Design of large-capacity lithium battery management system based on ISL9208

Abstract: This paper introduces the working principle of the large-capacity lithium battery pack management system with ISL9208 as the core, and gives the method of using ISL9208 to detect the voltage, current, temperature and other parameters of the single battery in the battery pack in real time, and using the single chip microcomputer to perform overvoltage, overcurrent, undervoltage, temperature protection and charge and discharge balance on the single battery.
Keywords: ISL9208; P87LPC768; battery pack management system; battery balance

0 Introduction
Lithium-ion batteries have been widely used in practice for their excellent performance. With the improvement of battery manufacturing level, the safety performance of lithium batteries has gradually improved, and the price has continued to decline. Therefore, lithium batteries are increasingly used as power sources in some large-capacity energy storage devices. Lithium
batteries themselves have high requirements for charging and discharging. Once overcharged, it is easy to cause explosion, and over-discharge will cause permanent damage to the battery. Therefore, improper use is very likely to cause personal and property losses. Especially in the use of large-capacity series lithium battery packs, relevant monitoring and control equipment must be designed and installed to prevent the above situation from happening. In addition, due to the inconsistency of single cells in the production process, multiple charging and discharging after series use will aggravate this inconsistency, thereby greatly affecting the life of the entire battery pack. Therefore, the balanced control of the battery pack is extremely important.
To this end, this paper uses Intersil's lithium battery microcontroller analog front-end chip ISL9208 and Philips' small package series control chip P87LPC768 (OTP microcontroller) as the main components, and gives a design method for a large-capacity lithium battery pack management system. The system can realize voltage monitoring and overcharge and over-discharge protection of single cells in the lithium battery pack, as well as overcharge current protection of the lithium battery pack during charging and discharging. At the same time, it can monitor the temperature of the lithium battery pack to ensure a maximum charging balance current of 200 mA for each battery.

1 System Hardware Design
The hardware structure of the battery pack management system given in this paper is shown in Figure 1. Figure 2 shows its actual circuit connection diagram.

When the system is connected to an external load or charger, an external switch is used to pull the WKUP pin of ISL9208 to a high level, thereby waking up the microcontroller analog front end ISL9208. After waking up, the ISL9208 outputs 3.3 V voltage from the RGO port through the built-in 3.3 V regulator to drive the control chip P87LPC768. In this way, the entire system can start to operate after the MCU is powered on.
The MCU can communicate with the ISL9208 through the I2C interface to set the internal registers of the ISL9208, monitor the voltage status of the single cell, and judge the battery status according to the specific parameters of each battery, and then protect the single cell through the balancing module to prevent overcharge and over discharge.
1.1 Control chip P87LPC768
The P87LPC7XX series is a small OTP microcontroller based on the 80C51 accelerated processor structure produced by Philips. Its performance is twice that of the standard 80C51 MCU, and it is low-priced and easy to control costs. P87LPC768 has 4KB OTP program memory and programmable I/O port, 4-channel multiplexed 8-bit A/D converter and I2C communication interface. Since ISL9208 has I2C interface, it can be directly connected with P87 LPC768 without software simulation, so it is more convenient.

1.2 ISL9208
ISL9208IRZ is a multi-cell lithium battery pack overcurrent protection device and microcontroller analog front end produced by Intersil, which can support 5 to 7 series battery packs. It integrates overcurrent protection circuit, short circuit protection, internal 3.3 V regulator, cell balance switch, voltage monitoring level converter and I2C communication interface. The internal structure of ISL9208 is shown in Figure 3.

(1) ISL9208 voltage measurement and charge and discharge voltage protection
ISL9208 can directly measure the voltage of each battery through VCELL1~7. However, the voltage of each battery is higher than the voltage of the regulator, especially the voltage of the battery at the high end may be higher than the voltage that the MCU can accept. Therefore, level conversion and voltage division must be performed during MCU measurement and external A/D conversion. In order to enter the voltage range required by the external circuit, the battery voltage can be divided by 2 based on VSS using a level converter. This allows the voltage of a typical 4.2 V lithium battery at the I/O port to be converted to 2.1 V and output to the outside.
During the charging process, the MCU will periodically measure the voltage of each single battery and compare it with the initial setting value. If it is greater than the initial setting value, the MCU can turn off the external N-channel FET by controlling the CFET pin voltage of the ISL9208 to stop charging and protect the battery pack.
During the discharge process, when the MCU detects that any battery is undervoltage, a control bit can also be written to the ISL9208 to control the voltage of the DFET pin and turn off the external FET to prevent overdischarge.
(2) Current measurement and overcurrent protection of ISL9208
The ISL9208 can choose two ways to charge and discharge, one is to integrate the charging and discharging circuits together, and the other is to separate them. The difference between the two is that the integrated charging and discharging circuits share a circuit, while the separate method uses two pins to detect the charging and discharging currents. The discharge detection stops during charging, and vice versa. This system adopts an integrated solution, so the CSENSE pin is directly grounded, and the DSENSE pin measures the voltage through an external resistor to measure the charging and discharging currents. It should be noted that the reference ground at this time is the DSREF pin.
During the charging process, when the measured voltage of DSENSE exceeds the set value and the time exceeds the set delay, the ISL9208 will enter the overcurrent protection and short-circuit protection mode. At this time, the MCU will control the CFET pin voltage through the chip to turn off the external FET, thereby disconnecting the circuit and avoiding battery pack safety accidents caused by overcurrent. Similarly, during discharge, if a discharge short circuit is detected, the system also controls the voltage of the DFET pin to turn off the external FET to achieve the purpose of control.
Since this design is aimed at large-capacity lithium batteries used in series, the charging current and discharge current are relatively high. Therefore, the external FET in the circuit is recommended to use IRF540NS, which can pass large currents and has good stability.
(3) Measurement and control of the internal temperature of the chip and the temperature of the external battery pack
The internal overheating of the chip is mainly caused by the heat energy generated by the built-in balancing current. The ISL9208 itself integrates the function of stopping battery balancing after the internal IC overheats, so there is no need for an external circuit to monitor the temperature of the chip itself.
The normal operating temperature range of lithium batteries is between 0℃ and 50℃. If the temperature is too low, the battery will not work, and if it is too high, it is easy to cause an explosion. Therefore, temperature control of the battery pack is particularly important. Once the temperature reaches a certain level, an external heat dissipation device must be used to dissipate the heat of the battery pack. If the warning temperature is exceeded, the circuit should be disconnected immediately to ensure safety.
ISL9208 comes with a temperature detection module (TEMP3V pin and TEMPI pin), and the circuit will be turned on repeatedly (TEMP3V is turned on for 5 ms every 640 ms). The TEMP3V pin detects the temperature of the battery pack by connecting an external resistor divider and a thermistor. The TEMPI pin is used to measure the voltage across the thermistor. When the voltage drops to the set threshold value, it indicates external overheating. At this time, the MCU will interrupt the circuit and start the cooling device to wait for the battery pack to dissipate heat and return to normal temperature. The voltage of TEMPI can be output to the MCU through the AO port and the MCU sets the multiplexer.
1.3 Balancing module
Battery balancing can be defined as the application of differential current to a single battery in a battery pack. The battery pack receives the same current, and each battery requires additional electronic components and circuits to achieve battery balancing. Battery balancing directly affects the service life of the entire battery pack, especially in the application of large-capacity lithium battery packs. The battery pack itself has a high cost. If the service life is very short, it is difficult to promote.
ISL9208 can achieve battery balancing with only a small amount of external resistors. It should be noted that this balancing method belongs to voltage balancing. Due to the difference in internal resistance and capacity between batteries, even if the voltage of each single battery is consistent, it does not mean that the capacity of each battery can be consistent. The actual design uses CB1~7 pins and uses the internal FET to bypass a single battery and shunt a small amount of current during charging; when discharging, the current is diverted from the battery. This function can reduce the voltage of the single battery. Its maximum current can reach 200 mA, and the size of the balancing current can be adjusted down according to the shunt resistance. When the balancing current is relatively small, multiple balancing FETs can be turned on, but the overall power consumption limit of the device cannot be exceeded. Excessive balancing current will cause the internal IC to overheat and interrupt charging and discharging.

2 System software design
The software part of this system is to periodically measure various parameters through MCU and compare them with the set values ​​at the time of initialization to determine whether it is necessary to enter the protection state or the balance state. The software of the entire system can adopt a modular design method.
2.1 System initialization module
The system initialization module mainly completes the initialization of ISL9208, mainly setting the system's over-discharge protection voltage, over-charge protection voltage, over-discharge current, the status of DFET and CFET pins, and TEMP3V temperature module, etc.
2.2 Parameter measurement module
The parameter measurement module is mainly used to periodically measure parameters such as voltage, current and temperature under the operation state of lithium batteries. Because the measurement method has been set for each parameter, as long as the MCU sends a command to the SDA pin of ISL9208 through the I2C communication interface and modifies the value of the register (address 03H) A03: A01 of the internal multiplexer of ISL9208, the AO pin can output the various voltage values ​​required to the MCU.
2.3 Status judgment module
The measured value obtained by the parameter measurement module is appropriately converted and then compared with the initial setting value by the MCU. If it exceeds the upper and lower limits, it enters the protection mode. If not, it enters the balancing mode.
2.4 Protection and balancing module
When the MCU determines that the system enters the protection mode, the MCU can control the external FET by setting the value of the last two digits of the FET Control register (address: 04H) of the ISL9208.
If all the parameters measured periodically meet the requirements of the normal working range, then the charge and discharge balancing mode is entered. If the voltage of the battery with the lowest voltage in the current battery pack is used as the reference, the balancing range is ±50 mV (the balancing phase difference voltage can be adjusted by resistors according to actual needs), then it can be determined whether other batteries need to be balanced one by one, and then the MCU can control the voltage of the CB1~7 pins by modifying the value of the Cell Balance (address: 02H) register to turn on or off the balancing module of each battery.

The main program flow chart of this system is shown in Figure 4.

3 Test results
The following results were obtained by testing the parameters of the entire system through simulation circuits:
(1) Overcharge protection voltage: 4.2 V ± 25 mV; Overcharge recovery voltage: 4.0 V ± 25 mV.
(2) Overdischarge protection voltage: 2.7 V ± 25 mV; Overdischarge recovery voltage: 3.0 V ± 25 mV.
(3) Equalization voltage of single cells: 50 mV (can be modified according to actual needs) The specific current parameters are listed in Table 1.

4 Conclusion
This article presents a design method for a 5-7 lithium battery series management system. This method has a simple structure and moderate accuracy, and can meet the management needs of most large-capacity lithium battery energy storage occasions. In addition, if the number of lithium batteries in series is larger, multiple ISL9208s can be connected in parallel to achieve greater expansion.