1. Introduction

18650 type lithium-ion batteries are used in energy storage applications such as renewable energy source buffers, electric transport propulsion, and portable devices like laptops, smartphones, and home appliances.

A 18650 battery is an industry-standard battery 18 mm in diameter and 65 mm long. The cell voltage ranges from 3.6 to 4.2 V, and the cell capacity can be 2600 mAh to 3300 mAh. Recently, lithium-ion batteries of greater capacity have been used. The 21700 (21 mm diameter, 70 mm length) or 26650 (26 mm diameter, 65 mm length) can have a capacity up to 6000 mAh. Even though they are small, lithium-ion batteries like the 18650, 21700, and 26650 have different internal resistances so their current may not be charged and discharged evenly ^(1-13).

The charging voltage of each battery cell in a series-parallel array is different; the charging current is concentrated in the array’s lowest voltage cell, and during discharge, the current is concentrated in the highest voltage cell. During charge, the current is concentrated in the cell with the lowest voltage, and during discharge, the current is conentrated in the cell with the highest voltage. Many battery cell balancing studies have been applied to this problem^(1-13). Fig. 1 classifies conventional battery balancing circuits.

Battery balancing research to date includes several approaches: switched capacitor ^(1-2), resistive shunt ⁽³⁾, buck-boost converter ^(4-5), bidirectional converter ⁽⁶⁾, and flyback converter ⁽⁷⁾ between the cells for directly balancing cell to cell. The switched capacitor method ^(1-2) is typical of energy shuttling methods that use capacitors for energy storage. In this topology, for N cells in a battery string, 2N switches and N-1 capacitors are required. A resistive shunt ⁽³⁾ is limited for resolving rapid cell unbalance and is inefficient due to energy loss. A buck-boost converter ^(4-5), a bidirectional converter ⁽⁶⁾, and a flyback converter ⁽⁷⁾ can each perform fast cell-to-cell balancing but have the disadvantage of complicated control.

The multiple secondary windings of a transformer ^(8-10) have been proposed as a method for balancing pack-to-cell. Also, a two-stage flyback converter ^(11-12) and a bidirectional flyback converter ⁽¹³⁾ have been proposed to balance the pack-to-cell and the cell-to-pack. These existing studies, however, have yielded complicated, expensive, and inefficient solutions with complicated control systems.

In this study, a battery management system (BMS) is proposed that has smoothly performed cell current balancing, a minimum voltage control function, and an overvoltage blocking function. The proposed method uses a voltage-sharing resistor as the balancing method for each pack and cell. This paper analyzes the charging and discharging current flow controlled by the proposed BMS for three parallel strings of 65 Wh class lithium-ion batteries (about 200 Wh capacity) for stable current balancing of their cells. The BMS prototype was fabricated and the validity of the proposed BMS was confirmed through a charging and discharging experiment using an LED and an electrical load.

2. Proposed Battery Management System

Fig. 2 shows the proposed indirect voltage detection BMS, and Fig. 3 shows the equivalent circuit of the BMS. The proposed BMS operates in a structure with lithium-ion batteries in a parallel array. The cell balancing MOSFETs (21,22,23,24,25) are inserted at the (-) terminals of the 1,2,3,4,5 lithium-ion battery cells connected in parallel. In particular, the balancing MOSFETs (21,22,23,24,25) are connected to the voltage distribution resistors (R1-R2, /R3-R4, /R5-R6, /R7-R8, and /R9-R10) at their respective gate terminals.

The voltage distribution resistors connect to the gate of the balancing MOSFETs (21,22,23,24,25), to the gate terminal in series with the upper resistor (R1, R3, R5, R7, R9), and to the lower resistor (R2, R4, R6, R8, R10) in series with the lithium-ion battery.

The gate voltage of the MOSFETs (21,22,23,24,25) can be expressed as

　$V_{con}$ : control voltage of Main Controller

　$V_{Batt}$ : voltage of lithium-ion battery

　$R_{H}$ : upper resistor (R1,R3,R5,R7,R9)

　$R_{L}$： coefficient

When the voltages of the 1,2,3,4,5 lithium-ion battery cells are different, different voltages are applied to the gate terminals of the balancing MOSFETs (21,22,23,24,25) through their upper and lower resistors.

When the voltage of a specific lithium-ion battery cell is high, the gate voltage of a specific balancing MOSFET connected to a lithium-ion battery cell is applied low. At the same time, when the voltage of a specific lithium-ion battery cell is low, the gate voltage of a specific balancing MOSFET connected to a lithium-ion battery cell is applied high. Accordingly, the charging voltage and charging current of the 1,2,3,4,5 lithium-ion battery cells are equalized.

Fig. 4 shows the lowest voltage(VLow) detection and BMS control.

The lowest voltage (VLow) represents the lowest voltage that can be supplied from a lithium-ion battery cell. The lowest voltage (VLow) detection and BMS control consist of a voltage sensor for detecting the lowest voltage, a TL431 circuit, a comparator 1 (OP-Amp1), and a TC4429 circuit (signal inversion circuit).

The lowest voltage (VLow) detection and the BMS controller operation are set so that the gate signal does not occur below the specified lowest voltage (VLow) but does occur above the minimum voltage. The reason for using the TL431 circuit for this paper is to eliminate the hysteresis phenomenon occurring in OP-Amp1.

The voltage divider resistance of the voltage sensor detects whether the set lowest voltage (VLow) has been reached. The TL431 circuit is set to a reference voltage of 2.5 [V] to generate an on signal at the gates of MOSFETs 12, 21, 22, 23, 24, and 25 with a pulse clearly above the lowest voltage (VLow).

The OP-Amp1/TL431 circuit exhibits hysteresis; pulse width modulation (PWM) is generated on and off repeatedly. This circuit was designed to generate an on pulse at the gates of MOSFETs 12, 21, 22, 23, 24, and 25 immediately above the minimum voltage. OP-Amp1 then determines whether the reference voltage (Vref) of 2.5 [V] is reached, and finally, through the TC4429 circuit, the output signal of OP-Amp1 is inverted for application to the MOSFET gates. MOSFET 12 blocks battery discharge below the lowest voltage (VLow).

Fig. 5 shows the cut-off voltage (VHigh2) detection and BMS control unit.

The cut-off voltage (VHigh2) represents the highest voltage at which charging is completely limited in the lithium-ion battery cell. The cut-off voltage (VHigh2) detection and the BMS control unit are composed of a voltage sensor, OP-Amp2, and a TC4429 circuit (signal inversion circuit) for detecting the cut-off voltage (VHigh2). The gate signal does not occur above the cut-off voltage (VHigh2) which is the highest voltage specified by the VHigh2 detection circuit and the BMS control unit. The gate signal occurs below the cut-off voltage (VHigh2). The voltage divider resistance of the voltage sensor for detecting the cut-off voltage detects whether the set maximum voltage has been reached.

When the battery voltage is between VHigh1 and VHigh2, OP-Amp2 generates a PWM stream. OP-Amp2 determines whether the reference voltage of 2.5 [V] (Vref) has been reached, and finally, the output signal of OP-Amp2 is inverted through the TC4429 circuit and applied to MOSFET 11 which completely blocks battery charging above the blocking voltage (VHigh2).

3. Charging and Discharging Mode of The Proposed Battery Management System

Fig. 6 shows the proposed BMS charging mode and charging stop mode.

Fig. 6(a) shows the charging mode below the lowest voltage (VLow), Fig. 6(b) shows the charging mode between the lowest voltage (VLow) and the cut-off voltage (VHigh2), and Fig. 6(c) shows the charging stop mode above the cut-off voltage (VHigh2). For this paper, the circuit was designed to output 21 [V] for LED light emission; the minimum voltage was set to 17.9 [V], and the cut-off voltage was set to 24.6 [V].

Therefore, below 17.9 [V], the circuit operates in charging mode below the lowest voltage (VLow) in Fig. 6(a), and MOSFET 11 is on, but MOSFETs 12, 21, 22, 23 are off. Current is charged to the battery through the anti-parallel diodes of MOSFETs 12, 21, 22, and 23. At 17.9 [V] to 24.6 [V], the charging mode is between the lowest voltage (VLow) and the cut-off voltage (VHigh2), all MOSFETs are in the on state, and the lithium-ion battery cell is monitored for balance. At a voltage of 24.6 [V] or higher, MOSFET 11 is turned off, so charging of the battery is completely blocked.

Fig. 7 shows the proposed BMS discharge mode and discharge stop mode.

Fig. 7(a) shows the discharge mode between the lowest voltage (VLow) and the cut-off voltage (VHigh2), and Fig. 7(b) shows the discharge stop mode below the lowest voltage (VLow).

Between 17.9[V] and 24.6[V], the discharge mode is between the lowest voltage (VLow) and the cut-off voltage (VHigh2); all the MOSFETs are on, the battery is discharging, and battery cell balancing is active. At or below 17.9[V], MOSFETs 12, 21, 22, and 23 are turned off, and the lithium-battery cell does not discharge further.

Fig. 8 shows the theoretical BMS control signal according to the charging or discharging voltage of the battery.

In this paper, the BMS operation of the battery is designed to operate as follows for LED light emission.

For this paper, the design was based on the following criteria for stable operation at 17.9 to 21 [V] to drive the LED module.

- Lowest voltage (Low) [VLow in Fig. 8] : 17.9[V]

- Cut-off voltage (High) [Vhigh2 in Fig. 8] : 24.6[V]

- PWM control(Vhigh1∼Vhigh2 in Fig. 8) : 21.6[V] - 24.6[V]

4. Experiment Result

Table 1 shows the experimental device and circuit parameters, and Fig. 9 pictures the experimental apparatus. In this study, the experiment was conducted based on a voltage of 21[V] for LED light emission, and charging and discharging tests of a 18650 lithium-ion battery were performed using a power supply, LED, and electrical load.

Fig. 10 shows the experimental waveform of the battery control signal according to the input voltage. Depending on the input voltage, at the lowest voltage (Low, 17.9[V]), MOSFETs 12, 21, 22, and 23 are turned off, and the lithium-battery cell is controlled to stop discharge below 17.9[V].

Between the lowest voltage (Low, 17.9[V]) and the lowest voltage of the PWM signal (Vhigh1, 21.6[V]), all of the MOSFETs 11, 12, 21, 22, and 23 are turned on, performing charging or discharging smoothly. Also, between the lowest voltage (Vhigh1, 21.6[V]) and the highest voltage (Vhigh2, 24.6[V]) of the PWM signal, the gate signal (Vgate 11) of MOSFET 11 operates on and off to gradually block the overcharging of the battery.

Above the maximum voltage of the PWM signal (High=Vhigh2, 24.6[V]), the gate signal (Vgate 11) of MOSFET 11 is turned off, and the battery is charged only up to 24.6[V].

Fig. 11 shows the waveforms of input voltage and current at the start of battery charging, and Fig. 12 shows the waveforms of input voltage (Vsys), input current (Isys), and charging current of cells 1, 2, and 3 (Ibatt1, Ibatt2, Ibatt3) when charging the battery.

From Fig. 12, it can be seen that the input current (Isys) is 1.25[A] during charging, and the currents of cells 1, 2, and 3 (Ibatt1, Ibatt2, Ibatt3) are well distributed due to the BMS. Fig.s 13 and 14 show the waveforms of the output voltage (Vsys), output current (Isys), and discharge currents (Ibatt1, Ibatt2, Ibatt3) of cells 1, 2, and 3 when the battery is discharged. Fig. 13 shows the case where the total discharge current is 2.4[A], and Fig. 14 shows the case where the total discharge current is 0.9[A] and generally shows good distribution characteristics.

Table 2 shows the voltage and current values of each cell when charging the battery measured every 3 minutes from the time of charging, and Table 3 shows the voltage and current values of each cell when the battery is discharged.

Table 2 confirms that the lithium-ion battery was charged while being controlled to within 0.194% of the maximum voltage error and 3.337% of the maximum current error.

Table 3 shows the output voltage and current according to the amount of discharge current. In discharges #1 to #5, as the total discharge current increases from 1.25A to 5.25A, the voltage and current are indicated. The table confirms that the discharge was controlled to within 0.164% of the maximum voltage error and 6.715% of the maximum current error. This is thought to be because as the energy stored in the battery is discharged, the energy storage level in the lithium-ion battery cells gradually differs.

Referring to Tables 2 and 3, it can be seen that the BMS proposed in this paper generally uniformly controls the voltages of all battery cells.

5. Conclusion

This paper proposed a BMS for cell balancing with a very simple structure. The proposed BMS cuts off battery discharge below the lowest allowed voltage (Low, 17.9[V]) and cuts off charge above the assigned cutoff voltage (Vhigh2, 24.6[V]). Charging and discharging are stably performed between the lowest voltage (Low, 17.9[V]) and the lowest voltage of the PWM signal (Vhigh1, 21.6[V]) PWM is generated between Vhigh1, 21.6[V]) and the cutoff voltage (Vhigh2, 24.6[V]) to gradually block battery overcharging.

The proposed BMS charge mode, charge stop mode, discharge mode, and discharge stop mode were analyzed. Experimental devices consisting of three parallel models of 65[Wh] class lithium-ion batteries were assembled, and it was confirmed that cell balancing was stably performed during a discharge experiment.