The protection board of polymer lithium battery is used to protect the battery from damage and prolong the battery life, as the name suggests. It only provides stable and effective protection to prevent accidents when the battery has extreme problems. It should not be operated under normal circumstances. Of course, monitoring work is necessary, just like the fuse or circuit breaker in our household appliances. In this article, we will tell you how manufacturers to protect lipo battery, like protecting over charging, over discharging, over-temperature and short circuit.
How to Protect Your Lipo Battery?
To Protect Battery Voltage
Overcharging and overdischarging vary depending on the materials used in polymer lithium batteries. Although it may seem simple, paying attention to the details requires experience and knowledge.
Overcharge protection in the past has set the protection voltage of single-cell batteries 50-150mV higher than the battery’s full charge voltage. However, polymer lithium batteries are different. If you want to extend the battery life, you should choose the battery’s full charge voltage or even a slightly lower voltage as the protection voltage.
For example, for manganese lithium batteries, you can choose 4.18V-4.2V. Because it is made up of multiple series of cells, the life capacity of the entire battery pack is mainly determined by the lowest-capacity cell, which always works at high currents and voltages, leading to faster attenuation. Larger capacity cells are charged and discharged more gently each time, so their natural attenuation is much slower. In order to ensure that smaller capacity batteries are also charged and discharged gently, the overcharge protection voltage should not be set too high. This protection delay can be set to 1 second to prevent the impact of pulses and provide protection.
Over-discharge protection is also related to the battery material, such as manganese lithium batteries, which are generally selected between 2.8V~3.0V. Try to set the over-discharge voltage slightly higher than that of a single battery. Because in domestically produced batteries, the discharge characteristics of each cell are completely different after the battery voltage drops below 3.3V, it is necessary to protect the battery in advance, which is a good protection for the battery life. Overall, it is helpful to the life of polymer lithium batteries to make each cell work under light charging and discharging.
To delay the over-discharge protection time, it needs to be adjusted according to different loads. For example, for electric tools, the starting current is generally above 10C, so it will quickly pull the voltage of the lithium polymer battery to the over-discharge voltage point to protect it. At this time, the battery cannot work.
The damage to MOS tubes is mainly caused by a rapid increase in temperature. Its heat generation is determined by the size of the current and its own internal resistance. Of course, small currents have little effect on MOS tubes, but for large currents, proper handling is required. When driving MOS tubes with a rated current of 10A or less, we can directly use voltage to drive them. For large currents, driver is necessary to provide MOS tubes with sufficient driving current.
There is a discussion on working current in MOS tube driving. When designing, there should not be more than 0.3W of power on MOS tubes. The calculation formula is I2*R/N. R is the internal resistance of MOS, and N is the number of MOS tubes. If the power exceeds this limit, the MOS tube will generate a temperature rise of more than 25 degrees. Even with a heat sink, the temperature will still rise during long-term operation because it is sealed and has nowhere to dissipate heat. Of course, there is no problem with the MOS tube itself, but the heat it generates will affect the battery, as the protection board is with the battery.
To Protect Battery Current
This is an essential and critical protection parameter for polymer lithium battery protection boards. The size of the protection current is closely related to the power of the MOS, so when designing, it is necessary to give enough margin to the MOS capability. When laying out the circuit board, the position of the current detection point must be carefully selected. It cannot be connected randomly, and this requires experience. Generally, it is recommended to connect to the middle terminal of the detection resistor, and attention should be paid to the interference problem of the current detection end because its signal is easily interfered with.
Overcurrent protection delay also needs to be adjusted according to different products.
To Prevent Short Circuit
The protective mechanism of the device is based on voltage comparison. In other words, it is designed to shut off or activate based on a direct comparison of voltages, without any additional processing. This feature ensures a precise and efficient operation, as it allows for the device to react quickly to any changes in the voltage levels.
Moreover, the setting of the short-circuit delay is crucial in the case of polymer lithium batteries. These batteries have a large input filter capacitor, which gets charged first when the battery is connected. This initial charging process is equivalent to a short-circuit of the battery, as the current flows rapidly through the capacitor. To ensure that the capacitor is charged properly and efficiently, the short-circuit delay must be set appropriately, taking into consideration the specific characteristics of the battery and the capacitor. This will help to prevent any potential damage to the battery and ensure a safe and reliable operation.
To Protect Lipo Battery Temperature
Smart batteries are very common in daily life, but even perfect batteries will have their shortcomings. We mainly disconnect the main switch by detecting the temperature of the battery to protect the battery itself or the load. Of course, if the battery works in a constant environmental condition, there will be no problem. However, the working environment of the battery is uncontrollable, and there are too many complicated changes, which makes it very difficult to choose the appropriate disconnect temperature. For example, in the winter in the north, what should be the disconnect temperature? And in the summer in the south, what should it be? Obviously, this range is too wide, and there are too many uncontrollable factors, so everyone will choose based on their own experience and knowledge.
To solve this problem, we need a more intelligent method to detect the temperature of the battery and automatically adjust the disconnect temperature according to the actual working environment of the battery. In this way, we can protect the battery while fully utilizing its performance and life, making our smart batteries more intelligent and efficient.