Semco university – All about the Lithium-Ion Batteries

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Lithium battery protection board – Semco University

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Introduction

The Lithium battery protection board is a small size board that provides protection against short-circuit, overcharge and over discharge. The board comes with pre-soldered Nickel strips which makes it a ready-to-use module with 18650 cells.

The Composition of the Protective Plate

Lithium battery protection circuit, the reason why the lithium battery protection board(rechargeable) needs to be protected is determined by its own characteristics. Since the material of the lithium battery protection board itself determines that it cannot be overcharged, over discharged, overcurrent, short circuit and ultra-high temperature charge and discharge, the lithium battery components of the lithium battery protection board will always be followed by a delicate protection board and a current insurance device appears. 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 circuits. It can accurately monitor the voltage of the battery cell and the current of the charging and discharging circuit at all times under the environment of -40°C to +85°C. Control the on-off of the current loop; PTC prevents bad damage to the battery in a high temperature environment.

Lithium battery protection boards usually include control ICs, MOS switches, resistors, capacitors and auxiliary devices NTC, ID memory, etc. Among them, the control IC controls the MOS switch to turn on when everything is normal, so that the cell communicates with the external circuit, and when the cell voltage or loop current exceeds the specified value, it immediately (tens of milliseconds) controls the MOS switch to turn off, Protect the safety of cells. NTC is the abbreviation of Negative temperature coefficient, which means negative temperature coefficient. When the ambient temperature rises, its resistance value decreases, and electrical equipment or charging equipment is used to respond in time and control the internal interruption to stop charging and discharging. ID memory is often a single-line interface memory, ID is the abbreviation of Identification, which means identification, and stores information such as battery type and production date. It can be used to limit the traceability and application of products.

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The Main Function of the Lithium Battery Protection Board

  • Overcharge protection: When the cell voltage rises to a certain value (below 4.4V), the protection board will cut off the charging circuit.
  • Over-discharge protection: When the cell voltage drops to a certain value (above 2.2V), the protection board will cut off the discharge circuit.
  • Short circuit protection: When the positive and negative terminals of the battery are short-circuited, the circuit can be quickly cut off to protect the cells.
  • Overcurrent protection: When the battery output current exceeds a certain value (about 2A), the protection circuit will cut off the output circuit.
  • Other auxiliary functions: There are usually NTC, identification resistor and decoding chip on the protection board, the main function is to ensure that the battery can be charged and used normally with the host and the original charger.
Lithium Battery Protection Board

Normal State

In the normal state, the “CO” and “DO” pins of N1 in the circuit both output high voltage, and both MOSFETs are in the on state, the battery can be charged and discharged freely, because the on-resistance of the MOSFET is very high. Small, usually less than 30milliohms, so its on-resistance has little effect on the performance of the circuit. The current consumption of the protection circuit in this state is μA level, usually less than 7μA

The Charging Method Required by the Lithium-Ion Battery for Overcharge Protection is Constant Current/Constant Voltage.

  • In the early stage of charging, it is charged with constant current. With the charging process, the voltage will rise to 4.2V (depending on the positive electrode material, some batteries require constant current.) voltage value is4.1V), switch to constant voltage charging until the current becomes smaller and smaller.
  • During the charging process of the lithium battery protection board battery, if the lithium battery protection board charger circuit is out of control, the battery voltage will continue to be charged with constant current after the battery voltage exceeds 4.2V.
  • In a battery with a protection circuit, when the control IC detects that the battery voltage reaches 4.28V (this value is determined by the control IC, different ICs have different values), its “CO” pin will change from high voltage to zero voltage, Turn T1 from on to off, thus cutting off the charging circuit, so that the charger can no longer charge the battery, which plays an overcharge protection role. At this time, due to the existence of the body diode VD1 of T1, the lithium battery protection board battery can discharge the external load through this diode.
  • There is still a delay time between the control IC detects that the battery voltage exceeds 4.28V and sends the turn-off T1 signal. The length of the delay time is determined by C2 and is usually set to about 1 second to avoid errors caused by interference. judge.

During the Discharge Process of the Over-Discharge Protection Battery

Its voltage will gradually decrease with the discharge process. When the battery voltage of the lithium battery protection board drops to 2.5V, its capacity has been completely discharged. At this time, if the battery is allowed to Continuing to discharge the load will cause permanent damage to the battery.

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  • In the process of battery discharge, when the control IC detects that the battery voltage is lower than 2.3V (this 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 Turning from on to off, thus cutting off the discharge circuit, so that the lithium battery protection board battery can no longer discharge the load, which plays the role of over-discharge protection
  • Since the battery voltage can no longer be reduced in the over discharge protection state, the current consumption of the lithium battery protection board is required to be extremely small. At this time, the control IC will enter a low power consumption state, and the power consumption of the entire protection circuit will be less than 0.1μA. There is also a delay time between when the control IC detects that the battery voltage is lower than 2.3V and sends the turn-off T2 signal. The length of the delay time is determined by C2 and is usually set to about 100 milliseconds to avoid errors caused by interference. judge.

Overcurrent Protection

Due to the chemical characteristics of lithium-ion batteries, the battery manufacturer stipulates that the maximum discharge current cannot exceed 2C (C=battery capacity/hour). When the battery discharges at a current exceeding 2C, it will cause permanent damage to the battery. or security issues.

  • During the normal discharge process of the lithium battery protection board to the load, when the discharge current passes through the two MOSFETs in series, a voltage will be generated at both ends of the MOSFETs due to the on-resistance of the MOSFETs. The voltage value is U=I*RDS*2, RDS is the on-resistance of a single MOSFET, and the “V-” pinon the control IC detects the voltage value. If the load is abnormal for some reason, the loop current will increase. When the loop current is large enough to make U>0.1V (The value is determined by the control IC, and different ICs have different values), its “DO” pin will change from high voltage to zero voltage, so that T2 is turned from on to off, thus cutting off the discharge loop and making the current in the loop. 0, play the role of overcurrent protection.
  • There is also a delay time between the control IC detects the occurrence of overcurrent and sends the turn-off T2 signal. The length of the delay time is determined by C2, usually about 13 milliseconds to avoid mis-judgement due to interference.
  • In the above control process, it can be seen that the value of the overcurrent detection value depends not only on the control value of the control IC, but also on the on-resistance of the MOSFET. the smaller the value.

Short Circuit Protection

During the process of discharging the load, if the loop current is so large that U>0.9V (this value is determined by the control IC, different ICs have different values), the control IC will judge that the load is short-circuited, its “DO” pin will quickly change from high voltage to zero voltage, so that T2 is turned from on to off, thereby cutting off the discharge circuit and playing a short-circuit protection role. The short-circuit protection has a very short delay time, usually less than 7 microseconds. Its working principle is similar to that of overcurrent protection, but the judgment method is different, and the protection delay time is also different.

Conclusion

Lithium-ion battery protection board, its role is to prevent the lithium-ion battery from being overcharged or overcharged and play a corresponding protective role. A protective plate can protect the battery itself. If not, first, the lithium-ion battery itself is easily damaged, and second, there is safety danger. 

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