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 continue telling you how manufacturers to protect lipo battery, like MOS protecting, balance lipo battery and self discharge
How to Protect Your Lipo Battery? and What should We Mind?
The main factors are the voltage, current, and temperature of the MOS. Of course, this also involves the selection of MOS transistors. The breakdown voltage of the MOS must exceed the battery pack voltage, which is necessary. When referring to current, the temperature rise on the MOS transistor body should not exceed 25 degrees during rated current flow. This is based on personal experience and is for reference only.
The MOS driving uses MOSFETs with low internal resistance and high current, but why is there still a high temperature?
This is because the MOSFET driver part is not well done. Driving MOSFETs requires sufficient current. The specific driving current depends on the input capacitance of the power MOSFET. Therefore, general overcurrent and short-circuit drivers cannot be directly driven by the chip and must be externally added. When working with high currents (over 50A), multiple-stage and multi-path driving must be achieved to ensure that the MOSFET opens and closes normally at the same time and under the same current. Because the MOSFET has an input capacitance, the larger the power and current of the MOSFET, the larger the input capacitance. If there is not enough current, complete control will not be achieved in a short time.
Especially when the current exceeds 50A, the current design needs to be more refined, and multiple-stage and multi-path driving control must be achieved. Only in this way can the normal overcurrent and short-circuit protection of the MOSFET be ensured.
MOS current balance mainly refers to the need to ensure that when multiple MOS transistors are used together, the current passing through each MOS transistor during opening and closing is consistent. This requires attention to the drawing board, ensuring that their inputs and outputs are symmetrical, and ensuring that the current passing through each transistor is consistent. This is the ultimate goal.
For polymer lithium batteries, this parameter should be as small as possible, ideally zero, but it is impossible to achieve. Everyone wants to make this parameter small, but some people’s demands are even lower, and there are chips on the protection board that need to work. They can achieve low self-discharge, but what about reliability? The problem of self-discharge should be considered only when the performance is reliable and completely okay. Some friends may have entered a misunderstanding, dividing self-discharge into overall self-discharge and self-discharge for each string.
For polymer lithium batteries, overall self-discharge of 100-500uA is not a problem because the capacity of polymer lithium batteries is already very large. For example, for a 5AH battery, if it discharges 500uA, it will take a long time, so it is very weak for the entire battery pack.
Self-discharge for each string is the most critical, and this cannot be zero either. Of course, this is also under the condition of completely feasible performance. However, one point to note is that the self-discharge for each string must be consistent, and the difference between each string should not exceed 5uA. As we all know, if the self-discharge for each string is not the same, then after a long period of storage, the capacity of the battery will definitely change.
Lipo Battery Balance
Balance is the focus of this article. Currently, the most commonly used balance methods are divided into two types: energy-consuming and energy-transferring.
A. Energy-consuming balance mainly uses resistors to dissipate excess energy from a battery cell with a higher charge or voltage in a multi-cell battery pack. It can be further divided into the following three types.
- During charging, it is balanced in real time. It mainly starts balancing when any battery voltage is higher than the average voltage. Regardless of the voltage range of the battery, it is mainly applied in intelligent software solutions. The advantage of this solution is that it can have more time to perform voltage balancing for polymer lithium batteries.
- Voltage-point balancing means starting balancing at a voltage point, such as manganese lithium batteries, many of which start balancing at 4.2V. This method is only performed at the end of battery charging, so the balancing time is short, and its usefulness can be imagined.
- Static automatic balancing can also be performed during the charging process of polymer lithium batteries or during discharge. What’s more, when the battery is statically placed, if the voltage is inconsistent, it will also be balanced until the battery voltage reaches consistency. However, some people think that if the battery is not working, why is the protection board still heating up?
The three methods mentioned above use reference voltage to achieve balancing. However, a high battery voltage does not necessarily mean a high capacity, it may be the opposite. Details are discussed below.
The advantage of this method is low cost and simple design. It can play a certain role when the voltage of polymer lithium batteries is inconsistent, mainly due to long-term self-consumption of batteries. Theoretically, it is slightly feasible.
Disadvantages of polymer lithium batteries include complex circuits, many components, high temperature, poor anti-static performance, and high failure rate.
When new single cells are combined into a PACK after capacity, voltage, and internal resistance are differentiated, there will always be individual cells with lower capacity. Usually, the cell with the lowest capacity will have the fastest voltage rise during charging, and it will be the first to reach the start balance voltage. At this point, the larger capacity cells have not yet reached the voltage point and have not started balancing, while the smaller capacity cell has already started balancing. With each cycle, this small capacity cell will always work in a state of full charge and full discharge, making it age faster and its internal resistance will gradually increase compared to other cells, forming a vicious cycle. This is a major drawback.
The more components a polymer lithium battery has, the higher its failure rate will naturally be.
As for temperature, as can be imagined, it is energy-consuming, meaning that it uses resistance to dissipate excess energy in the form of heat. It has indeed become a real heat source. High temperature is a fatal factor for the battery itself, which may cause the battery to burn or even explode. Originally, we were trying to find ways to reduce the temperature generated by the entire battery pack, but what about energy balance? At the same time, its temperature is surprisingly high, and you can test it in a fully enclosed environment. Overall, it is a heat-generating body, and heat is the battery’s mortal enemy.
When designing protection boards for personal use, I never use low-power MOSFETs, not even one. This is because I have suffered too much from the static electricity problem of MOSFETs. Leaving aside the working environment of small MOSFETs, in the production and processing of PCBA patches, if the humidity in the workshop is less than 60%, the defect rate of small MOSFETs produced will exceed 10% or more. However, when the humidity is adjusted to 80%, the defect rate of small MOSFETs will be zero. You can try it out. What does this indicate? Whether small MOSFETs can pass through in the winter of northern China for polymer lithium batteries requires time to verify. Furthermore, MOSFETs only suffer from short-circuit damage. If a short circuit occurs, it means that the battery group will be damaged immediately. Moreover, we also use small MOSFETs on our balancing board quite often.
B. Energy transfer balance is a method of transferring energy from large-capacity batteries to small-capacity batteries in the form of energy storage, which sounds very intelligent and practical. It also includes real-time balance based on capacity and fixed-point balance based on capacity. It balances the batteries by detecting their capacity, but it seems to have overlooked the battery voltage. For example, with a 10AH battery pack, if there is a battery with a capacity of 10.1AH and another with a smaller capacity of 9.8AH, and the charging current is 2A, while the energy balance current is 0.5A, the 10.1AH battery will transfer energy to the smaller 9.8AH battery for charging.
As a result, the charging current of the 9.8AH battery will be 2A+0.5A=2.5A, and its capacity will be replenished. However, what will its voltage be? Obviously, it will rise faster than the other batteries. If it reaches the end of the charging process, the 9.8AH battery will trigger overcharging protection much earlier. During each charging and discharging cycle, the smaller battery will always be in a state of deep charging and discharging. Whether the other batteries are fully charged or not is uncertain and has too many variables.