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1. Composition of lithium power lithium-ion battery protection board

Lithium ion battery protection board is an integrated circuit board designed for the protection of lithium-ion batteries. The protection of lithium-ion batteries is determined by their own characteristics. Due to the material of the lithium-ion battery itself, it cannot be overcharged, overdischarged, overcurrent, short circuited, or subjected to ultra-high temperature charging and discharging. Therefore, when designing a lithium-ion battery pack, a protective plate will be included.

Lithium ion battery protection boards are usually composed of control ICs, switch tubes, precision resistors, NTC, PTC, ID memory, etc. The control IC controls the switch tubes to conduct under normal conditions of the lithium-ion battery pack, allowing the battery cells to communicate with the external circuit. When the battery cell voltage, circuit current, or temperature exceeds the specified value, it immediately (tens of milliseconds) controls the switch tubes to turn off, protecting the safety of the battery cells.

NTC stands for Negative Temperature Coefficient, which is a resistor with a negative temperature coefficient. As the ambient temperature increases, its resistance decreases. ID memory is often a single wire interface memory, which stores information such as the type and processing date of lithium-ion battery packs for lithium-ion power, and can provide product traceability and service life information.

PTC stands for Positive Temperature Coefficient in English, which is a resistor with a positive temperature coefficient. PTC in lithium-ion battery pack products can prevent high-temperature discharge and unsafe high currents. Based on the voltage, current density characteristics, and usage environment of lithium-ion battery packs, there are specific requirements for PTC. PTC is a very important component in lithium-ion battery pack products, which plays a crucial role in ensuring the safety of lithium-ion battery packs. Its performance and quality are also critical factors in the performance and quality of lithium-ion battery packs.

2. Lithium power lithium-ion battery protection board function

The protection of lithium-ion batteries is composed of electronic circuits, which accurately monitor the voltage of the battery cells, the current of the charging and discharging circuit, and the temperature of the battery cells at all times in an environment of -40 ℃ to+85 ℃, and control the on/off of the current circuit in real time. The design of protective plates for single cell protection will be relatively simple, while the design complexity of protective plates for lithium-ion battery pack protection varies according to different requirements.

There are many factors to consider in the design of lithium-ion battery pack protection boards, such as voltage platform issues. Lithium ion battery packs often require a large platform voltage during use. Therefore, when designing lithium-ion battery pack protection boards, efforts should be made to ensure that the protection board does not affect the discharge voltage of the cells. This will result in high requirements for control ICs, sampling resistors, and other components. The current sampling resistor should meet high-precision, low temperature coefficient, and non inductive requirements. The circuit of the lithium-ion battery protection board is shown in Figure 1. In Figure 1, b+and b - respectively represent the positive and negative poles connected to the battery cell; P+and P - are the positive and negative poles of the protection board output, respectively; T is the temperature resistor (NTC) port. The essential functions of lithium-ion battery protection boards include overcharge protection, overdischarge protection, overcurrent protection, short circuit protection, and temperature protection.

(1) Overcharge protection

The meaning of overcharge protection for lithium-ion battery packs: When the voltage of a certain string of lithium-ion battery packs exceeds the maximum value (overvoltage) and reaches the protection delay time, the IC controls Q2 to turn off the charging circuit. Field effect transistors Q1 and Q2 can be equivalent to two switches. When the G-terminal voltage of Q1 or Q2 is greater than 1V, the switch tube conducts. The internal resistance between D and S of the conducting switch tube is very small (tens of milliohms), equivalent to opening and closing; When the voltage at the G pole is less than 0.7V, the switch tube is turned off, and the internal resistance between the D and S poles of the turned off switch tube is very high (several megaohms), which is equivalent to the switch being disconnected.

When charging the lithium-ion battery pack, when the lithium-ion battery pack is charged normally through the charger, as the charging time increases, the voltage at both ends of the battery cell will gradually rise. When the battery cell voltage rises to 4.4V (usually referred to as overcharge protection voltage), the control IC will determine that the battery cell is in an overcharge state, and the control IC will turn off Q2. At this time, the b-terminal of the battery cell and the P-terminal of the protection circuit are in a disconnected state and maintained, that is, the charging circuit of the battery cell is cut off, and charging is stopped.

When the P+and P - terminals of the protection circuit are connected to the discharge load, although the overcharge control switch Q2 is turned off, the positive direction of its internal diode is the same as the current direction of the discharge circuit, so it can still discharge the load. When the voltage across the battery cell is lower than 4.3V (usually referred to as the overcharge protection recovery voltage), the control IC will exit the overcharge protection state of Q2, that is, Q2 will conduct, that is, the b-terminal of the battery cell and the P-terminal of the protection circuit will be reconnected, and the battery cell can resume normal charging.

(2) Over release protection

The meaning of over discharge protection for lithium-ion battery packs: When the voltage of a certain string of lithium-ion battery packs is less than the maximum value (under voltage) and reaches the protection delay, the IC controls Q1 to turn off the discharge circuit.

When the battery cell is discharged through an external load, the voltage at both ends of the lithium-ion battery pack will gradually decrease. At the same time, the control IC will monitor the voltage of the lithium-ion battery pack in real time through resistor R1. When the voltage of a single battery cell drops to 2.3V (usually referred to as the over discharge protection voltage), the control IC considers that the single battery cell is in an over discharge state and will cut off Q1. At this time, the b - and P - of the battery cell are in a disconnected state, that is, the discharge circuit of the lithium-ion battery pack is cut off, and the battery cell will stop discharging.

The protection board is in an over discharge state and remains in this state until the voltage between P+and P - of the protection board rises to the threshold voltage of the IC (usually 3.1V, commonly referred to as the over discharge protection recovery voltage), and the control IC will make Q1 conduct again. The b - of the lithium-ion battery pack and the P - of the protection board are reconnected, and the lithium-ion battery pack is charged straight through the charger.

(3) Overcurrent protection

The meaning of overcurrent protection for lithium-ion battery packs: When the output current of the battery pack P+and P - exceeds the overcurrent/short-circuit current value and reaches the overcurrent delay, the control circuit controls the discharge switch tube to turn off the discharge circuit and stop discharging. The accumulation of heat caused by excessive current is a continuous process, so overcurrent generally has two levels of protection. The first level of protection has a relatively small set value and a longer delay time, while the second level of protection has a relatively large set value and a shorter delay time. When the overcurrent protection is activated, the circuit current instantly becomes 0A. To restore the protection state, there are generally two conditions:

1) Do not intervene manually. After a period of time, the circuit will automatically open. If it is still in an overcurrent state, the lithium-ion battery pack will enter protection again. If the overcurrent is released, the lithium-ion battery pack will enter working mode.

2) Manual intervention is required. After the load or charger is removed, the overcurrent protection should be manually reset.

During the normal discharge process of a lithium-ion battery pack to a load, when the discharge current passes through two series connected switching tubes, a voltage U=I&times will appear at both ends due to the conduction impedance of the switching tubes; RDS× 2 (RDS is the conduction impedance of a single switch tube), the control IC tests the voltage value. If the load is abnormal for some reason, causing an increase in the circuit current, when the circuit current is large enough to make U>0.1V (this value is determined by the control IC, and different ICs have different values), the control IC switches Q1 from conducting to off, thereby cutting off the discharge circuit and making the current in the circuit zero, providing overcurrent protection.

There is a delay time between the occurrence of overcurrent in the control IC test and the issuance of the shutdown signal. The length of this delay time is determined by C2, usually around 13 milliseconds, to guard against misjudgment caused by interference. In the above control process, the magnitude of the overcurrent test value depends not only on the control value of the control IC, but also on the conduction impedance of the switch tube. The larger the conduction impedance of the switch tube, the smaller the overcurrent protection value for the same control IC.

(4) Short circuit protection

Short circuit protection is actually a type of overcurrent protection. However, when the system is short circuited, the current theoretically becomes infinite, and the heat generated is also infinite. If we wait for the software to react before protecting, the lithium-ion battery pack may be damaged. Therefore, the short circuit protection is usually triggered automatically by hardware, and a signal is transmitted to the control IC after triggering.

When the P+and P - output currents of the lithium-ion battery pack exceed the short-circuit current value and reach the short-circuit delay, the control circuit controls the discharge switch to turn off the discharge circuit and stop discharging. Short circuit protection is an extreme form of overcurrent protection, and its control process and principle are the same as overcurrent protection. Short circuit only adds a small resistance (about 0&) between P+and P - to make the load current of the protection board instantly reach the set value, and the protection board immediately triggers short-circuit protection.

(5) Temperature protection

The meaning of temperature protection for lithium-ion battery packs: When the temperature of the lithium-ion battery pack reaches the temperature threshold and reaches the protection delay, the control circuit controls to turn off the charging and discharging switch tube and stop charging and discharging. Temperature protection is relatively simple, with upper and lower limits for temperature protection values. It can even be divided into temperature protection during charging and temperature protection during discharging, which need to be designed according to actual needs.

When designing, it should be noted that in actual detection, temperature is a value that is relatively prone to shaking (which is related to the selected sensor, such as using a thermistor). Therefore, when making judgments, a reasonable range must be set between the protection value and the recovery value, otherwise the system will be unstable.

The T port on the protection board is the over temperature protection terminal. A common over temperature protection circuit is to connect an NTC resistor (see R3 in Figure 1) between the T and P terminals, which is installed tightly against the battery cell. When the lithium-ion battery pack is in high-power operation for a long time, the temperature of the lithium-ion battery pack will rise, and the NTC resistance value will gradually decrease. The control IC of the lithium-ion battery pack protection board tests the NTC resistance value. When the resistance value drops to the set threshold, the control IC immediately sends a command to turn off the charge and discharge switch tube, thus achieving the purpose of protecting the lithium-ion battery pack.

3. Wiring of lithium power lithium-ion battery protection board

The lithium-ion battery protection board for lithium-ion batteries is divided into positive and negative plates. The negative plate is divided into negative same port plate and negative split port plate, both of which have the same function and do not support modifying the settings of the positive and negative protection boards through software. Therefore, the protection method can only be determined according to the wiring diagram of the protection board.

(1) Negative pole plate split connection

The wiring of the negative electrode plate is shown in Figure 2, and the wiring sequence of the negative electrode plate is shown in Table 1.

(2) Negative pole plate with same port connection

The same port connection of the negative electrode plate is shown in Figure 3, and the sequence of the split port connection of the negative electrode plate is shown in Table 2.

(3) Positive electrode plate same port wiring

The wiring of the positive electrode plate at the same port is shown in Figure 4, and the wiring sequence of the positive electrode plate at different ports is shown in Table 3.

(4) Wiring steps for same port protection board

Due to the fact that the wiring of the protection board for lithium-ion battery packs from different manufacturers is not universal, it is necessary to ensure the use of matching wiring when wiring. The wiring steps for the same port protection board are as follows:

1) Do not insert the cable into the protective board before connecting it.

2) Connect the B-line (thick blue line) of the protective board to the negative terminal of the power lithium-ion battery.

3) The first thin black wire of the cable is connected to b -, the second wire (thin red wire) is connected to the positive pole of the first string of batteries, and then connected to the positive pole of each string of batteries in sequence until the last string b+.

4) After the wiring is connected, measure the voltage between every two adjacent metal terminals on the back of the plug. If it is a ternary polymer battery, the voltage should be between 2.8-4.2V, an iron lithium ion battery should be between 2.5-3.65V, and a titanium lithium ion battery should be between 1.6-2.8V.

5) After testing the voltage, insert the protection board into the protection board socket.

6) Measure whether the b+, b - voltage of the battery is consistent with the P+, P - voltage. If the two voltages are consistent, it indicates that the protection board is working properly (the protection board is equivalent to a switch, the switch has been opened, and the current can pass safely). If not, check if the wiring is correct according to the wiring sequence above.

The wiring method for the same port and split port protection board cables is the same, but the difference lies in the wiring method for the discharge and charging wires. The negative electrodes of the same charging and discharging port are all connected to the P-line; The charging cable is connected to the C-line, and the discharging cable is connected to the P-line.

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