Lithium-ion batteries are more "squeaky". If the charging voltage is higher than the specified voltage during charging, the charging current exceeds the specified current; or there is excessive discharge current during the discharge process; if the discharge is terminated until the discharge voltage continues to discharge, it will be damaged. Batteries or scrap them. Since the current price of lithium ion batteries is relatively expensive, various protection components, protectors and monitors have been developed, which can effectively protect the lithium ion battery from being damaged due to improper charging or use, and monitor the signal output of the battery energy.
The PTC polymer protection component is the simplest protector that protects the lithium-ion battery from damage caused by 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 during the charging process (overcharge) or the battery voltage during the discharging process is too low (below the termination discharge voltage, called overdischarge), so a fully functional protector integrated circuit has been developed.
Basic parameters of lithium-ion batteries <br> Lithium-ion batteries are rated at 3.6V (a few are 3.7V). The termination charge voltage at full charge is related to the battery anode material: 4.2V for graphite; 4.1V for coke. The accuracy required to terminate the charging voltage during charging is within ±1%. The termination discharge voltage of a lithium ion battery is 2.4 to 2.7 V (the parameters given by the battery manufacturer for the operating voltage range or the termination discharge voltage are slightly different). The battery is damaged above the termination charging voltage and below the termination discharge.
There are certain requirements for use: charging temperature 0 ~ 45 ° C; discharge or storage temperature -20 ~ +60 ° C. Lithium-ion batteries are not suitable for charging and discharging large currents. Generally, the charging current is not more than 1C, and the discharging current is not more than 2C (C is the capacity of the battery, such as C=950mAh, and the charging rate of 1C is 950mA).
Charging and discharging are better at around 20 °C, can not be charged at negative temperature, and the discharge effect is poor. (The discharge effect is the worst at -20 °C, not only the discharge voltage is low, but the discharge time is less than half of that at 20 °C discharge) .
Lithium-ion battery protector IC
Lithium-ion battery protector ICs are available in single-cell and 2- to 4-cell battery packs. The requirements for this type of protector are described here, with a focus on the single-cell Li-Ion battery protector circuit.
Basic requirements for lithium ion battery protectors:
1. When charging, it should be full, and the accuracy of terminating the charging voltage should be protected by ±1%;
2. In the process of charging and discharging, it does not flow, and there is short circuit protection;
3. It is forbidden to continue discharging until the termination discharge voltage is reached, and the accuracy of the termination discharge voltage is about ±3%;
4. A deeply discharged battery (below the termination discharge voltage) is pre-charged in a turbulent manner prior to charging;
5. In order to ensure stable and reliable operation and prevent interference from transient voltage changes, there are internal delay circuits for overcharge, overdischarge and overcurrent protection to prevent malfunction caused by transient interference;
6. When charging a plurality of battery packs connected in series, it is necessary to protect the matching balance of the battery voltages of each section, and the matching accuracy is required to be about ±10%;
7. The power consumption is saved (the protector is energized when charging or discharging). The power consumption of a single-cell battery protector is generally less than 10μA, and the multi-section is generally around 20μA; when it reaches the termination discharge, it is in a closed state, generally consumes less than 2μA;
8. The protector circuit is simple, has few peripheral components, and takes up a small space, and can be used in a battery or a battery pack;
9. The price is low.
Single-cell lithium-ion battery protector Here, the AIC1811 single-cell lithium-ion battery protector is taken as an example to illustrate the circuit and working principle of the protector. The main features of the device: termination charging voltage is 4.35V, 4.30V and 4.25V (represented by the model suffixes A, B, C respectively), the charging voltage accuracy can reach ±30mV (±0.7%); power consumption is saved at 3.5V The working current is typically 7μA at the working voltage, and the power consumption is only 0.2μA after reaching the termination discharge; there are overcharge, overdischarge, overcurrent protection, and there is a delay to avoid transient interference; the overdischarge voltage is 2.4V, the accuracy is ±3.5%; Small size 5-pin SOT-25 package; operating temperature range -20 to +80 °C.
The single-cell lithium-ion battery protection circuit composed of AIC1811 is shown in Figure 1. The simplified internal structure and external components are shown in Figure 2. V1 is the MOSFET that controls the discharge, V2 is the MOSFET that controls the charging, R1 and C1 are used to eliminate the ripple and interference voltage of the charger input voltage, R2 is to prevent the resistance of the CS terminal when the charger power supply is reversed, and R3 is the bias of V2. Resistance, 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 turned on. The charging current flows from BATT+, charges the battery through the fuse, and flows out through BATT- after V1 and V2. During normal discharge, the current flows from BATT+ through the load RL (not shown in Figure 1), and flows through the BATT- and V2, V1 to the negative pole of the battery, and its current direction is opposite to the direction of the charging current. Since the on-resistance R DS(ON) of V1 and V2 is extremely small, the loss is small.
The working states of several protections are as follows (see Figure 2):
1. Overcharge protection P1 is a comparator with hysteresis for controlling overcharge. R6 and R7 form a voltage divider connected to both ends of the lithium ion battery. The middle of the battery detects the voltage of the battery and is connected to the non-inverting terminal of R1. Connect to the 1.2V reference voltage. When the battery voltage is lower than the overcharge threshold voltage during charging, the voltage of the inverting terminal of P1 is greater than the voltage of the non-inverting terminal, P1 outputs a low level, which makes Q1 turn on, and the biasing resistor R3 of V2 has a current flowing to make V2 also turn on ( V1 is conductive when charging), thus forming a charging circuit. When the charge reaches and exceeds the charging threshold voltage, the P1 non-inverting terminal voltage exceeds 1.2V, P1 outputs a high level, and after 100ms delay, Q1 is turned off, R3 has no voltage to make V2 cut off, and the charging circuit is disconnected to prevent overcharging.
2. Over-discharge protection <br> The over-discharge protection circuit is composed of a voltage divider composed of R4 and R5, a comparator with hysteresis P2, a delay circuit of 100ms, an OR gate, and a CMOS output circuit composed of Q2 and Q3. When the battery discharge reaches 2.4V, P2 outputs a high level. After the delay, the OD output is low, V1 is cut off, the discharge loop is disconnected, and discharge is prohibited.
3. Overcurrent protection <br> Taking discharge current overcurrent protection as an example, the CS terminal is the discharge current detection terminal, which continuously detects the discharge current. This is based on the voltage V CS at the CS terminal and the discharge current IL, as shown in Figure 3. If the turned-on V1 and V2 are regarded as one resistor, that is, R V1DS(ON) and R V2DS(ON) , the discharge circuit is as shown by the dashed line in FIG. If the voltage drop on R2 is neglected, the voltage at VCS to ground is: V CS =[R V1DS(ON) +R V2DS(ON) ]×I L
That is, V CS is proportional to the discharge current IL.
The overcurrent protection circuit is composed of a comparator P3, a delay circuit or a gate. When the discharge current exceeds the set threshold and VCS exceeds 0.2V, P3 outputs a high level, and as a result, as in the case of overdischarge, V2 is turned off, and discharge is prohibited. The device has additional features and will not be covered here.
3- to 4-cell lithium-ion battery protector <br> The MAX1894/MAX1924 is used as an example to illustrate its functions and features. The MAX1894 is designed for 4-cell Li-Ion battery packs, while the MAX1924 is suitable for 3-cell or 4-cell battery packs. Two protectors monitor the voltage of each of the series connected batteries to avoid overcharging and overdischarging, thus effectively extending battery life. In addition, it also prevents excessive current or short circuit during charging and discharging.
The protector circuit composed of two devices is shown in Figure 4. It is a protector for a 4-cell lithium-ion battery pack. It differs from FIG. 1 in that in order to make the voltage of each battery in the series of four batteries charged substantially equal (equal voltage), it increases the internal circuit and external resistance, capacitance and other components; in addition, it is controlled by micro (μC) to control, it can output the status signal of the battery to make the function more perfect.
The main features of the two devices: the overvoltage threshold of each battery is set by the factory, its voltage accuracy can reach ±0.5%; the termination discharge voltage threshold is set by the factory, the accuracy can reach ±2%; there is off mode, off state The power consumption is 0.8μA, which can prevent deep discharge of the battery; the working current is typically 30μA, the small size 16-pin QSOP package; the operating temperature range is -40～+85°C.
Lithium-ion battery monitor <br> Lithium-ion battery monitor not only has protection circuit (protects the battery from overcharging, over-discharging and overheating during charging and discharging), and can output residual energy signal of the battery (with LCD display Visually showing the remaining energy of the battery), the user can always know the remaining energy status of the battery in order to charge or replace the battery 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.
Here, the DS2760 is taken 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 detects bidirectional current, and a battery voltage detector. A 12-bit ADC converts the analog quantity into a digital quantity; there are a variety of memories that enable calculation of the remaining energy of the battery. It combines data collection, information computing and storage, and security. In addition, it features fewer external components, simple circuitry, and small device package size (3.25mm x 2.75mm die-type BGA package).
The DS2760 has a 25mΩ sense resistor inside, which can detect bidirectional (charge and discharge) current (but its own resistance is very small, and the loss is very small); the current resolution is 0.625mA, the dynamic range is 1.8A, and there is current accumulation calculation; The measurement resolution is 48mV; the temperature measurement resolution can reach 0.125°C; 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 supply composed of the lithium ion battery, that is, Enables read/write access and control with internal memory. The device consumes low power, with a maximum current of 80μA during operation and less than 2μA for power-saving states (sleep mode).
The functional block diagram of the DS2760 is shown in Figure 5. It consists of temperature sensor, 25mΩ sense resistor, multiplexer, reference voltage, ADC, various memories, current accumulator time base, state/control circuit, single line interface and address with main system, lithium ion protector and so on.
DS2760 has EEPOM, lockable EEPROM and SRAM three kinds of memory, EEPROM protects important data of battery, lock EEPROM is used as ROM, SRAM is used for temporary data storage.
The application circuit is not complicated, and the circuit is shown in Figure 6. Two P-channel power MOSFETs control charging and discharging respectively. BAT+ and BAT- are connected to the lithium-ion battery. 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 the chip-type components occupy a small space, they can be used in batteries.
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