Lithium battery protection board composition and precautions overview
What is a lithium battery protection board? What are its precautions? The lithium battery protection board is mainly composed of maintenance IC (overvoltage maintenance) and MOS tube (overcurrent maintenance), which is used to protect the safety of lithium battery cells. Lithium batteries are widely used by people with large discharge current, low internal resistance, long life, no memory effect, etc. Lithium-ion batteries are prohibited from overcharging, overdischarging, and short-circuiting, otherwise it will cause the battery to catch fire, blast and other death defects Therefore, when using rechargeable lithium batteries, there will be a maintenance board to maintain the safety of the batteries.
Function of lithium battery protection board
1. Voltage protection capability
Overcharge protection board: The protection board must have the ability to prevent the cell voltage from exceeding the preset value. Over-discharge maintenance: The protection board must have the ability to prevent the cell voltage from falling below the preset value.
2. Current capability
As a safety protection device for lithium batteries, the protection board must not only work reliably within the normal operating current scale of the equipment, but also be able to act quickly when the battery is accidentally short-circuited or over-current, so that the batteries are protected.
Definition: When the charging current is 500mA, the on-resistance of the MOS tube.
Due to the high operating frequency of communication equipment, the low bit error rate required for data transmission, and the steep rising and falling edges of the pulse train, the current output capability and voltage stability of the battery are required to be high, so the MOS switch of the protection board is turned on The time resistance should be small, and the single-cell battery protection board is usually "70mΩ. If it is too large, it will cause abnormal operation of the communication equipment, such as the sudden disconnection of the mobile phone during a call, the phone cannot be connected, and noise.
4. Self-consumption current
Definition: The operating voltage of the IC is 3.6V. Under no-load conditions, the operating current flowing through the protection IC is generally very small.
The self-consumption current of the protection board directly affects the standby time of the battery. Usually, the self-consumption current of the protection board is less than 10 microamps.
5. Mechanical function, temperature adaptability, antistatic ability
The protection board must be able to pass the national standard test and impact test; the protection board can work safely at 40 to 85 degrees, and can withstand the ±15KV non-touch ESD electrostatic test.
Characteristics and working principle of lithium battery charge and discharge protection circuit
The protection function of the lithium battery is usually completed by the protection circuit board and the PTC. The protection board is composed of electronic components. The voltage of the battery cell and the current of the charging and discharging circuit can be accurately monitored at all times under the environment of -40℃～+85℃, and timely Control the on and off of the current loop; the main function of PTC is to protect in high temperature environments to prevent the battery from burning, explosion and other vicious accidents.
[Hint] PTC is the abbreviation of PosiTIve temperature coefficient in English, which means positive temperature coefficient resistance (the higher the temperature, the greater the resistance). This component can play a role in over-current protection, that is, to prevent high-temperature discharge of the battery and unsafe high-current charging and discharging. The PTC device is made of high molecular material polymer through a rigorous process, consisting of a polymer resin matrix and conductive particles distributed inside. Under normal circumstances, conductive particles constitute a conductive path in the resin, and the device exhibits low impedance; when there is an overcurrent phenomenon in the circuit, the heat generated by the large current flowing through the PTC causes the volume of the polymer resin matrix to expand, thus Cut off the connection between conductive particles, so as to protect the circuit from overcurrent. When the fault is solved, the component can automatically return to the initial state to ensure the normal operation of the circuit.
Lithium battery charge and discharge requirements
1. Charging of Lithium Battery
The maximum end-of-charge voltage of a single-cell lithium battery is 4.2V, and it cannot be overcharged, otherwise the battery will be scrapped due to the loss of too much lithium ions in the positive electrode. When charging the lithium battery, a dedicated constant current and constant voltage charger should be used. First, charge with constant current until the voltage at both ends of the lithium battery is 4.2V, then switch to constant voltage charging mode; when the constant voltage charging current drops to 100mA, Charging should be stopped.
The charging current (mA) can be 0.1 to 1.5 times the battery capacity, for example: a 1350mAh lithium battery, the charging current can be controlled between 135mA and 2025mA. The normal charging current can be selected at about 0.5 times the battery capacity, and the charging time is about 2 to 3 hours.
2. Discharge of Lithium Battery
Due to the internal structure of the lithium battery, all lithium ions cannot move to the positive electrode during discharge, and a part of the lithium ions must be retained in the negative electrode to ensure that lithium ions can be inserted into the channel smoothly during the next charge. Otherwise, the battery life will be shortened. In order to ensure that some lithium ions remain in the graphite layer after discharge, it is necessary to strictly limit the minimum discharge termination voltage, which means that the lithium battery cannot be over-discharged. The discharge termination voltage of a single-cell lithium battery is usually 3.0V, and the minimum cannot be lower than 2.5V. The length of battery discharge time is related to battery capacity and discharge current. Battery discharge time (hours) = battery capacity/discharge current, and the lithium battery discharge current (mA) should not exceed 3 times the battery capacity, for example: a 1000mAh lithium battery, the discharge current should be strictly controlled within 3A, otherwise it will The battery is damaged.
The composition of the protection circuit
The protection circuit is usually composed of control IC, MOs switch tube, blown fuse, resistor, capacitor and other components, as shown in Figure 2. Under normal circumstances, the control IC output signal controls the MOs switch tube to conduct, so that the cell and the external circuit are connected. When the cell voltage or loop current exceeds the specified value, it immediately controls the MOS tube to turn off to protect the cell. Safety. The control IC has a built-in high-precision voltage detection circuit and a multi-level current detection circuit. Among them, the voltage detection circuit first detects the charging voltage, once it reaches its set threshold (usually 3.9V ~ 4.4V), it immediately enters the overcharge protection state; the second is to detect the discharge voltage, once it reaches its set threshold (Usually 2.0V～3.0V), immediately enter the over-discharge protection state.
In this circuit, the MOS switch tube mostly adopts the thin TSSOP -8 or SOT23 -6 package form, its appearance is shown as in Fig. 3. Some of these MOS switch tubes contain an N-channel field effect transistor, such as FDMC7680, whose pins ①～③ are S poles, ④pins are G poles, and ⑤～⑧ are D poles. The internal structure is shown in Figure 4. Some include two N-channel field effect transistors, such as FDW9926A, 8205A, etc., whose pin function is related to the package form, as shown in Figure 5.
If there is a small circular concave point on the control IC and MOs switch tube, the pin to which the concave point corresponds is the ① pin; if there is no concave on the surface, the first pin on the left side of the component model label is ① The other pins are arranged in a counterclockwise direction. In addition, when switching to a MOS switch tube, it is necessary to judge its internal circuit according to the actual line direction, so as to perform the correct substitution.
In addition, NTC and ID signal forming components are also installed in some lithium battery protection circuits. NTC is the abbreviation of NegaTIvetemperature coefficient in English, which means negative temperature coefficient resistance. This element mainly plays a role of overheating protection in this circuit, that is, when the temperature of the battery itself or its surrounding environment rises, the resistance of the NTC element decreases, and the use of electrical equipment or charging equipment responds in time. If the temperature exceeds a certain value, the system enters Protection state, stop charging and discharging. ID is the abbreviation of IdenTIficaTIon, that is, the meaning of identity recognition. Its information recognition components are divided into two types: one is a memory, often an animal line interface memory, which stores information such as battery type and production date; the other is an identification resistor, both Both can play a role in product traceability and application restrictions.
3. Analysis of the working principle of the protection circuit
The normal output voltage of a single lithium battery is about 3.7V, which can be directly used as a power source for mobile phones, MP3/MP4 and some small-screen tablet computers. For electrical appliances that require higher voltage, such as mobile DVD/EVD or large-screen tablet computers, you can use multiple lithium batteries in series to obtain the required voltage. For a tablet computer that requires 11.1V power supply, the battery assembly is Three lithium batteries connected in series. The protection circuit of a single-cell lithium battery is different from that of a multi-cell series lithium battery. The above are the precautions for the lithium battery protection board. I hope to help everyone.