Nickel Hydride No. 5 battery.Series lithium battery balanced charging battery pack protection board solution

Electronic enthusiasts provide you with a series-connected lithium battery balanced charging battery pack protection board solution. This article is aimed at power lithium batteries used in groups. Each lithium battery requires protection from charging overvoltage, discharge undervoltage, overcurrent, and short circuit. During the charging process To achieve balanced charging of the entire battery pack, a single-cell lithium battery protection chip is designed to charge any string of batteries.

When charging a group of lithium batteries in series, each battery should be charged evenly, otherwise the performance and life of the entire battery group will be affected during use. Commonly used balancing charging technologies include constant shunt resistor balancing charging, on-off shunt resistor balancing charging, average battery voltage balancing charging, switched capacitor balancing charging, buck converter balancing charging, inductor balancing charging, etc. However, the existing single-cell lithium battery protection chips do not contain the balanced charge control function; the balanced charge control function of the multi-cell lithium battery protection chip requires an external CPU and is implemented through serial communication with the protection chip (such as I2C bus), which increases the This increases the complexity and design difficulty of the protection circuit, reduces the efficiency and reliability of the system, and increases power consumption.

This article aims at the use of power lithium batteries in groups. Each lithium battery requires protection from charging overvoltage, discharge undervoltage, overcurrent, and short circuit. During the charging process, it is necessary to achieve balanced charging of the entire group of batteries. A single-cell lithium battery is designed. The protection chip protects any number of series-connected lithium batteries in a battery pack protection board with a balanced charging function. Simulation results and industrial production applications prove that the protection board has perfect protection functions, stable operation, high cost performance, and the balanced charging error is less than 50mV.

Basic working principle of balanced charging of lithium battery pack protection board

The schematic diagram of a lithium battery pack protection board with balanced charging capability designed using a single-cell lithium battery protection chip is shown in Figure 1. Among them: 1 is a single-cell lithium-ion battery; 2 is a charge overvoltage shunt discharge branch resistor; 3 is a switching device for shunt discharge branch control; 4 is an overcurrent detection protection resistor; 5 is an omitted lithium battery protection chip and circuit Connection part; 6 is a single-cell lithium battery protection chip (generally including charge control pin CO, discharge control pin DO, discharge overcurrent and short circuit detection pin VM, battery positive terminal VDD, battery negative terminal VSS, etc.); 7 is The charging overvoltage protection signal is isolated by an optocoupler and forms a parallel relationship to drive the gate of the charging control MOS tube in the main circuit; 8 is the discharge undervoltage, overcurrent and short circuit protection signal, which is isolated by an optocoupler and forms a series relationship to drive the discharge in the main circuit. Control MOS tube gate; 9 is the charge control switching device; 10 is the discharge control switching device; 11 is the control circuit; 12 is the main circuit; 13 is the shunt discharge branch. The number of single-cell lithium battery protection chips is determined based on the number of cells in the lithium battery pack, and they are used in series to protect the corresponding single-cell lithium battery from the charge and discharge, overcurrent, and short-circuit conditions. While charging protection, the system uses the protection chip to control the on and off of the shunt discharge branch switching device to achieve balanced charging. This solution is different from the traditional approach of achieving balanced charging at the charger end and reduces the design of lithium battery pack chargers. Application cost.

Figure 1 Schematic diagram of a lithium battery pack protection board with balanced charging capability

When the lithium battery pack is charging, the positive and negative poles of the external power supply are connected to the positive and negative poles BAT+ and BAT- of the battery pack respectively. The charging current flows through the positive pole BAT+ of the battery pack, the single lithium batteries 1~N in the battery pack, and the discharge control switching device. , charging control switch device, battery pack negative electrode BAT-, the current flow direction is shown in Figure 2.

Figure 2 Charging process

The charging overvoltage protection control signal of the single-cell lithium battery protection chip in the control circuit part of the system is isolated by an optocoupler and output in parallel to provide the gate voltage for the conduction of the charging switch device in the main circuit; such as a certain or several lithium batteries During the charging process, the overvoltage protection state is entered first, and the overvoltage protection signal controls the discharge of the shunt discharge branch connected in parallel at the positive and negative ends of the single-cell lithium battery. At the same time, the corresponding single lithium battery connected in series in the charging circuit is disconnected. Leave the charging circuit.

When charging lithium battery packs in series, the impact of the difference in capacity of single cells is ignored. Generally, the battery with smaller internal resistance is fully charged first. At this time, the corresponding overvoltage protection signal controls the switching device of the shunt discharge branch to close, and a shunt resistor is connected in parallel to both ends of the original battery. According to the pNGV equivalent circuit model of the battery, the shunt branch resistance at this time is equivalent to the load of the single-cell lithium battery that is fully charged first. The battery is discharged through it to maintain the battery terminal voltage within a very small range near the full state. Assuming that the first lithium battery is charged first and enters the overvoltage protection state, the current flow in the main circuit and the shunt discharge branch is as shown in Figure 3. When all single-cell batteries are charged and enter the over-voltage protection state, the voltages of all single-cell lithium batteries are completely equal within the error range. The charge protection control signals of each protection chip become low, and the charge control switching device in the main circuit cannot be used. The gate bias is provided to turn it off and the main circuit is disconnected, that is, balanced charging is achieved and the charging process is completed.

Figure 3 Diversion and equalization process

When the battery pack is discharging, the external load is connected to the positive and negative terminals BAT+ and BAT- of the battery pack respectively, and the discharge current flows through the negative electrode BAT- of the battery pack, the charge control switch device, the discharge control switch device, and the single-cell lithium battery N in the battery pack. ~1 and the positive electrode BAT+ of the battery pack, the current flow direction is shown in Figure 4. In the control circuit part of the system, the discharge under-voltage protection, over-current and short-circuit protection control signals of the single-cell lithium battery protection chip are output in series after being isolated by optocoupler to provide the gate voltage for the conduction of the discharge switching device in the main circuit; once the battery pack When a single-cell lithium battery encounters special conditions such as undervoltage, overcurrent, and short-circuit during the discharge process, the corresponding single-cell lithium battery discharge protection control signal becomes low and cannot provide gate bias for the discharge control switching device in the main circuit. It is turned off and the main circuit is disconnected, which ends the discharge process.

Figure 4 Discharge process

Generally, lithium batteries adopt constant current-constant voltage (TApER) charging control. During constant voltage charging, the charging current decreases approximately exponentially. The switching devices of the main charging and discharging circuit in the system can be selected according to the maximum operating current and operating voltage that the external circuit requirements meet.

The single-cell lithium battery protection chip for the control circuit can be selected according to the voltage level, protection delay time, etc. of the single-cell lithium battery to be protected.

The discharge branch resistance connected in parallel at both ends of a single battery can be calculated based on the charging voltage of the lithium battery charger, the parameters of the lithium battery, and the discharge current. The balancing current should be selected reasonably. If it is too small, the balancing effect will not be obvious; if it is too large, the energy loss of the system will be large, the balancing efficiency will be low, and the thermal management requirements of the lithium battery pack will be high. Generally, the current size can be designed between 50 and 100mA.

The shunt discharge branch resistance can be implemented using a power resistor or a resistor network. It is more reasonable to use a resistor network to realize the shunt discharge branch resistance, which can effectively eliminate the influence of resistance deviation. In addition, it can also reduce thermal power consumption.