5A Mode transition voltage 14V Charging pulse frequency 100Hz Charging pulse duty-ratio 50% Charging voltage 14V Charging ending current 100mA CC PS (a) Pulse charging waveform RX CV (b) Reflex charging waveform until the charging current reduced to 100mA, and then the charging process stopped. The total charging time is 411 min. (1: VBǺ2V/div, 2: IoǺ5A/div, 4: IBǺ5A/div, TimeǺ2ms/div) Fig. 5 Charging waveforms controller to computer and were recorded every 15 seconds. The experiment begins with constant-current mode. After the battery voltage reached the transition point, that is, 13.75V, charging mode changed into pulse charging mode. Since duty ratio of the charging current waveform is 50%, the average charging current reduced; hence, the battery voltage decreased. At the instant, battery voltage touched 13.75V again; it turned into reflex charging mode. Due to Another multi-stage charging marked CC-RX-CV was also performed, with the experiment settings listed in Table V. The experimental results of both of the multi-stage charging are calculated and show in Table VI. From the results, it seems CC-RX-CV is better than CC-PS-RX-CV. However, this is not the final conclusion. There are a lot of combinations of multi-stage charging, and much more study on transition point selections. Many experiments are to be carried out to find out the best charging strategy. the reduction of the current magnitude, the battery voltage fell. While the battery voltage climbed to 14V, the final 6. Conclusion mode, constant-voltage mode started. This mode continued A multi-mode charging circuit for rechargeable batteries is proposed in this paper. This circuit is composed of an Charging current (A) Battery voltage (V) 3.5 15 Battery voltage Table V 3 14 2.5 13 2 12 1.5 Charging current 11 Stage CC 1 0.5 RX 0 10 0 Fig. 6 100 200 300 400 Time (min) CV Battery voltage and current in a multi-stage charging experiment 1573 Experiment settings of CX -RX-CV charging Settings Values Charging current 2A Mode transition voltage 13.4V Charging current magnitude 1A Mode transition voltage 14V Charging pulse duty-ratio 50% Charging voltage 14V Charging ending current 100mA The 2005 International Power Electronics Conference Table VI (3) J. C. M. Chen, ³Rapid Charging of Battery by Applying Alternating Pulsed Large Current Without A High Temperature,´ U. S. Pat. No. 6060865, May 09, 2000. Experiment results of multi-stage chargings CC -RX-CV CC-PS-RX-CV Results Charging time(min) 284 411 Input capacity(Ah) 3.17 3.48 Output capacity(Ah) 2.96 2.84 Stabilized battery voltage after charging (V) 13.25 13.23 (4) ) % 'LQL] / ( 3 %RUJHV DQG % GH % 1HWR ³$ Comparative Study of Pulsed Current Formation for 3RVLWLYH 3ODWHV RI $XWRPRWLYH /HDG $FLG %DWWHULHV´ Journal of Power Sources, Vol.109, No.1, June 2002, pp. 184-188. (5) L. Jun, M. Edward, W. Jack, and K. Paul, ³7KHEffects of Pulse Charging on Cycling Characteristics of Commercial Lithium-Ion Batteries,´ -RXUQDO RI Power Sources, Vol. 102, No.1-2, Dec. 2001, pp. 302-309. adjustable energy supplying part and a revised reflex charging circuit proposed earlier. By this charging circuit, four basic charging modes can be implemented; they are constant-voltage, constant-current, pulse charging and reflex charging. The controller can be programmed to perform solely one of the four charging modes, or even to charging the batteries with hybrid charging modes within one charging cycle. A multi-stage charging experiment with hybrid charging modes was carried out to verify the feasibility of this proposed circuit. By making use of this multi-mode charging circuit, charging strategy can be intensively investigated. It is expected that a more efficient charging strategy can be exploited out to conform to the necessity of modern lifestyles. (6) K. C. Tseng, T. J. Liang, J. F. Chen, and M. T. Chang, ³+LJK)UHTXHQF\3RVLWLYH1HJDWLYH3XOVH&KDUJHUZLWK Power )DFWRU &RUUHFWLRQ´ 7KH rd IEEE Power Electronics Specialists Conference, PESC2002, pp. 671-675. (7) P. H. Cheng and C. L. Chen, ³High Efficiency and Nondissipative Fast Charging Strategy,´ IEE Proc.-Electr. Power Appl., Vol. 150, No. 5, Sep. 2003, pp. 539-545. References (1) +DQV%RGH-RKQ:LOH\6RQV³/HDG-$FLG%DWWHULHV´ 1977. (2) K. Wang, F. C. Lee, and W. Dong, ³A New Soft-Switched Quasi-Single-Stage (QSS) Bi-Directional Inverter/Charger,´ IEEE Industry Application Society Annual Meeting 1999, ,$6¶99, pp. 2031-2038. 1574 (8) T. Ikeya etc., ³Multi-Step Constant-Current Charging Method for an Electric Vehicle Nickel/Metal Hydride Battery with High-Energy Efficiency and Long Cycle Life´, Journal of Power Sources, Vol. 105, No. 1, March 2002, pp. 6-12. (9) Y. C. Hsieh, C. S. Moo, C. K. Wu, J. C. &KHQJ ³$ non-dissipative reflex charging circuit,´ 25th IEEE International Telecommunications Energy Conference, ,17(/(&¶2FW, pp. 679 ± 683.