Lithium-ion battery protector and monitor (Fig.)

Rechargeable lithium-ion batteries have many advantages, such as high output voltage, high specific energy, stable discharge voltage, wide operating temperature range, low self-discharge rate, long storage life, no memory effect, and so on. They are used more and more widely, especially with the miniaturization and popularization of mobile phones, which makes the use of lithium-ion batteries increase dramatically. In order to meet the needs of various portable electronic products, in addition to a single lithium-ion battery, there are 2-4 lithium-ion battery packs.

Lithium-ion batteries are more delicate. If the charging voltage is higher than the specified voltage during charging, the charging current exceeds the specified current. Or excessive discharge current during discharge; If the discharge voltage continues until the end, the battery will be damaged or discarded. Due to the high cost of lithium-ion batteries, various protection elements, protectors and monitors have been developed to effectively protect the lithium-ion batteries from damage due to charging or improper use and to monitor the signal output of the battery energy.

PTC polymer protection element is the simplest protector to protect lithium-ion batteries from battery damage due to excessive charging, discharging current or short circuit during charging or discharging. However, it can not solve the problem that the charging voltage is too high (over-charging) or the battery voltage is too low (under the termination discharge voltage, called over-discharge) during the charging process, so a fully functional protector integrated circuit has been developed.

Basic parameters for lithium-ion batteries
The rated voltage of lithium-ion batteries is 3.6V (a minority is 3.7V). The termination charging voltage at full charge is related to the battery anode material: 4.2V of graphite; 4.1V of coke. When charging, it is required that the end charging voltage be within (+) 1%. The termination discharge voltage of lithium-ion batteries is 2.4 to 2.7V (the parameters given by the battery manufacturer for the operating voltage range or termination discharge voltage are slightly different). Batteries can be damaged when they are higher than the termination charging voltage and lower than the termination discharge.

There are certain requirements for use: charging temperature 0-45 C; The discharge or storage temperature is -20 to+60 C. Lithium-ion batteries are not suitable for high current charging and discharging. Generally, the charging current is not more than 1 C and the discharging current is not more than 2 C (C is the capacity of the battery, e.g. C=950 mAh, the charging rate of 1C is 950 mA).

The effect of charging and discharging is better at about 20 C, but it can not be charged at negative temperature, and the effect of discharging is poor. (The effect of discharging at -20 C is the worst, not only the discharging voltage is low, but the discharging time is less than half of that at 20 C.)

Li-ion battery protector IC
The lithium-ion battery protector IC is suitable for single and 2-4 battery packs. This paper describes the requirements of this type of protector, and focuses on the circuit of a single lithium-ion battery protector.

Basic requirements for lithium-ion battery protectors:
1. When charging, it should be fully charged and the termination charging voltage should be kept at +1%;
2. During charging and discharging, there is no current and short-circuit protection;
3. It is forbidden to continue discharging when the end discharging voltage is reached, and the end discharging voltage is about +3%.
4. Precharge batteries with deep discharge (lower than termination discharge voltage) by trickle flow before charging;
5. In order to work stably and reliably and prevent the interference of transient voltage change, there are internal delay circuits with charge, discharge and overcurrent protection to prevent the misoperation caused by transient interference.
6. When charging multiple series batteries, the matching balance of the voltage of each batteries should be maintained, and the matching requirement is about (+) 10%.
7. Save your own power consumption (the protector works on power when charging or discharging). Single battery protectors generally consume less than 10 power μ A, multi-section typically at 20 μ Around A; When the termination discharge is reached, it is turned off and generally consumes 2 μ Below A;
8. The protector has a simple circuit, few peripheral components and takes up less space, and can be used in batteries or battery packs.
9. Low price.

Single lithium-ion battery protector
This paper takes AIC1811 single lithium ion battery protector as an example to illustrate the circuit and working principle of the protector. The main features of the device are as follows: the termination charging voltage is 4.35V, 4.30V and 4.25V (represented by model suffixes A, B and C respectively), and the charging voltage can reach +30mV (+0.7%); Power consumption, typical operating current value 7 at 3.5V operating voltage μ A, only 0.2 power consumption after termination of discharge μ A; Over-charge, over-discharge, over-current protection, and delay to avoid transient interference; The over-discharge voltage is 2.4V, +3.5%; Small size 5 pin SOT-25 package; Operating temperature range - 20 to + 80 C.

A single lithium-ion battery protection circuit consisting of AIC1811 is shown in Figure 1, and its internal structure simplified diagram and external components are shown in Figure 2. V1 is the MOSFET to control discharge, V2 is the MOSFET to control charging, R1 and C1 are used to eliminate ripple and interference voltage of the charger input voltage, R2 is used to protect the resistance of CS end against backward connection of the charger power supply, R3 is the bias resistance of V2, FU is the fuse, BATT+ and BATT - are the positive and negative poles of the battery pack (this protector circuit is placed in the battery).

During normal charging and discharging, both V1 and V2 are conducting. The charging current flows in from BATT+, charges to the battery through the fuse, and flows out from BATT-after V1 and V2. When discharging normally, the current flows from BATT+ to the battery negative through the load RL (not illustrated in Fig. 1), and then to the battery negative through BATT-and V2 and V1, with the current direction opposite to the charging current direction. Because the conduction resistance RDS (ON) of V1 and V2 is very small, the loss is small.

The working status of several protections is as follows (see Figure 2):

　　1. Overcharge protection
P1 is a comparator with lag to control overcharging. The voltage divider consisting of R6 and R7 is connected to both ends of the Li-ion battery. The middle end detects the voltage of the battery and connects to the same end of R1. The reverse end of P1 is connected to the 1.2V reference voltage. When the battery voltage is lower than the overcharge threshold voltage during charging, the reverse voltage of P1 is greater than the same-phase voltage, P1 outputs a low level, which leads to Q1 conduction, and the bias resistance R3 of V2 flows through to make V2 conductive (V1 is conductive during charging), thus forming a charging circuit. When the charging reaches and exceeds the charging threshold voltage, the P1 ipsilateral voltage exceeds 1.2V, P1 outputs a high level, Q1 is cut off after 100ms delay, V2 is cut off by R3 no voltage, and the charging circuit is disconnected to prevent overcharging.

　　2. Overdischarge protection
The over-discharge protection circuit consists of a voltage divider composed of R4 and R5, a comparator with lag P2, a 100ms delay circuit, or a gate, and a CMOS output circuit composed of Q2 and Q3. When the battery discharge reaches 2.4V, P2 outputs a high level, after delay, OD outputs a low level, V1 cuts off, the discharge circuit is disconnected, forbidding discharge.

　　3. Overcurrent protection
For example, the CS end is the detection end of the discharge current, which continuously detects the discharge current. This is due to the relationship between the voltage VCS at the CS end and the discharge current IL, as shown in Figure 3. If the conducting V1 and V2 are considered as resistors, that is, RV1DS (ON) and RV2DS (ON), the discharge circuit is shown in the dashed line of figure 3. If the very small voltage drop on R2 is ignored, the VCS voltage to the ground is:
VCS=[RV1DS(ON)+RV2DS(ON)] × IL
That is, the VCS is proportional to the discharge current IL.
The overcurrent protection circuit is composed of a comparator P3, a delay circuit or a gate. If the discharge current exceeds the set threshold and the VCS exceeds 0.2V, P3 outputs a high level, the result is that the V2 is cut off and the discharge is prohibited in the same way as the over discharge. Other features of this device are not described here.

3-4 Lithium Ion Battery Protectors
This paper takes MAX1894/MAX1924 as an example to illustrate its functions and features. MAX1894 is designed for four lithium-ion battery packs, while MAX1924 is designed for three or four battery packs. The two protectors monitor the voltage of each battery in the series battery to avoid overcharging and discharging, thereby effectively prolonging the battery life. In addition, it can also prevent excessive current or short circuit during charging and discharging.

The protector circuit consisting of two devices is shown in Figure 4. It is a protector for four lithium-ion battery packs. It differs from Figure 1 in that the voltage of each of the four batteries in series is essentially equal (equal voltage) when charging, so it adds internal circuit, external resistance, capacitance and other components. In addition, it is controlled by a microcontroller ( μ C) Control, it can output the status signal of the battery, make the function more perfect.

The main features of the two devices are that the overvoltage threshold of each battery is set by the factory and the voltage of each battery can reach (+) 0.5%; The threshold value of termination discharge voltage is set by the factory and can reach (+) 2%; For off mode, power consumption is 0.8 when off state μ A, to prevent deep battery discharge; Typical value of working current 30 μ A, small size 16 pin QSOP package; Operating temperature range - 40 to + 85 C.

Lithium Ion Battery Monitor
In addition to the protective circuit (which protects the battery from overcharging, overdischarging and overheating during charging and discharging), the lithium-ion battery monitor can output the remaining energy signal of the battery (LCD display can visually show the remaining energy of the battery). Users can always know the remaining energy status of the battery in order to charge or replace the battery in time. It is mainly used for μ C or μ P in portable electronic products, such as mobile phones, cameras, cameras, medical devices or audio and video devices.

This paper takes DS2760 as an example to illustrate the characteristics, internal structure and application circuit of the device. The device has a temperature sensor, a current detector capable of detecting bidirectional current and a battery voltage detector, and a 12-bit ADC converts analog quantities to digital quantities. There are a variety of memory that can calculate the remaining battery energy. It combines data collection, information calculation, storage and security protection. In addition, it has few peripheral elements, simple circuit, small device packaging size (3.25mm) × Features of 2.75mm tube-core BGA package.

DS2760 has a 25 mm_internal resistance to detect bidirectional (charging and discharging) currents (with minimal self-resistance and minimal loss); The current resolution is 0.625mA, the dynamic range is 1.8A, and there is a current accumulation calculation. The voltage measurement resolution is 48 mV. The resolution of temperature measurement can reach 0.125 C. The amount of digits converted by ADC is stored in the corresponding memory, and connected to the main system through a single-line interface, the power supply composed of lithium-ion batteries can be managed and controlled, that is, read/write access and control with the internal memory. The device consumes low power and operates at a current of 80. μ A, less than 2 in energy-saving state (sleep mode) μ A.

The functional structure of DS2760 is shown in Figure 5. It is composed of temperature sensor, 25m detection resistance, multiplexer, reference voltage, ADC, multiple memory, current accumulator and time base, status/control circuit, single line interface with main system, address, lithium ion protector, etc.

DS2760 has three types of memory: EEPOM, Lockable EEPROM and SRAM. EEPROM protects the important data of batteries, Lockable EEPROM is used as ROM, and SRAM is used to store temporary abundant data.

The application circuit is not complex, as shown in Figure 6. Two P-channel power MOSFETs control charging and discharging respectively. Lithium-ion batteries are inserted between BAT+, BAT-. PACK+, PACK- are the positive and negative ends of the batteries, and DATA terminal is the interface with the system. The circuit is suitable for a single lithium battery. If the use of patch components takes up little space, it can be used in the battery.

Source:Xiang Xueqin