Composition of lithium power battery pack protection board and overcurrent protection!

Composition of lithium power battery protection board

Lithium power battery protection board is an integrated circuit board designed for lithium power batteries to provide protection.

 The need for protection of lithium power batteries is determined by their own characteristics. 

Due to the material of lithium power batteries, they cannot be overcharged, overdischarged, overcurrent, short circuited, or subjected to ultra-high temperature charging and discharging. 

Therefore, when designing a lithium power battery pack, a protective plate will be included.

Lithium power battery protection board is usually composed of control IC, switch tube, precision resistor, NTC, PTC, ID memory, etc.

 The control IC controls the switch tube to conduct under normal conditions of the lithium power battery pack, allowing the battery cell to communicate with the external circuit. 

When the battery cell voltage or circuit current, temperature exceeds the specified value, it immediately (tens of milliseconds) controls the switch tube to turn off, protecting the safety of the battery cell.

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 line interface memory that stores information such as the type and production date of lithium power battery packs, 

which can serve as traceability and service life information for products.

PTC stands for Positive Temperature Coefficient in English, which is a resistor with a positive temperature coefficient. 

PTC in lithium power battery pack products can prevent high-temperature discharge and unsafe high current from occurring. 

According to the voltage, current density characteristics, and application environment of lithium power battery packs, there are specific requirements for PTC. 

PTC is a very important component in lithium power battery pack products, responsible for the safety of lithium power battery packs. 

Its performance and quality are also important factors in the performance and quality of lithium power battery packs.

Overcurrent protection

Definition of overcurrent protection for lithium power battery pack: 

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 requires a continuous process, so overcurrent generally has two levels of protection. 

The first level of protection has a relatively small set value and a long delay time, while the second level of protection has a relatively large set value and a short 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) No manual intervention is required. After a period of time, the circuit will automatically open. 

If it is still in an overcurrent state, the lithium power battery pack will enter protection again. 

If the overcurrent is released, the lithium power battery pack will enter working mode.

2) Manual intervention is required to reset the overcurrent protection manually after the load or charger is removed.

During the normal discharge process of the lithium power battery pack to the load, 

when the discharge current passes through two series connected switching tubes, a voltage is generated at both ends due to the conduction impedance of the switching tubes. 

The voltage value U=I × RDS × 2 (RDS is the conduction impedance of a single switching tube). The control IC detects this voltage value. 

If the load is abnormal for some reason and the circuit current increases,

 when the circuit current is greater than U>0.1V (this value is determined by the control IC, and different ICs have different values), 

the control IC switches Q1 from conduction to cutoff, thereby cutting off the discharge circuit and making the current in the circuit zero, playing an overcurrent protection role.

There is a delay time between the detection of overcurrent by the control IC and the issuance of the shutdown signal.

 The length of this delay time is determined by C2, usually around 13 milliseconds, to avoid misjudgment caused by interference.

 In the above control process, the magnitude of the overcurrent detection 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.

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 power battery pack may be damaged.

 Therefore, for short circuit protection, hardware is generally used to automatically trigger it, and a signal is transmitted to the control IC after triggering.

When the P+and P - output currents of the lithium power battery pack exceed the short-circuit current value and reach the short-circuit delay, 

the control circuit controls the discharge switch tube 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 Ω) 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.