Lithium batteries have become ubiquitous in today's technology-driven world. Essentially, a finished lithium battery comprises two essential components: the lithium battery cell and the PCM (Protection Circuit Module to provide safeguard).
Part1: Why does a Lithium Battery Need PCM?
The longevity and security of lithium batteries are reliant on their constituent materials, making them vulnerable to damage from over-charging, over-discharging, overcurrent, or overheating. Thus, to safeguard both the battery's lifespan and user safety, proper protection measures should be taken. PCM and BMS both serve as this protective role.
Part2: Functions of PCM
Over-charging protection: Over-charging refers to the charging voltage being applied above the rated voltage of a lithium cell of 4.2V (for NCM). For example, directly charging a lithium cell with a 5V voltage (without a PCM) can result in the battery bulging and potentially exploding and catching fire. In this case, if there is a PCM, it will cut off the charging.
Over-discharging protection: Over-discharging refers to discharging when the voltage reaches the discharge cut-off voltage and continues to discharge. For example, if the discharge cut-off voltage is 3V and continues to discharge below 3V, it is called over-discharging. Over-discharging can cause the reversibility of positive and negative active substances to be disrupted, and these damages are irreversible, resulting in a significant reduction in capacity. In this scenario, the PCM will promptly respond by disabling over-discharge and safeguarding the battery against further depletion, ensuring a consistent voltage is maintained.
Over–current protection: Both charging over-current and discharging over-current can cause overheating, leading to a potential risk of combustion and explosion. In this scenario, the PCM takes action and switches off the over-current.
Over-temperature protection: High temperature during charging and discharging can disrupt the chemical balance inside the battery cell, degrade material properties, and greatly reduce its lifespan.
Part3: Differences Between PCM and BMS
PCM and BMS (Battery Management System) are both designed for safeguarding the performance and longevity of lithium batteries during charging and discharging. Although they share some similarities, there are notable differences between them.
PCM provides basic protection functions such as over-charging, over-discharging, over-current, and over-temperature. It is commonly utilized in small battery packs found in digital devices like power tools, electric shavers, and mobile phones.
On the other hand, BMS provides advanced features beyond the basic protection functions offered by PCM. This includes cell balancing, estimation of SOC (State of Charge), and communication capabilities like CAN or RS485. BMS plays a crucial role in large battery cells used in applications such as drones, where real-time battery status and data transmission are critical.
Essentially, PCM is a simplified design of BMS that successfully implements over-charging, over-discharging, over-current, and other protection functions through hardware solutions. In summary, while PCM provides basic protection, BMS offers advanced features to ensure optimal performance and safety of lithium batteries.
Part4: Types of PCM
According to the control method, PCM is divided into 2 types：
Hardware PCM: The protection parameters of the circuit are pre-set within the IC and cannot be externally altered. This design ensures stable and reliable performance, while maintaining a low cost. However, it is important to note that the protection parameters must be set on a one-to-one basis, without the ability to make further adjustments.
Software PCM: The intelligent protection chip on this circuit board can have its protection parameters set and programmed by the host computer, allowing for the achievement of the desired parameters for protection.
Advantages: This board has a broad range of applications, making it versatile and scalable. It can be used for communication protocols, Bluetooth, mobile phone apps, and GPS positioning.
Disadvantages: However, it is worth noting that this board can come at an expensive price. Additionally, the software operation may be slightly complicated for some users.
According to the way of use, PCM are divided into another two types:
Common port PCM: also known as same port PCM. The PCM's charging and discharging functions share a single pair of ports, requiring only two wires. During the discharge process, the current will flow through the charging control MOS, leading to increased costs, internal resistance, and heat generation.
Separate port PCM: also known as split port PCM. The charging and discharging ports of the PCM are separate and require 3 wires.
The emergence of common port and separate port PCM is mainly driven by cost reduction. Typically, the charging current for electric vehicle lithium battery packs is lower than the discharge current. Consequently, if two MOSFET (abbreviated as MOS) tubes are connected in series on the same line, both MOS tubes need to be purchased based on the larger one, which is more expensive. On the other hand, the separate port design enables the use of a smaller MOSFET as the charging current is lower. This also leads to lower costs for separate port PCMs.
| Common port PCM | Separate port PCM | |
| Advantage | • The wiring is simple and convenient, saving space. | • Lower cost. • Lower failure rate. • Suitable for larger capacity battery packs. |
| Disadvantage | • Higher cost • Suitable for smaller capacity battery packs. |
• An additional cable is required, wiring is complicated. |
Conclusion
PCM and BMS are both used to protect lithium batteries. Batteries without PCM/BMS are prohibited from use and can arise significant safety hazards.
