An Isolated Resonant Voltage Balancing Charger of Series-Connected Lithium-Ion Batteries Based on Multi-Port Transformer

Comments 1: [The main issue that is not addressed is that the output of the converter is connected directly to the batteries. It should have been connected to capacitors and an inductance to filter the output ripple current that should not reach the batteries. The batteries are not happy with ripple currents, see https://www.mdpi.com/2313-0105/8/2/11 for example. So this aspect should be investigated, the ripple to the batteries should be compared with other topologies.]

Response 1: [The leakage inductance of the transformer serves as a filtering inductor, and a capacitor can also be connected to the output. Since this circuit is designed for voltage equalization, no capacitor is connected.] Thank you for pointing this out. We agree with this comment.

Comments 2: [Other aspects that need to be addressed are formulations of resonant cavity and switch tube that are not correct, it should be resonant circuit and transistor.]

Response 2: Agree. We have, accordingly, modified the formulations from “resonant cavity and switch tube” to “resonant circuit and transistor” to emphasize this point.  (Page 1, line 18,20; page 2, line 61,62,86,88; page 6, line 176; page 8, line 254;page 17, line 479.)

Comments 3: [How is the transformer windings realized? From the experimental pictures is not clear. Bigger and clearer pictures should be used.]

Response 3:  [As depicted in Fig. 12b,The designed planar multi - winding transformer has a 16 - turn primary of four - layer PCBs and secondary of double - layer FPCs. With a flat EE core, it can support 32 battery packs with four extra windings at positions 7 and 8. ] Thank you for pointing this out. We agree with this comment. Therefore, we have modified the bigger and clearer pictures to emphasize this point.(Page 12, line 347.)

“[As depicted in Fig. 12b,The designed planar multi - winding transformer has a 16 - turn primary of four - layer PCBs and secondary of double - layer FPCs. With a flat EE core, it can support 32 battery packs with four extra windings at positions 7 and 8. It offers a cost - effective solution by improving the voltage equalization capability and reducing unit cost.]”(Page 12, line 356-360.)

Comments 4: [In 5.2, how are the optimizations realized and what elements are optimized?]

Response 4:  [The parasitic inductance L_si on the secondary side has been optimized. A theoretical analysis was conducted in 3.2. A multi-layer printed circuit board design was employed to reduce the circuit length and the loop area of the current, thereby minimizing the parasitic inductance. ] Thank you for pointing this out. We agree with this comment.

“[The interface board shortens lines and reduces parasitic parameters..]”(Page 12, line 353-254.)  

Comments 1: [The paper states that lithium-ion batteries were used in the experiments, but the voltage range of 1.9V to 2.5V appears lower than the typical range for commercial LIB cells (usually 3.0–4.2V). Could the authors clarify whether these are custom low-voltage LIBs, or whether the cells were operated under intentionally reduced voltage windows for experimental purposes? Like, ref 21.]

Response 1: [In the experiments，Lithium Titanate Oxide (LTO) batteries were employed. The LTO battery box features a 24 - series configuration. Each individual battery has a normal voltage range of 1.9 V to 2.5 V, a capacity of 15 Ah, and a rated charge - discharge current of 100 A. (Page 12, line 252-253.)] Thank you for pointing this out. We agree with this comment.

Comments 2: [While using a multi-port transformer enables effective galvanic isolation and flexible energy transfer among cells, it inherently operates with AC, requiring AC–DC conversion before interfacing with the battery cells. This is because the battery is a DC device. This additional stage may introduce power loss, increase circuit complexity, and raise EMI concerns. It would strengthen the paper if the authors could briefly discuss how the AC–DC conversion is handled in the proposed design and what measures are taken to minimize its impact on system efficiency and reliability.]

Response 2:  [The voltage equalization circuit employs isolated DC - DC conversion. Isolation and energy conversion are achieved through the transformation of a high - frequency AC transformer in the middle. In this way, the system efficiency and reliability are improved.] Thank you for pointing this out. We agree with this comment.

Comments 3: [In references 21, 22, and 8, the transformer, especially the multi-port transformer, was also studied. Although the authors systematically demonstrated parameter fitting of the circuit, the method and fitting results show no advancements. Could the authors emphasize the originality of this work?]

Response 3:  [Compared with References 21, 22 and 8, in this study, there is no bus bar directly connecting the primary-side capacitor and the secondary-side battery, achieving full isolation. ] Thank you for pointing this out. ]

“[As illustrated in Figure 1, the proposed voltage - balancing circuit demonstrates two crucial features that enhance its isolation capabilities. Firstly, no direct transmission bus is present between the capacitor and the battery string. Secondly, there is a single connection path between the primary and secondary sides, which is established via a multi - port transformer. The combination of these characteristics substantially improves the isolation effect. ]”(Page 2, line 80-85.)

Comments 1: [ There are a lot of typing layout mistakes (missing the space between the commas and the words, as example:  On page 1, line 5, you write“Xifeng Xie, Chunjian Cai,Jianglin Nie1,Shijie Jiao and Zeliang Su”, the right one must be: “Xifeng Xie, Chunjian Cai, Jianglin Nie, Shijie Jiao1 and Zeliang Su”. Such mistakes are repeated at different places in the text., as example: Lines 26, 27, 121, 230, ….]

Response 1:Thank you for pointing this out. We agree with this comment. Therefore, we have revised the layout mistakes. (Lines 5,26, 27, 121, 230, ….])

· Comments 2: [The references by the equations are missing.]

Response 2: Thank you for pointing this out. We agree with this comment. Therefore, we have revised equations (6), (7) and (9) which are cited from Reference 8.(Page 8, line 243,247,255.)

· Comments 3: [Please check Equation 9. I think, it must be Cp= 1/[(2Pif)².Lr]?]

Response 3:  Thank you for pointing this out. We agree with this comment. Therefore,.we have revised Equation 9 according to the suggestions . (Page 8, line 255.)

· Comments 4: [On page 4, line 115, you write “In (2), the most extreme ..”. It would be better to write “in equation (2), the most extreme ..”.]

Response 4:  Thank you for pointing this out. We agree with this comment. Therefore,we have revised equation according to the suggestions.

· Comments 5: [Damping Resistance Req to be 3.1.1 Damping Resistance Req; 3.1.2 Resonant   Inductance Lp; 3.1.3 Transformer turns ratio n; 1.4 Distribution of the average voltage Vave and the target battery voltage VBi1. The same is in section 3.2.]

Response 5:  Thank you for pointing this out. We agree with this comment. Therefore,we have revised the format according to the suggestions.

· Comments 6: [By section 4. “The proposed control strategy”, it would be better to write the steps numbers on the Block diagram (Fig. 11).]

Response 6:  Thank you for pointing this out. We agree with this comment. Therefore,we have revised equation according to the suggestions. 

· Comments 7: [By section 4. “The proposed control strategy”, it would be better to write the steps numbers on the Block diagram (Fig. 11).]

Response 7:  Thank you for pointing this out. We agree with this comment. Therefore,we have written the steps numbers on the Block diagram (Fig. 11). (Page 11, line 322.)

· Comments 8: [In section 5.3 “Voltage equalization control experiment”, it would be better to set subsections for the following parameters: 5.3.1 “Static voltage equalization”; … till 5.3.3 Discharging voltage equalization.]

Response 8:  Thank you for pointing this out. We agree with this comment. Therefore,we have set subsections for the following parameters:  “5.3.1, 5.3.2, 5.3.3” .(Page 14, line 413;page 15, line 430 and 439.)

· Comments 9: [In the conclusion, it is recommended to add more details about the gained results through simulation and experimental investigations. At this point, it is recommended to mention the advantages of the proposed method over the known studies of the state of the art.]

Response 9:  [with the highest efficiency reaching 98.2%.(Page 17, line 489.)] Thank you for pointing this out. We agree with this comment.

Comments 1: [Switching tube is still used.]

Response 1: Agree. We have, accordingly, modified the formulations from “ switch tube” to “ transistor” to emphasize this point.  (Page 2, line 58,61,62,87,89........)

Comments 2: [The details of the transformer design are not presented. In Fig. 12b the coil structure and position is not clear. If the number of primary turns is 16 and the ratio n is 10 how is the secondary coil made with 1.6 turns? Also, the primary coils are not clear marked. From the connections is not clear, a PCB of the coils copper layer should be added.]

Response 2: [1.6 turns can be achieved by adjusting the winding length of the secondary winding. Thus, the degree of winding coupling and turns ratio can be reduced].Thank you for pointing this out.

Comments 3: [The leakage inductance of the transformer does not "serves as a filtering inductor" as replied by the authors as this is a resonant converter. The filter inductor and capacitor should be at the battery after the transistors. ]

Response 3:  [The leakage inductance of the transformer cannot "serve as a filtering inductor" because its value is too small, only 1 - 2 μH, making it difficult to achieve the effect of an 18 - μH filtering inductor.]

Figure1 The topology of the LLC converter（See  author-coverletter-45905720.v1.docx）

The topology of the LLC converter is shown in the Figure 1. The resonant inductor L_r and the resonant capacitor C_r are located between the transformer and the transistors.

Comments 4: [The control of the battery current and switch signals is not clear. For example, the Si1, Si2, SA, SB in Fig. 11 , the PWM signals, are not marked in Fig 10, Fig. 8 or  Fig. 3. If the converter is LLC then control should be with variable frequency.]

Response 4:  [the Si1, Si2, SA, SB are marked in Figures 8 and 10. Specifically, SA and SB conduct in turn, and Si1 and Si2 conduct synchronously with SA and SB respectively, depending on whether the corresponding batteries need voltage equalization. (Page 7 Figrure 8;Page 9 Figrure 10).] Thank you for pointing this out. We agree with this comment.

Additionally, the proposed converter is mainly based on the LLC converter and it is controlled by pulse frequency modulation (PFM). We have supplemented the relevant instruction into (Page 2  Line 74-76)

Comments 5: [Also, the secondary side transistors are used for synchronous rectification so their activation signal affects only the efficiency and the voltage drop of the diode, how are they used to control the balancing is not clear.]

Response 5:  [As shown in Figure 1, when the battery voltage is lower than the average voltage, the corresponding transistor conducts to charge the corresponding battery.] Thank you for pointing this out. We agree with this comment. We have supplemented the relevant instruction into (Page 4  Line 125-126)

Comments 6: [From Fig. 3 it seems that the battery is in use with a discharge current and the proposed balancing circuit is charging the battery or compensates the discharge current.  Is the proposed circuit intended as a charger? If so, the title should be changed, considering the large current available in the circuit.]

Response 6:  [An Isolated Resonant Voltage Balancing charger of Series-connected Lithium-ion batteries Based on Multi-Port Transformer.(Page 1,Line 2)] Thank you for pointing this out. We agree with this comment.

Comments 7: [In Fig 14 the optimization of the parasitic elements is presented but what is the actual optimization consisting of is not presented, if there is an optimization, then a before and after should be presented also of the component that is optimized, not only the result.  ]

Response 7:  [Figure 8 illustrates the schematic of i_Lsi with parasitic inductance considered, and figure 14 presents the waveforms before and after optimizing parasitic parameters. The component-level optimizations primarily focus on minimizing circuit parasitic inductance by reducing the length of transformer windings and the wiring distance from batteries to the equalization circuit.] Thank you for pointing this out. We agree with this comment. We have supplemented the relevant instruction into (Page 13  Line 404-406)

Comments 8: [For the voltage experiments, the currents are not clear specified. The parameters of the battery are not complete in Table 1, like the capacity. The slow variation of the battery voltage in 1-2 hours is not explained. The charging at 3A for 9470s generates a small variation of the voltag]

Response 8:  [(1) A parameter has been added to Table 1: the capacity is 15 Ah.(Page 10 Line 300).(2) In the first 1020 seconds, the circuit was in a static state, and the voltage remained almost unchanged. Charging only started after 1020 seconds.(3) The instantaneous current fluctuated at any time during the whole process, and the average current was 3 A . Moreover, within 9470 seconds, the voltage had the fastest rate of change within its range. The reason for the low voltage variation is mainly due to the large battery capacity, whose voltage is difficult to change quickly] Thank you for pointing this out. We agree with this comment.