In recent years, lithium polymer (LiPo) batteries have gained immense popularity due to their high energy density and lightweight design, making them a preferred choice in various portable electronic devices. However, their inherent characteristics also pose certain risks, such as thermal runaway, overcharging, and short circuits, which can lead to battery failure and potential safety hazards. To address these concerns and ensure the safe and prolonged use of LiPo batteries, manufacturers and researchers have implemented a range of protecting measures. This article will delve into the key protection mechanisms employed in LiPo batteries, including overcurrent protection, overvoltage protection, temperature protection, and more. By exploring these measures, we can gain a better understanding of how LiPo batteries are safeguarded against potential risks, promoting their reliability and enhancing user confidence in utilizing these powerful energy storage solutions.
Lipo Battery Protection Measures & Precautions
The protection board of a Lipo battery, as the name suggests, is used to protect the battery from damage and extend its lifespan. It only comes into stable and effective protection when the battery faces extreme issues to prevent accidents. It should not be manipulated under normal circumstances. However, monitoring is necessary, similar to the fuses or circuit breakers in our household appliances.
Lipo Battery Voltage Protection
Overcharging and overdischarging vary depending on the materials of the Lipo battery. Although it may seem simple, it requires experience and knowledge to pay attention to the details.
For overcharging protection, in the past, the protection voltage for a single cell battery would be set 50-150mV higher than the fully charged voltage of the battery. However, Lipo batteries are different. If you want to extend the battery life, you should choose a protection voltage that matches the fully charged voltage of the battery or even slightly lower. For example, for manganese lithium batteries, the range can be selected as 4.18V-4.2V. Since they are connected in series, the overall lifespan and capacity of the battery pack depend mainly on the lowest-capacity cell. Cells with smaller capacity always work under high current and high voltage, resulting in faster degradation. On the other hand, cells with higher capacity undergo gentle charging and discharging, leading to slower degradation. To ensure that even cells with smaller capacity undergo gentle charging and discharging, the overcharging protection voltage should not be set too high. This protection delay can be set to 1 second to prevent the influence of pulses and provide protection.
For over-discharging protection, it also depends on the battery material. For example, for manganese lithium batteries, the range is usually set as 2.8V-3.0V, slightly higher than the over-discharge voltage of a single cell. In domestically produced batteries, once the battery voltage drops below 3.3V, the discharge characteristics of each cell become completely inconsistent. Therefore, it is necessary to protect the battery in advance, which is beneficial for prolonging the battery life. In summary, the goal is to ensure that every cell operates under gentle charging and discharging conditions, which is helpful for the lifespan of Lipo batteries.
The delay time for overdischarge protection needs to be adjusted according to different loads. For example, in power tools, the startup current is generally above 10C, which means that the voltage of the Lipo battery will be pulled down to the overdischarge voltage point within a short period, triggering protection. In this case, the battery cannot work.
The damage to MOSFET (metal-oxide-semiconductor field-effect transistor) is mainly caused by a rapid increase in temperature. Its heat generation depends on the magnitude of the current and its own internal resistance. Low currents have little impact on MOSFET, but high currents require proper handling. Under the rated current, for currents below 10A, we can directly drive the MOSFET with voltage. For high currents, sufficient driving current should be provided to the MOSFET. The working current is discussed in the MOSFET driving section. When designing, the power dissipation on the MOSFET should not exceed 0.3W. The formula for calculation is I2*R/N, where R represents the internal resistance of the MOSFET and N represents the quantity of MOSFETs. If the power exceeds this limit, the MOSFET will generate a temperature rise of over 25 degrees. Also, as they are enclosed, even with heat sinks, the temperature will rise during long-term operation because there is no place for heat dissipation. It is worth noting that the MOSFET itself is not the problem; the issue is the heat it generates, which can affect the battery since the protection board is placed together with the battery.
Overcurrent protection for Lipo batteries (maximum current)
This is an essential and critical protection parameter for the Lipo battery protection board. The size of the protection current is closely related to the power of the MOSFET, so when designing, it is important to provide sufficient margin for the MOSFET’s ability. When routing the board, the current detection point must be carefully selected and not simply connected. This requires experience. It is generally recommended to connect it to the middle terminal of the current detection resistor. Pay attention to the interference issue at the current detection terminal, as its signal is easily interfered with.
The overcurrent protection delay also needs to be adjusted accordingly based on different products.
Short circuit protection for Lipo batteries
Strictly speaking, it is a voltage comparison-based protection, which means it is directly turned off or driven by comparing voltages without unnecessary processing.
The setting of the short circuit delay is also crucial because in Lipo batteries, the input filtering capacitor is usually large, and when it makes contact, the capacitor is charged immediately, which is equivalent to short-circuiting the battery to charge the capacitor.
Temperature protection for Lipo batteries
It is commonly used in smart batteries, but its perfection often brings other shortcomings. We primarily detect the battery’s temperature to disconnect the main switch to protect the battery itself or the load. If it is in a constant environmental condition, there should be no problem. However, since the working environment of the battery is uncontrollable and subject to many complex changes, it is difficult to make a precise selection. For example, in the winter in the northern region, what temperature is appropriate? Similarly, in the southern region during summer, what temperature is suitable? Obviously, the range is too wide and there are too many uncontrollable factors, so the choice depends on different opinions and perspectives.
And in order to prevent the article being too long, we will continue to give you more information about lipo battery safety measure in next blog. See you then.