Overview of 3.7v lithium battery protection board

What is a lithium battery protection board? What are the precautions for it? The lithium battery protection board is mainly composed of a protection IC (overvoltage protection) and a MOS tube (overcurrent protection). It is a device used to protect the safety of lithium battery cells. Lithium batteries are widely used due to their large discharge current, low internal resistance, long life, and no memory effect. Lithium-ion batteries are prohibited from overcharging, over-discharging, and short circuiting during use, otherwise they may cause fatal defects such as fire or explosion. , Therefore, when using rechargeable lithium batteries, there will be a protective board to protect the safety of the battery cells.

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 protection: The protection board must have the ability to prevent the cell voltage from falling below the preset value.

2. Current capability

(over current protection current, short circuit protection)

As a safety protection device for lithium batteries, the protective board must not only work reliably within the normal operating current range of the equipment, but also act quickly when the battery is accidentally short-circuited or overcurrent to protect the battery core.

3. On-resistance

Definition: When the charging current is 500mA, the on-resistance of the MOS tube.

Since the working frequency of communication equipment is high, the data transmission requires a low bit error rate, and the rising and falling edges of the pulse train are steep, so the battery's current output capability and voltage stability are required to be high, so the MOS tube switch of the protection board is turned on. When the resistance is small, a single cell protection plate is usually <70mΩ. If it is too large, it will cause the communication equipment to operate abnormally, such as the mobile phone suddenly disconnecting during a call, the phone cannot be connected, noise, etc.

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. Generally, the self-consumption current of the protection board is less than 10 microamps.

5. Mechanical function, temperature adaptability, anti-static ability

The protective board must be able to pass the vibration and impact test specified by the national standard; the protective board can work safely at 40 to 85 degrees and can withstand the non-touch ESD static test of ±15KV.

Characteristics and working principle of lithium battery charge and discharge protection circuit

The protection function of lithium batteries is usually completed by the protection circuit board and PTC. The protection board is composed of electronic components. It accurately monitors the voltage of the battery core and the current of the charge and discharge circuit at all times in an environment of -40°C to +85°C, and provides timely Control the on and off of the current loop; the main function of PTC is to protect the battery in high temperature environments and prevent battery combustion, explosion and other vicious accidents.

[Tip] PTC is the abbreviation of PosiTIvetemperature coefficient in English, which means positive temperature coefficient resistance (the higher the temperature, the greater the resistance). This component can act as over-current protection, that is, to prevent high-temperature discharge of the battery and unsafe high-current charge and discharge. PTC devices are made of polymer materials through strict processes. They are composed of a polymer resin matrix and conductive particles distributed inside. Under normal circumstances, the conductive particles form a conductive path in the resin vinegar, and the device exhibits low impedance; when an overcurrent occurs in the circuit, the heat generated by the large current flowing through the PTC causes the polymer resin matrix to expand in volume, thus Cut off the connection between conductive particles, thereby protecting the circuit from overcurrent. When the fault is resolved, the component can automatically return to its initial state to ensure normal operation of the circuit.

1. Charging and discharging requirements of lithium batteries

1.Charging of lithium battery

The maximum charge termination voltage of a single lithium battery is 4.2V. It cannot be overcharged, otherwise the battery will be scrapped due to too much loss of lithium ions in the positive electrode. When charging lithium batteries, 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 0.1~1.5 times the battery capacity. For example: for a 1350mAh lithium battery, the charging current can be controlled between 135mA~2025mA. The conventional charging current can be selected to be around 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. A part of the lithium ions must be retained at the negative electrode to ensure that the lithium ions can be inserted into the channel smoothly during the next charge. Otherwise, battery life will be shortened. In order to ensure that some lithium ions remain in the graphite layer after discharge, the minimum discharge termination voltage must be strictly limited, 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: for a 1000mAh lithium battery, the discharge current should be strictly controlled within 3A, otherwise it will cause The battery is damaged.

2. Composition of protection circuit

The protection circuit usually consists of control IC, MOs switch tube, 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 battery core is connected to the external circuit. When the battery core voltage or loop current exceeds the specified value, it immediately controls the MOS tube to turn off to protect the battery core. Safety. The control IC has built-in high-precision voltage detection circuit and multi-stage 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), it immediately enters the over-discharge protection state.

In this circuit, MOS switch tubes are mostly packaged in thin TSSOP-8 or SOT23-6, and their appearance is shown in Figure 3. Some of these MOS switch tubes contain an N-channel field effect transistor, such as the FDMC7680, whose ①~③ pins are S poles, ④ pins are G poles, and ⑤~⑧ pins are D poles. Its internal structure is shown in Figure 4. ; Some contain two N-channel field effect transistors, such as FDW9926A, 8205A, etc., and their pin functions are related to the packaging form, as shown in Figure 5.

[Tip] If there is a small circular concave spot on the control IC and MOs switch tube, the pin opposite to the concave spot is pin ①; if there is no concave spot on the surface, the first pin on the left side of the component model label is ① pins, and the remaining pins are arranged in a counterclockwise direction. In addition, when replacing MOS switching tubes, it is necessary to judge its internal circuit according to the actual line direction, so as to make the correct replacement.

In addition, some lithium battery protection circuits are also equipped with NTC and ID signal forming components. NTC is the abbreviation of Negative temperature coefficient in English, which means negative temperature coefficient resistance. This component mainly plays the role of overheat protection in this circuit. That is, when the temperature of the battery itself or its surrounding environment increases, the resistance of the NTC component decreases, and the 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, which means identity recognition. Its information recognition components are divided into two types: one is memory, often animal wire interface memory, which stores battery type, production date and other information; the other is identification resistor, both of which All can achieve product traceability and application restrictions.

3. Analysis of working principle of protection circuit

The normal output voltage of a single lithium battery is about 3.7V, which can be directly used as a power supply 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, multiple lithium batteries can be connected in series to obtain the required voltage. For example, a tablet computer that requires 11.1V power supply, the battery component is: Three lithium batteries connected in series. The protection circuits of single-cell lithium batteries and multi-cell lithium batteries in series are different. The above are the precautions for lithium battery protection boards. I hope it can help you.