Lithium battery protection board

1. The composition of the protection board

Li-ion battery protection circuit, Li-ion battery Li-ion battery protection board (rechargeable type) is the reason why it needs to be protected, is determined by its own characteristics. Because the material of the lithium battery protection board itself determines that it can not be overcharged, overdischarged, overcurrent, short circuit and ultra-high temperature charging and discharging, so the lithium battery lithium battery protection board lithium battery components will always follow a delicate protection board and a current fuse appear. The protection function of Li-ion battery is usually accomplished by the protection board and PTC in cooperation. The protection board is composed of electronic circuit, which accurately monitors the voltage of the battery cell and the current of the charging and discharging circuit at all times under the environment of -40℃ to +85℃, and instantly controls the on/off of the current circuit; PTC prevents the battery from bad damage under the high temperature environment.

Lithium battery management system usually includes control IC, MOS switch, resistor, capacitor and auxiliary device NTC, ID memory, etc.. Among them, the control IC, under all normal conditions to control MOS switch on, so that the core and external circuit communication, and when the core voltage or circuit current exceeds the specified value, it immediately (tens of milliseconds) control MOS switch off, to protect the safety of the core. ID memory is often a single-wire interface memory, ID is the abbreviation of Identification, that is, the meaning of identification, storage of battery type, production date and other information. Can play a product traceability and application of the restrictions.

2. The main role of the lithium battery protection board

Lithium battery protection circuit BMS generally requires Control (IC) detects the voltage of the control cell and charging and discharging circuit work current, voltage, in all normal circumstances C-MOS switching tube on, so that the cell and protection circuit board in a normal working state, and when the cell voltage or circuit work current exceeds the control IC in the comparison circuit preset value, in 15 ~ 30ms (different control IC and C-MOS have different response time), the CMOS off, that is, to close the cell discharge or charging circuit, to ensure the safety of the user and the cell.

（1）the normal state

In the normal state of the circuit N1 "CO" and "DO" pin output high voltage, the two MOSFETs are in the on state, the battery can be charged and discharged freely, due to the MOSFET on impedance is very small, usually less than 30 milliohm, so its on resistance on the performance of the circuit is very small.

The current consumption of the protection circuit in this state is μA level, usually less than 7 μA.

（2）Overcharge protection

Lithium-ion batteries require a constant current/voltage charging method, in the early stages of charging, for constant current charging, with the charging process, the voltage will rise to 4.2V (depending on the cathode material, some batteries require a constant voltage value of 4.1V), turning to constant voltage charging, until the current becomes smaller and smaller.

Li-ion battery protection board battery in the process of being charged, if the Li-ion battery protection board charger circuit loses control, it will cause the battery voltage to exceed 4.2V and then continue to charge at constant current, at which time the battery voltage will continue to rise, when the battery voltage is charged to more than 4.3V, the chemical side reactions of the battery will intensify, which will lead to battery damage or safety problems.

In the battery with protection circuit, when the control IC detects that the battery voltage reaches 4.28V (the value is determined by the control IC, different ICs have different values), its "CO" pin will change from high voltage to zero voltage, so that T1 will turn from on to off, thus cutting off the charging circuit, so that the charger can no longer charge the battery. The charger can no longer charge the battery and play the role of overcharge protection. At this time, due to the existence of T1's own body diode VD1, the Li-ion battery protection board battery can be discharged to the external load through the diode.

In the control IC detects that the battery voltage exceeds 4.28V to issue a shutdown T1 signal, there is a delay time, the length of the delay time is determined by C2, usually set to about 1 second, to avoid interference and cause misjudgment.

（3）Over-discharge protection

Battery in the process of discharging the external load, its voltage will gradually decrease with the discharge process, when the lithium battery protection board battery voltage down to 2.5V, its capacity has been completely discharged, at this time, if the battery continues to discharge the load, will cause permanent damage to the battery.

In the process of battery discharge, when the control IC detects that the battery voltage is lower than 2.3V (the value is determined by the control IC, different ICs have different values), its "DO" pin will change from high voltage to zero voltage, so that T2 turns from on to off, thus cutting off the discharge circuit, so that the lithium battery protection board battery can no longer discharge to the load. discharge, to play a role in over-discharge protection. At this time, the charger can charge the battery through the diode due to the existence of the body diode VD2 of T2.

As the battery voltage cannot be lowered in the over-discharge protection state, the current consumption of the Li-ion battery protection board is required to be very small, and the control IC will enter a low-power state at this time, and the power consumption of the whole protection circuit will be less than 0.1μA. Between the time the control IC detects that the battery voltage is lower than 2.3V and sends a signal to turn off T2, there is a delay time, and the length of the delay time is determined by C2, which is usually set to 100 milliseconds or so, in order to avoid misjudgment caused by interference.

（4）Overcurrent protection

Due to the chemical characteristics of lithium-ion batteries, battery manufacturers specify that the maximum discharge current can not exceed 2C (C = battery capacity / hour), when the battery exceeds 2C current discharge, will lead to permanent damage to the battery or safety problems.

Lithium battery protection board in the normal discharge process of the load, the discharge current in series through the 2 MOSFETs, due to the MOSFET on impedance, will produce a voltage at both ends, the voltage value U = I * RDS * 2, RDS for a single MOSFET on impedance, the control IC "V-" pin The voltage value to detect, if the load for some reason caused by abnormal, so that the loop current increases, when the loop current is large enough to make U> 0.1V (the value is determined by the control IC, different ICs have different values), the "DO" pin will be changed from high voltage to zero voltage, so that T2 from conduction to turn off, thus cutting off the discharge circuit, so that the current in the circuit is zero, playing a role in overcurrent protection.

In the control IC detects the occurrence of overcurrent to issue a shutdown T2 signal between, there is also a delay time, the length of the delay time is determined by C2, usually about 13 milliseconds to avoid interference caused by misjudgment.

In the above control process can be seen, the size of its overcurrent detection value depends not only on the control IC control value, but also depends on the MOSFET conduction impedance, when the MOSFET conduction impedance is larger, for the same control IC, the smaller the overcurrent protection value.

（5）Short circuit protection

Lithium battery protection board in the process of discharging the load, if the circuit current is large enough to make U>0.9V (the value is determined by the control IC, different ICs have different values), the control IC will judge the load short circuit, its "DO" pin will quickly change from high voltage to zero voltage, so that T2 from conduction to turn off, thereby cutting off the discharge circuit The short-circuit protection function is played. The delay time of short-circuit protection is very short, usually less than 7 microseconds. The working principle is similar to overcurrent protection, but the judgment method is different, and the protection delay time is also different.