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BMS plays a very important role in Lithium ion Battery Pack building

Date:7/15/2020

To have higher power and more capacity on a battery, we can assmble the battery cells into one big pack, simply assembly cells into one big pack is easy, but keep the pack safe and get most use of this pack's power are not easy, except for the main part - cells, BMS also plays a very important role too.
 
What is BMS ? Does the BMS necessary for lithium-ion Battery pack building? 
 
BMS: The abbreviation of the Battery Management System. Purpose to improve the use efficiency of rechargeable battery. Avoid appearing over charger, over-discharge, over-current, 
and short circuit. Lengthen the cycle life of the battery, and do supervisory control of the battery status.
What can BMS do? 
 
1.Exactly Estimate of SOC:
Exactly estimate battery pack of state of charge, define the remaining capacity of the battery. Make sure the battery’s SOC maintain in a valid range. Avoid damage to the battery due to overcharge or over-discharge current. And present the status of battery visible by display monitor. 
 
2.BMS with balancing.
That is to realize balance charge for every battery cell of the battery pack, let the power of each battery cell reach to balancing. It is a keypoint skill that the entire world devoted to develop. 
 
3.Negative Temperature Coefficient(NTC) - Heat sensitive resistance. 
The key point of NTC is its characteristic of negative temperature resistivity performance. At the moment when the circuit power is turned on, a surge current that is many times higher than that is continuous powered will be generated in the circuit, and the initial resistivity of the NTC thermistor is large, which can suppress the excessive current in the circuit and thereby protect its BMS circuit and load. When the circuit enters the normal working state, the thermistor's temperature rises due to the continuous passage of current, and the resistivity drops to a very small value. This characteristic decision the NTC provide protection for battery BMS and load circuit. Generate relative bigger resistance at a lower temperature at the power-on moment and relative less resistance at the raised of temperature due to continuous powered. Avoid the BMS or load circuits be damaged by momentary high power. Which will not affect the normal working of the circuit.
 
4.Positive Temperature Coefficient(PTC)- Another Heat sensitive resistance
PTC is the abbreviation of positive temperature coefficient, meaning positive temperature coefficient, which refers to semiconductor materials or components with large positive temperature coefficient. Usually we mentioned PTC means positive temperature coefficient thermistor, or PTC thermistor for short. The PTC thermistor is a typical temperature-sensitive semiconductor resistor. When the temperature exceeds a certain temperature (Curie temperature), its resistance value conversion temperature increases in a stepwise increase.
PTC utilizes the variable resistance characteristics of polymers and is doped into the conductor based on the polymer. When the current increases rapidly, the temperature of the PTC element rises instantaneously, the impedance increases and the current flowing becomes smaller within a few milliseconds. As an open circuit, to achieve the purpose of suppression, when the abnormal current disappears, it instantly returns to a low impedance conductor.
 
5.BMS with conmmunication protocol: 
Bluetooth, WIFI, CamBus, SmBus, RS485/RS232, RSUART, HDQ , etc. 
All the communication protocols have various of applications. Through integrated with these communication protocols,  we can realize to connect the battery to the smart devices, electric vehicle, PC, and consumer electrical. To check the status of battery pack. 
 
BMS provide shield for battery pack safety
 
1.Over charge protection. 
Mainly refers to the battery continues to charge after reaching the full state when it is charging. Such practices may lead to increased battery internal pressure, battery deformation, liquid leakage, etc., and battery performance will be significantly reduced and damaged. BMS has the function to cut off over voltage and stop charging. 
 
2.Over discharge protection.
That is, excessive discharge. When the battery is discharged, the stored electrical energy is gradually released, and the voltage slowly decreases. When the voltage drops to a specified value, the discharge should be stopped and recharged to restore the battery's energy storage state. Continue to discharge below the specified value, that is, over-discharge, over-discharge may cause damage to the electrode active material, loss of reaction capacity, and shorten the battery life. BMS can effectively avoid this appear.
 
3.Over current protection.
It is a protection method that makes the protection device operate when the current exceeds a predetermined maximum value. When the current flowing through the protected original exceeds a certain value set in advance, the protection device is activated, and the time limit is used to ensure the selectivity of the action, so that the circuit breaker trips or gives an alarm signal.
 
4.Short circuit protection.
Protect short-circuit faults caused by electrical short-circuiting of equipotential conductors in the power supply system.
 
Battery pack to keep 100 percent safe is difficult. 
 
Three reasons cause the battery to generate heat: Short circuit, overcharge, and overload.
The actual reasons cause Battery short circuit is complex, both the reason for battery its self questions, but the reason for actually use. with the defects of metal impurity reason in the origin material. Reverse the terminals of cathode and anode in the process of workmanship. Continuous working at a high power load environment generate a lot of heat reach to a 90-celsius degree causes the cover films to shrink, The battery cathode and anode with losing isolate protection. Thereby cause the battery short circuit.
 
In actual use, Big current charging, lower degree charging, and crash dropped also the reasons short circuit. Triggering battery malfunction or explosion is the reason for the degree。For Lithium-ion battery to working normally has a degree limited. Between -20 Celsius degree and 45 celsius degree. Too lower degree will cause the power can't be output and input as well. But working in a high degree environment is more dangerous. If reach to 90-100 celsius degree.Will causing the reaction of battery self generate heat. If the degree reach to 150-250 celsius, will lose the control of generating heat. In means the isolating film internal battery the cell will be damaged. The electrolyte internal battery cell will overflow from the packing materials. Even burn up. When the degree reaches to 900-celsius degree. Will causing the safely valve of battery cell break, electrolyte overflow or heating ignition, If the safety valve can't be broken to release heat and press. The battery cell even explode. 
 
Except for some unpredicted and can’t avoid accident cases causing the battery damaged, the BMS plays a very important role in protecting battery pack and electric apparatus. Battery cell or battery pack integrated with BMS, Will has the functions of over-temperature protection, Overcharge protection, Over-discharge protection, and short circuit protection.
 
In conclusion, BMS is very important in battery pack building.