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The Importance of MOS in LiPo Batteries


LiPo (Lithium Polymer) batteries have gained significant popularity in various consumer electronics, from smartphones to drones, due to their high energy density and compact design. While these batteries offer numerous advantages, it is crucial to understand the role of MOS (Metal Oxide Semiconductor) in ensuring their safety and efficient operation. In this post, we will explore the importance of MOS in LiPo batteries.

What are MOS circuits in LiPo batteries?

MOS circuits, also known as protection circuits, are an essential component of LiPo batteries. These small electronic systems are designed to monitor and control the voltage, current, temperature, and other parameters during charging and discharging processes. Their primary function is to ensure the safe and optimal operation of the battery, protecting it from overcharging, over-discharging, and excessive temperature.

Why are MOS circuits necessary?

Overcharge protection

MOS circuits constantly monitor the battery’s voltage levels during charging. Once the battery reaches its maximum voltage threshold, the MOS circuit cuts off the charging current, preventing overcharging. Overcharging can lead to thermal runaway, reduced battery life, or even catastrophic failure.

Over-discharge protection

Similarly, MOS circuits prevent the battery from being discharged beyond its safe lower limit. When the battery voltage drops to a predetermined threshold, the MOS circuit disconnects the load from the battery, preserving its capacity and preventing damage. Over-discharging can result in irreversible damage to the battery, reducing its overall lifespan.

Short-circuit protection

MOS circuits also provide protection against short circuits. If a short circuit occurs, the MOS circuit swiftly interrupts the flow of current, preventing excessive discharge or overheating that could lead to a fire hazard.

Temperature monitoring

MOS circuits monitor the battery’s temperature throughout its operation. If the temperature exceeds the safe operating range, the MOS circuit triggers protective measures, such as reducing the charging or discharging rate, to prevent thermal runaway and maintain the battery’s integrity.

Cell balancing

In multi-cell LiPo batteries, MOS circuits help balance the voltage levels between individual cells. This ensures that each cell is charged and discharged evenly, maximizing the battery’s overall performance and preventing capacity imbalances that can reduce the battery’s effectiveness and lifespan.

Why do a Lipo Battery Need Protection Board?

Although lithium batteries have many advantages as mentioned above, due to the highly reactive nature of the metal lithium and its low ignition point, they are prone to ignite and burn when exposed to high temperatures in the air. Therefore, lithium batteries are quite sensitive to overcharging, over-discharging, and overcurrent during their application. Once they are not used correctly, accidents such as fires, burns, and even explosions may occur, causing personal injury to the user.

Overcharging and over-discharging of lithium batteries can cause permanent damage to the battery cells, resulting in a reduced lifespan and capacity that cannot be reversed.

We know that the terminal voltage of a single lithium battery when fully charged is DC4.2V. During use, the terminal voltage of the battery will gradually decrease with the depletion of the battery’s charge. When it drops below DC3.0V, the main chip and peripheral devices on the protection board will detect it, and the main chip will send a low-level signal to quickly turn off the power MOS tube, thereby preventing the lithium battery from being permanently damaged due to over-discharging.

To prevent the problem of overcharging lithium batteries, it is usually controlled by the battery protection board and charging module working together. However, when users use lithium batteries, safety issues caused by momentary over-discharge due to accidentally short-circuiting the positive and negative terminals (VBAT+ and P-) may arise.

According to theoretical estimation, the internal resistance R0 of a lithium battery cell is in the milliohm level, and the terminal voltage V0 of a lithium battery is in the volt level. When the positive and negative terminals (VBAT+ and P-) are short-circuited directly, the instantaneous current according to Ohm’s law is:

I = V0/R0 = 3.0v/10mR ~= * x 10^3 A

Therefore, it is estimated that the short-circuit current at this moment will reach the level of several hundred amperes. Of course, in actual tests, due to the limit of the actual capacity of the battery, the instantaneous current may not be that large, but it will be at least tens of amperes or more, enough to cause the battery pack to catch fire, burn or explode.

Single polymer lithium batteries are generally used in industries such as mobile phones, Bluetooth headsets, tablet computers, laptops, etc. The capacity varies greatly depending on the industry. Bluetooth headsets are usually between tens of mAH and 100mAH, while mobile phones and tablets are usually between 4000mAH and 6000mAH+. Laptops are above 10000mAH.

However, in industries such as power tools, electric bicycles, and emergency power supplies for cars, the capacity of a single lithium battery cell is limited, and the ability to carry loads and the instantaneous discharge ability cannot meet the requirements. Therefore, multiple single-cell lithium batteries are often used in series. Generally, there are 4 to 13 series, and the application of 4-series iron lithium batteries is the most common. The capacity of the battery pack is usually tens of ampere-hours (AH) or larger. Therefore, the instantaneous current caused by short-circuiting the positive and negative terminals (VBAT+ and P-) of the battery pack will reach the level of thousands of amperes, and the resulting losses cannot be ignored.


As various types of lithium batteries are increasingly being used, the demand for lithium battery protection boards continues to increase. In the application field of multi-series battery protection boards, there are differences in the capacity of battery packs and the number of parallel MOS tubes, so it is necessary to appropriately adjust the driving circuit according to the requirements in practical applications.

The inclusion of MOS circuits in LiPo batteries is crucial for maintaining their safety, performance, and longevity. These protective circuits guard against overcharging, over-discharging, short circuits, and excessive temperature, mitigating potential risks associated with LiPo batteries. The monitoring and control functions performed by MOS circuits ensure that the battery operates within safe parameters and maximizes its efficiency during use.

It is important to recognize the significance of MOS circuits and to choose LiPo batteries equipped with reliable and high-quality protection systems. Proper care and handling of LiPo batteries, including avoiding extreme temperatures, using compatible chargers, and following manufacturer guidelines, further enhance their safety and lifespan. Remember, responsible usage and adherence to recommended practices are key to enjoying the benefits of LiPo batteries while minimizing risks.

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