Design of Li-ion battery monitor and system thermal change and control


Lithium-ion battery monitor

Lithium-ion battery monitor


In addition to the protection circuit (can protect the battery from overcharging, overdischarging and overheating during charging and discharging), the lithium-ion battery monitor can also output the remaining battery energy signal (the LCD display can visually display the remaining battery energy), so that the remaining energy status of the battery can be known at any time, so that the battery can be charged or replaced in time. It is mainly used in portable electronic products with μC or μP, such as mobile phones, video cameras, cameras, medical instruments or audio and video devices.

Now take DS2760 as an example to illustrate the features, internal structure and application circuit of the device. The device has a temperature sensor, a current detector that can detect bidirectional current, and a battery voltage detector, and a 12-bit ADC converts analog to digital; there are multiple memories that can calculate the remaining battery energy. It integrates data collection, information calculation and storage, and security protection. In addition, it has the characteristics of fewer peripheral components, simple circuits, and small device package size (3.25mm2.75mm, die-type BGA package).

DS2760 has a 25mΩ detection resistor inside, which can detect bidirectional (charging and discharging) current (but its own resistance is extremely small and loss is extremely small); the current resolution is 0.625mA, the dynamic range is 1.8A, and there is current accumulation calculation; the voltage measurement resolution is 48mV; the temperature measurement resolution can reach 0.125℃; the digital quantity converted by the ADC is stored in the corresponding memory, and is connected to the main system through a single-wire interface, which can manage and control the power source composed of the lithium-ion battery, that is, realize the read, write access and control of the internal memory. The device has low power consumption, with a maximum current of 80μA in the working state and less than 2μA in the energy-saving state (sleep mode).

The functional structure block diagram of DS2760 is shown as in Figure 1. It is composed of temperature sensor, 25mΩ current detection resistor, multiplexer, reference voltage, ADC, multiple memories, current accumulator and time base, status/control circuit, single-wire interface and address with main system, lithium ion protector, etc.


Figure 1 - Schematic diagram of the functional structure of DS2760

Figure 1 - Schematic diagram of the functional structure of DS2760


DS2760 has three types of memory: EEPOM, lockable EEPROM and SRAM. EEPROM is used to protect important battery data, lockable EEPROM is used as ROM, and SRAM can be temporarily used for data storage.

The application circuit is not complicated, as shown in Figure 2. Two P-channel power MOSFETs respectively control charging and discharging. Li-ion batteries are connected between BAT+ and BAT-. PACK+ and PACK- are the positive and negative terminals of the battery pack, and the DATA terminal is connected to the system. This circuit is suitable for single-cell lithium batteries. If SMD components occupy a small space, it can also be used in batteries.


Figure 2 - Schematic diagram of application circuit

Figure 2 - Schematic diagram of application circuit


Thermal change and control of lithium-ion battery system

Thermal change and control of lithium-ion battery system


The temperature change of the battery system is determined by two factors: heat generation and dissipation. The heat generation can be caused by thermal decomposition and/or reaction between battery materials.

When a certain part of the battery is deviated, such as internal short circuit, high current charge and discharge, and overcharge, a large amount of heat will be generated, resulting in an increase in the temperature of the battery system. When the battery system reaches a certain temperature, it will cause a series of reactions such as decomposition, which will cause the battery to be thermally damaged. At the same time, since the liquid electrolyte in the lithium ion battery is an organic compound and is flammable, the battery will catch fire when the temperature of the system reaches a high level. When the heat generated is not large, the temperature of the battery system is not high, and the battery is in a safe state at this time. The reason for the heat generated inside the lithium-ion battery is mainly as follows.

(1) The reaction between the battery electrolyte and the negative electrode Although there is an interface protective film between the electrolyte and the metal lithium or carbon material, the existence of the protective film limits the reaction between them. However, when the temperature reaches a certain height, the reaction activity increases, and the interface film is not enough to prevent the reaction between the materials, and the reaction can only be prevented when a thicker protective film is formed. Since the reaction is an exothermic reaction, the temperature of the battery system will increase. For example, during the thermal test of the battery, it will indicate that the system has an exothermic reaction. Put the battery in a warmer. When the air temperature rises to a certain level, the temperature of the battery system rises and is higher than the temperature of the surrounding air, but after a period of time, it returns to the surrounding air temperature. It shows that when the protective film reaches a certain thickness, the reaction stops. There is no doubt that different types of protective films are related to the reaction temperature.

(2) Thermal decomposition in the electrolyte When the lithium-ion battery system reaches a certain temperature, the electrolyte will decompose and generate heat. For EC-PC/LiAsF6 electrolyte, the decomposition temperature is about 190℃. After adding 2-methyltetrahydrofuran, the decomposition temperature of the electrolyte began to drop.

For example, for the EC-(2-Me-THF)(50/50)/LiAsP6 and PC-EC-(2-Me-THF)(70/15/15) LiAsP6 systems, their decomposition temperatures are 145°C and 155°C, respectively. But replacing LiAsP6 with LiCF3SO3, the thermal stability is significantly improved. The decomposition temperature of PC-EC-(2-Me-THF)(70/15/15)/LiCF3SO3 is 260℃. When it is oxidized, the thermal stability of the electrolyte system is significantly reduced.

(3) The reaction between the electrolyte and the positive electrode Since the decomposition voltage of the lithium-ion battery electrolyte is higher than the voltage of the positive electrode, the reaction between the electrolyte and the positive electrode rarely occurs. However, when overcharge occurs, the positive electrode will become unstable and will oxidize with the electrolyte to generate heat.

(4) Thermal decomposition of negative electrode material As a negative electrode material, metallic lithium will absorb heat and melt at 180°C. When the negative electrode is heated to above 180°C, the battery temperature will stay at about 180°C. It must be noted that the molten lithium is easy to flow and cause a short circuit.

For carbon anodes, lithium carbide decomposes at 180°C to generate heat. Acupuncture experiments show that the safety limit of lithium insertion is 60%. When the insertion amount is too much, it is easy to cause the negative electrode material to undergo exothermic decomposition at a lower temperature.

(5) Thermal decomposition of the positive electrode material When the working voltage is higher than 4V, the positive electrode material will be unstable, especially when it is in a charged state, the positive electrode material will decompose at 180°C. Compared with other cathode materials, the V2O5 cathode is relatively stable, with a melting point (endothermic) of 670°C and a boiling point of 1690°C. For 4V cathode materials, when they are in a charged state, their decomposition temperature decreases in the following order: LiMnO4>LiCO2>LiNiO2. The reversible capacity of LiNiO2 is high, but it is unstable. By doping with elements such as Al, Co, Mn, etc., its thermal stability can be effectively improved.

(6) The enthalpy change of the positive electrode active material and the negative electrode active material Lithium ion battery absorbs heat when charging, and releases heat when discharging, mainly due to the change of the enthalpy of lithium inserted into the positive electrode material.

(7) Heat is generated by current passing through internal resistance. The battery has internal resistance (Rc). When current passes through the battery, the heat generated by the internal resistance can be calculated using I2Rc. The heat is sometimes polarized heat. When the battery is short-circuited externally, the heat generated by the battery's internal resistance is dominant.

(8) Others For lithium-ion batteries, the negative electrode potential is close to the electrode potential of metal lithium, so in addition to the above reactions, the reaction with adhesives must also be considered. Such as the heat generated by the reaction between fluorine-containing adhesives (including PVDF) and the negative electrode. When other adhesives such as phenolic resin-based adhesives are used, the heat generation of the battery can be greatly reduced. In addition, solvents and electrolyte salts also cause the generation of reaction heat.

Reduce the heat of the battery system and improve the high temperature resistance of the system, and the battery system is safe. In addition, the use of non-flammable or non-flammable electrolytes, such as ceramic electrolytes, molten salts, etc., in the battery manufacturing process can also improve the high temperature resistance of the battery.