Isolated Balance Charger for Series-connected Batteries Based on Dual-output Converter

An isolated balance charger for series-connected batteries is proposed in this paper. With the proposed charger, the balanced charging power is directly provided to the series-connected batteries without any additional energy-moving process between the batteries. The proposed charger is composed of an isolated dual half-bridge DC converter and two dual-output converters. Operational principles and control strategy are illustrated in this paper. A lowcost microcontroller unit is adopted to implement the digital controller with fed-back voltage and current signals of the batteries. A prototype for four 12 V/22 Ah series-connected batteries is constructed. From the experimental results, the performance and validity of the proposed charger can then be verified. Compared with the conventional charger, the charging efficiency can be improved by 8.3%.


Introduction
In the modern era, with the development of technology, the consumption of fossil fuel has grown rapidly. The environmental pollution caused by fossil fuel is becoming increasingly serious. Therefore, with rising awareness of environmental protection, green energy is being widely researched and developed. (1)(2)(3) According to statistics, 20% of CO 2 emission comes from transportation and 80% comes from automobiles and motorcycles. For saving energy and CO 2 reduction, the government of Taiwan encourages people to walk or to use electric motorcycles for short-distance transportation and also provides a subsidy for the purchase of such vehicles. Hence, the market demand for electric motorcycles is increasing daily. Moreover, the CO 2 emission of electric motorcycles is only one-fifth that of traditional motorcycles. (4)(5)(6) The study of electric vehicles has therefore become one of the attractive research topics from among recent technologies. Lead-acid and LiFePO 4 batteries are widely used in most electric motorcycles. (7)(8)(9)(10) In the common battery power system, several batteries are usually connected in series to provide higher voltage for driving the motors. However, overcharge and over-discharge due to the use of a conventional charger without charge balance would cause serious damage to the batteries. In recent studies, several kinds of topology have been proposed to equalize the state of charge in each battery cell of the series-connected battery pack. (11)(12)(13)(14)(15)(16) However, the energy moving between the battery cells would result in additional power losses in power conversion processes. In this paper, an isolated balance charger for series-connected batteries is proposed to avoid overcharging the batteries. Also, power is directly charged into each battery equally, and no secondary charge equalization between batteries is required.

Circuit Topology and Operation Principles
The topology of the proposed isolated balance charger is shown in Fig. 1. The proposed charger is composed of a dual half-bridge converter and two balance charging circuits. The dual half-bridge converter with a multiwinding transformer is used to provide two charge powers for two balance charging circuits at the secondary sides. Each balance charging circuit is used to switch the charging current to an individual battery in the series-connected batteries for balance charging. The gating signals and relative waveforms of the dual half-bridge converter are shown in Fig. 2 There are six operation modes in one switching cycle, which are divided by t 0 , t 1 , t 2 , t 3 , t 4 , t 5 , and t 6 as shown in Fig. 2. The corresponding circuit operation of each mode is shown in Fig. 3. The battery set of V B1 and V B2 is represented as the load in subcircuit A and the battery set of V B3 and V B4 is represented as the load in subcircuit B.
Mode 1 (t 0 -t 1 ): The corresponding equivalent circuit is shown in Fig. 3(a). Switches Q 1 and Q 3 are turned on, and switches Q 2 and Q 4 are turned off, and diodes D 2 and D 5 are reverse biased. Inductors L 1 and L 2 are charged.
Mode 2 (t 1 -t 2 ): Switch Q 1 is turned on, and the remaining switches are turned off, as shown in Fig. 3(b). The oscillation on V DS3 and V DS4 results from the resonance of leakage inductance L lk2 and buffer capacitors C 1 and C 2 , as well as the parasitic capacitors C ds3 and C ds4 . Inductor L 1 remains charged and the energy stored in inductor L 2 is then delivered to the output side.
Mode 3 (t 2 -t 3 ): In this mode, all the switches are turned off, as shown in Fig. 3(c). Voltage V DS1 is equal to capacitor voltage V C1 , which is equal to V DC /2. The same as subcircuit A in mode 2, the energy prestored in the inductor L 2 in subcircuit B is released to the output side.
Mode 4 (t 3 -t 4 ): Switches Q 2 and Q 4 are turned on, and switches Q 1 and Q 3 are turned off, as shown in Fig. 3(d). The prestored energy on the inductors L 1 and L 2 are both released to the output sides.
Mode 5 (t 4 -t 5 ): As shown in Fig. 3(e), switch Q 2 remains turned on, whereas the other switches are turned off. In this mode, the operation of subcircuit A is similar to mode 4, and the operation of subcircuit B is similar to mode 2. Mode 6 (t 5 -t 6 ): All switches are turned off and the operation in this mode is similar to mode 3. The end of this mode is also the end of one switching cycle.
In the conventional charging topology, the charging current is directly used to charge seriesconnected batteries. However, the proposed dual-output charging circuit shown in Fig. 1 is used to provide a current switching capability for series-connected batteries. The dual-output charging circuit with an equivalent input current source, as shown in Fig. 4, is composed of two switches and two diodes. By controlling the two switches, the charging current I in can be switched to the corresponding battery. Therefore, each battery in the series-connected batteries can then be balance-charged. Basically, the operation modes of the dual-output charging circuit can be divided into two modes as shown in Fig. 5. In the first mode, the two batteries are connected in series with two switches Q 5 and Q 6 turned off, to be charged, as shown in Fig.  5(a). In the other mode, only one battery is charged. One of the two switches is turned on to bypass the corresponding battery, as shown in Fig. 5(b).

Balance Charging and Control Strategy
The batteries are charged by the constant-current/constant-voltage (CC/CV) charging strategy. The charging mode, i.e., CC or CV, for each battery is controlled in accordance with the voltage of each battery. While the battery voltage is lower than the rated charging voltage,  the battery is charged in the CC mode. When the voltage is higher, the battery is charged in the CV mode. In the CV mode, the charging current would be reduced to maintain a constant charging voltage. The charging process would be stopped when the charging current is reduced to the cutoff current.
The series-connected batteries are divided into two groups, group A (batteries B 1 , B 2 ) and group B (batteries B 3 , B 4 ), as shown Fig. 1. These two groups are charged by the two secondary windings of the dual half-bridge converter. The charging strategy of each group is shown in Fig. 6. While one battery voltage is higher than the other one, the corresponding switch is controlled to switch off the charging current for the battery with the higher voltage. As shown in Fig. 6, switch Q 6 is controlled to reduce charge to battery B 2 while voltage V B2 is higher than V B1 . When switch Q 6 is turned on, battery B 2 is bypassed and the charging current is therefore switched off as well. The charging current for battery B 1 is still equal to the input charging current I in . As a result, the equivalent charge for battery B 2 is less than the charge for battery B 1 and balance charging is achieved. The equivalent charge for battery B 2 is proportional to the duty ratio of the gating signal turn-off time of switch Q 6 . Once the two battery voltages become closer, the control of switch Q 6 is halted, i.e., always be turned off, and the batteries are then series-charged with the same charging current I in .
The closed-loop control block diagram of the charging strategy of series-connected batteries is shown in Fig. 7. The proportion-integration (PI) controller is integrated for current and voltage control. V * is the rated charging voltage of the two series-connected voltages taken from the specifications of the batteries. The actual V T is first obtained as the sum of the two battery voltages V B1 and V B2 . Then the charging current command can be obtained from the error between V * and V T . The charging current command would be limited to the designed maximum charging current for the batteries. Finally, the pulse width modulation (PWM) control signals for the active switches Q 1 and Q 2 in the half-bridge converter can be obtained using the current PI controller. The control signals of Q 3 and Q 4 can also be provided by the controller for the other battery group, i.e., B 3 and B 4 . Figure 8 shows the constructed hardware of the proposed balance charger, and the corresponding circuit parameters of the constructed hardware are shown in Table 1. The gating signals (V GS1 , V GS2 ) and the drain-to-source voltages (V DS1 , V DS2 ) of switches Q 1 and Q 2 are shown in Fig. 9.

Experimental Results and Discussion
The secondary winding voltage V LS1-1 , inductor current i L1 , and diode currents i D1 and i D2 are shown in Fig. 10. Figure 11 shows the measured charging data of each battery with the balance charger. Batteries B 2 , B 3 , and B 4 are precharged with 1.   200 μH Primary side L P1 , L P2 2.7 μH Secondary side L S1-1 , L S1-2 , L S2-1 , L S2-2 300 μH Capacitor C 1 , C 2 330 μF Output capacitor C 3 -C 6 470 μF VGS1:10 V/div VGS2:10 V/div VDS1:100 V/div VDS2:100 V/div It can be seen that once the voltage of one battery in each group is higher, it is bypassed, as mentioned in the previous section. While the battery with a higher voltage is being bypassed, the charging current is reduced to zero to charge only the other battery with the lower terminal voltage. For performance comparison, the charging test with a conventional nonbalance charger is also carried out. In the conventional nonbalance charger, the series-connected batteries are charged with the same charging current and only the terminal voltage of the entire battery module is fed back. Therefore, the state of each battery in the module cannot be controlled in the conventional charger. The charging data for each battery with the conventional charger is shown in Fig. 12. The four batteries are also precharged under the same conditions as in the charging test with the proposed charger. Because there is no feedback signal of each battery in the series-connected batteries, battery B 4 is seriously overcharged to about 16 V and, therefore,   would be damaged. Table 2 shows the results obtained with the proposed charger and the conventional charger. The series-connected batteries are discharged with the same discharging current. From the results, it can be seen that the proposed charger can provide well-balanced charging for series-connected batteries. Moreover, the charging efficiency is about 8% higher than that in the case of the conventional nonbalance charger.

Conclusions
A new isolated balance charger was proposed in this paper. A dual half-bridge converter with multiwindings and a dual-output charging circuit were integrated to provide balance charging for four series-connected batteries. With the proposed charger, the charging power was directly delivered to each battery; therefore, there was no second power conversion required between the batteries. A prototype for four 12 V/22 Ah lead-acid batteries was constructed. Corresponding experiments were also carried out to verify the validity and performance. From the experimental results, it can be seen that the series-connected batteries can be well charged by using our proposed charger. Moreover, the charging efficiency can be improved by 8.3% compared with that of the conventional nonbalance charger.