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The difference between PCM and BMS

The difference between PCM and BMS

PCM (protective circuit modul) means battery protection module, which is composed of a battery protection chip and dual MOSFETs, with functions of overcharge, overdischarge, overcurrent, overvoltage and temperature protection. Improve battery safety and extend life cycle.

BMS (battery management system) battery system is commonly known as battery nanny or battery steward, mainly for intelligent management and maintenance of each battery unit, to prevent the battery from overcharging and overdischarging, to prolong the service life of the battery, and to monitor the state of the battery

1. Function of PCM
1.1Overcharge protection: If there is an abnormal situation in the battery charging (fault, error, incorrect charging), it may be charged above 4.2V, in this case, the PCM action shuts down the charging.
1.2. Over-discharge protection: Over-discharge of the battery will cause errors and malfunctions in electrical equipment (telephones, personal computers). In this case, PCM acts and turns off over-discharge and protects the battery from discharging and maintains a certain voltage.
1.3. Over-current protection: The abnormally high current of the battery will cause errors and malfunctions of small electrical equipment. In this case the PCM operates and shuts down the overcurrent.
1.4. Short circuit protection: It protects the battery from external short circuit (coins, keys, etc.).

2. The working principle of PCM
When the battery is over-discharged or over-charged, the battery voltage is detected and the MOSFET is cut off.

When the battery is over-current or short-circuited, detect the Vcc and V-voltage of the IC: if the voltage exceeds Vocd, turn off the over-current; if the voltage exceeds Vscd, turn off the short-circuit.

Remarks. Vocd: overcurrent protection voltage, Vscd: short circuit current detection voltage

Three, the role of each component

   IC: Detect cell voltage, FET input current, charger, load and drive FET on and off.
   FET: Accepts IC control and performs switching functions. There are usually three legs, which are called drain D, source S, and gate G respectively. When the gate G gets a high  level, the switch is closed; the current passes between D.S. When the gate G gets low level instead of high level, the switch between D and S is regarded as an open circuit, and the current cannot pass through.
   Resistor: voltage detection, current limiting, identification capability (ID).
   Capacitors: Set up delay, filter (noise removal), ESD removal.
   Fuse: cut off (destroy) when the input is abnormal
   PTC: The function is similar to that of the fuse, but it can be restored after being turned off.
   NTC: Detects the internal temperature of the battery.
   Varistor: ESD protection.
   PCB: Connect various components and transmit battery signals to chargers or electrical appliances.

PCM Basic Knowledge Questions and Answers
1. What is Li-ion battery protection IC?
Answer: During the use of lithium-ion batteries, over-charging and over-discharging will have a certain impact on the electrical performance of lithium-ion batteries. It is miniaturized and becomes a chip, which is commonly known as a lithium battery protection IC.

2.What is the shape of the protection IC?
There are two commonly used protective IC shapes: one is called SOT-23-5 package; the other is thinner, called TSSOP-8 package.

3.On the PCB for lithium battery protection current, besides the protection IC, what other components are needed to form a complete protection PCB?
Answer: Two field effect transistors, several resistors, and capacitors are also needed as a switch function.

4.What does a FET look like?
Answer: Field effect transistors are also called MOSFETs. They are used in pairs on the lithium battery protection PCB. Let’s ask the manufacturer whether two independent MOSFETs are packaged together. There are usually two shapes: one is SOP-8 package; another package is thinner, called TSSOP-8.

5.I often hear people say what is the internal resistance of a MOSFET, and what is the internal resistance of a MOSFET?
Answer: There is always a certain resistance when the switch between D and S is closed. This resistance is equivalent to the internal resistance of the MOSFET. Generally, this resistance is very small, and it is between 10 and 30mΩ. It can be seen that when the current passes through the MOSFET, due to the internal resistance, according to Ohm's law, there must be a voltage drop, thereby losing part of the electric energy. It can be seen that the internal resistance of the MOSFET should be as small as possible.

6.What is the power consumption of the IC? How to measure?
Answer: IC is a complete electronic circuit. It consumes part of the power when it is working. When the battery block is working in the mobile phone, the IC will draw power from the lithium battery. It can be seen that the smaller the power consumption of the IC is required, the higher the power consumption. good. The power consumption of the IC is measured by the consumed current, which is generally between 3uA and 6uA. The IC draws current from the battery through the resistor R1, so as long as the voltage drop V1 across R1 is measured, the power consumption of the IC can be calculated according to Ohm's law, and the current value is I=V1/R1.

7.Are the protective PCB boards for lithium batteries interchangeable?
Answer: The answer is no, mainly because:
  The performance of lithium batteries produced by different lithium battery manufacturers is endless, so the ICs selected are also different, mainly referring to the overcharge detection voltage.
  Using different MOS tubes Due to their different internal resistances, different ICs should be selected according to the working current.
  The identification resistors are different.

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