Mos in Lithium Battery-Roles and Functions

Lithium batteries are a class of batteries that are made of lithium metal or lithium alloy as the positive/negative electrode material and use a non-aqueous electrolyte solution.In 1912, lithium metal batteries were first developed by Gilbert N. Lewis proposed and researched.In the 1970s, M. S. Whittingham proposed and began to study lithium-ion batteries.Because the chemical properties of lithium metal are very active, the processing, preservation and use of lithium metal have very high environmental requirements.With the development of science and technology, lithium batteries have become the mainstream.

Lithium batteries can be roughly divided into two categories: lithium metal batteries and lithium-ion batteries.Lithium-ion batteries do not contain lithium in the metallic state and can be charged.The fifth-generation product of rechargeable batteries, lithium metal batteries, was born in 1996, and its safety, specific capacity, self-discharge rate, and performance-to-price ratio are better than lithium-ion batteries.

Advantages of lithium batteries

The biggest advantage of lithium batteries is environmental protection

The demand for electric vehicles has increased sharply in recent years, and a large part of the reason is that environmental issues have become more and more prominent.The world has always attached great importance to environmental protection issues.Even if the lithium battery is in a waste state, it will not cause harm to the environment. During the production process, there will be no chemicals that pollute the environment.

Lithium batteries have strong battery life and longer service life

Battery life is also the most worrying issue for many consumers for electric vehicles.The length of service life is related to the cost of using the car. The life of the lithium battery is about five years, which is much higher than that of other batteries.

Lithium battery has high voltage and light weight

The average voltage of a single battery can reach 3.7V or 3.2V. It is relatively equal to the series voltage of 2-4 nickel-metal hydride batteries or nickel-metal hydride batteries.If the customer requires the voltage of the battery to pass the customs, then the selection of lithium batteries is also easy and convenient to form a lithium battery pack, which is convenient and quick to increase the voltage of the lithium battery.

The energy and density of lithium-ion batteries are also very high for other batteries

Lithium-ion batteries have a high energy storage density. The density of commonly used lithium-ion batteries can reach 450-620Wh/kg, which is 5-7 times that of lead-acid batteries.

With the introduction of the new national standard and the support of national policies, the sales of lithium batteries have hit a boom, but the number of accidents caused by the safety of lithium batteries has also risen, and the consequences of many problems are shocking. Frequent lithium battery fires and explosions have once again sounded the alarm about the safety of lithium batteries.Therefore, lithium batteries have a safe voltage range for use. The highest and lowest voltages are generally called the charge-discharge termination voltage or cut-off voltage. When the actual operating voltage of the battery is lower than the discharge termination voltage for a long time or higher than the charge termination voltage for a long time, irreversible damage will occur inside the battery, seriously harming the battery, resulting in performance degradation, commonly known as battery attenuation, the performance of battery attenuation is the increase of the internal resistance of the battery, the capacity decreases, etc.Therefore, generally, there will be a small PCB board inside the lithium-ion battery, which is packaged with the battery. As shown in the figure below, the main function is to protect the battery.

What is MOS in Lithium Battery?

Lithium batteries are mainly composed of two major components, the battery cell and the protection board PCM (the power battery is generally called the battery management system BMS). The battery cell is equivalent to the heart of the lithium battery, and the management system is equivalent to the brain of the lithium battery.The general lithium battery protection board is composed of a control IC, a MOS tube, a resistor capacitor, a fuse, and a PCB board. The normal operation of the battery is largely due to the protection board, so the protection board is also known as the god of battery protection.

The MOS tube uses 2pcs as charge and discharge protection on the lithium battery protection board, but in actual work, according to different applications, multiple power MOS tubes will be used to work in parallel to reduce the conduction internal resistance and enhance heat dissipation performance.In terms of selecting MOS, the BVDSS of MOS is selected according to the battery voltage of the lithium battery in series, and the current of MOS is selected according to the requirements of the battery capacity for the current discharge current and the requirements for overcurrent and short circuit protection.

Function of MOS in Lithium Battery

(1) Detect overcharging

(2) Detected discharge

(3) Detect over-current during charging

(4) Detect the over-current during discharge

(5) Detect over-current during short circuit

(6) Battery CELL longevity

(7) Restrictions of the external environment

(8) Over-current detection during charge and discharge and the rated current of the power MOSFET for charge and discharge control

After MOS is turned on, there is an on-resistance, so that the current will consume energy on this resistance. This part of the energy consumed is called conduction loss.Choosing a MOS tube with a small on-resistance will reduce the on-loss.The current on-resistance of low-power MOS tubes is generally around tens of milliohm, and there are also a few milliohm.

When MOS is turned on and off, it must not be done in an instant.The voltage across the MOS has a process of falling, and the current flowing through has a process of rising. During this period of time, the loss of the MOS tube is the product of voltage and current, which is called switching loss.Usually the switching loss is much greater than the conduction loss, and the faster the switching frequency, the greater the loss.

The product of voltage and current at the moment of conduction is very large, and the loss caused is also very large.Shortening the switching time can reduce the loss each time it is turned on; reducing the switching frequency can reduce the number of switches per unit time.Both of these methods can reduce the switching loss.

The working process of the MOS tube is very complicated, and there are many variables in it. In short, slow switching is not easy to cause Miller shock (introduction to Miller capacitance, Miller effect, etc., very detailed), but the switching loss will increase and the heat generation will be large; the switching speed is fast, and the loss will be reduced, but the Miller shock is very severe, which will increase the loss.The wiring of the drive circuit and the wiring of the main circuit are very demanding. In the end, it is to find a balance point. The general opening process does not exceed 1us.

In all, the lower the internal resistance of the protection plate, the better, and the lower the lower the less heat is generated.The current limiting thickness of the protection board is determined by the sampling resistance of the Kang copper wire, but the continuous current is determined by the mos, so it is very important to choose the appropriate mos model.