Discussion and Analysis on the design of power battery protection board

Due to the rapid development of power lithium batteries in recent years, there have been considerable breakthroughs in production process, material technology improvement, or price advantages, so it also lays a solid foundation for many parallel and many series. The era of replacing lead-acid batteries is approaching. It is an undeniable fact that the market share of electric bicycles and backup power supplies has naturally begun to expand wildly. Then, for the safety and service life of the battery, the effective protection of lithium battery is also indispensable. At this time, the protective plate is also one of the core components in the battery pack.
Theoretically, the power multi string battery protection board has not much electronic technology content, such as circuit and software processing, there are too many choices. It is mainly about how to make the protection part stable, reliable, safer and more practical. Of course, the price is also one of them. If you really want to do it well, it is a very complex, careful and long cycle work. If the ratio of experience to technical value is to be considered, technology only accounts for 20%. Experience accounts for 80%. Without three to five years of experience in doing a good job of power battery protection board, it is still difficult. Of course, doing well and being able to do well are two different things. Why is there such a conclusion? This is well founded. To be honest, the circuit of the protection board is not complicated. As long as I have worked in the battery electronics industry for a year or two, it is not difficult to design a circuit and copy another circuit. For example, multi string power batteries mainly work with high voltage, high current and high internal resistance (micro current), and consider the working environment of battery packs, which involve many years of comprehensive experience in electronics. As big as understanding the whole pack, as small as the selection of a resistor, capacitor or transistor, or paying attention to details when distributing boards. In a word, the protection board is mainly to protect the battery pack stably, reliably and safely, so as to ensure the normal and safe use or longer use of the battery pack. Other added unique technologies and functions are floating clouds. Now let's discuss it.) Power battery protection board, as the name suggests, is used to protect the battery from damage and prolong the service life of the battery. And it only provides the most stable and effective protection against accidents when the battery has extreme problems. You should not act at ordinary times. Of course, monitoring is necessary, just like the fuse or safety switch in our household appliances. This is the purpose of this paper.
Protection items and precautions
1. Voltage protection: overcharge and over discharge, which should be changed according to the material of the battery. This seems simple, but in terms of details, it is still experienced and learned.
For overcharge protection, in our past, the protection voltage of a single battery will be 50~150mv higher than the full battery voltage. But the power battery is different. If you want to prolong the battery life, your protection voltage should choose the full charge voltage of the battery, even lower than this voltage. For example, manganese lithium battery, you can choose 4.18v~4.2v. Because it is multi string, the lifetime capacity of the whole battery pack is mainly based on the battery with the lowest capacity. The small battery always works at high current and high voltage, so the attenuation is accelerated. However, the large capacity is filled and placed gently every time, and the natural attenuation is much slower. In order to charge and discharge the small capacity battery with care, the overcharge protection voltage point should not be too high. This protection delay can be achieved for 1s to prevent the impact of pulses and thus protect.
Over discharge protection is also related to the material of the battery. For example, manganese lithium battery is generally selected at 2.8v~3.0v. Try to be slightly higher than the over discharge voltage of a single battery. Because the discharge characteristics of domestic batteries are completely different when the battery voltage is lower than 3.3V, so it is a good protection for the battery life to protect the battery in advance.
The general point is to try to make every battery work in light charge and light down, which must be a help to the battery life.
The delay time of over discharge protection should be changed according to different loads. For example, for electric tools, the starting current is generally above 10C, so the battery voltage will be pulled to the over discharge voltage point in a short time to protect. The battery cannot be operated at this time. This is noteworthy.
2. Current protection: it is mainly reflected in that the working current and overcurrent disconnect the switch MOS to protect the battery pack or load.
The damage of MOS tube is mainly caused by the sharp rise of temperature, and its heating is also determined by the size of current and its own internal resistance. Of course, small current has no impact on MOS, but for large current, it needs to be handled well. When passing the rated current, when the small current is below 10a, we can directly drive MOS tube with voltage. High current must be driven to give MOS enough driving current. The following is described in MOS tube drive
The working current, when designed, cannot exceed 0.3w power on the MOS tube. Calculation formula: i2*r/n. R is the internal resistance of MOS and N is the number of MOS. If the power exceeds, MOS will produce a temperature rise of more than 25 degrees. Because they are sealed, even if there are heat sinks, the temperature will rise after long-time operation, because there is no place to heat. Of course, there is no problem with the MOS tube. The problem is that the heat generated by it will affect the battery. After all, the protective plate is placed with the battery.
Overcurrent protection (maximum current), which is an essential and critical protection parameter of the protection board. The magnitude of protection current is closely related to the power of MOS, so the margin of MOS capacity should be given as much as possible during design. When arranging the board, the current detection point must be located properly, not just connected, which requires empirical value. It is generally recommended to connect it to the middle end of the detection resistance. Also pay attention to the interference at the current detection end, because its signal is easily disturbed.
The time delay of overcurrent protection should also be adjusted according to different products. I won't say more here.
3. Short circuit protection: strictly speaking, it is a voltage comparison protection, that is to say, it is directly turned off or driven by voltage comparison, and no unnecessary treatment is required.
The setting of short-circuit delay is also very critical, because in our products, the input filter capacitor is very large. Charging the capacitor at the first time when contacting is equivalent to charging the capacitor when the battery is short circuited.
4. Temperature protection: it is generally used on smart batteries, and it is also indispensable. But often its perfection will always bring deficiencies on the other hand. We mainly detect the temperature of the battery to disconnect the main switch to protect the battery itself or the load. If it is under a constant environmental condition, of course, there will be no problem. Because the working environment of the battery is beyond our control, there are too many complex changes, so it is not a good choice. If it's winter in the north, how much time is suitable for us? Another example is the southern region in summer, how much is appropriate? Obviously, the scope is too wide and there are too many uncontrollable factors. Different people have different opinions and wise people choose.
5. MOS protection: mainly the voltage, current and temperature of MOS. Of course, it involves the selection of MOS tubes. Of course, the withstand voltage of MOS must exceed the voltage of the battery pack, which is necessary. The current refers to the temperature rise on the MOS body when passing the rated current, which generally does not exceed 25 degrees. Personal experience value is only for reference.
Some people may say that I use MOS tubes with low internal resistance and high current, but why is there a very high temperature? This is because the driving part of the MOS tube is not done well. The driving current of the MOS tube should be large enough. The specific driving current should be determined according to the input capacitance of the power MOS tube. Therefore, general over-current and short-circuit drives cannot be directly driven by chips, and must be added. When working with high current (more than 50a), multi-stage and multi-channel drive must be achieved to ensure that the MOS can be opened and closed normally at the same time with the same current. Because the MOS tube has an input capacitor, the higher the power and current of the MOS tube, the larger the input capacitor. If there is not enough current, the complete control will not be made in a short time. Especially when the current exceeds 50a, the current design should be more refined, and multi-level and multi-channel drive control must be achieved. In this way, the normal overcurrent and short circuit protection of MOS can be guaranteed.
MOS current balance mainly refers to that when multiple MOS are used together, the current passing through each MOS tube should have the same opening and closing time. This needs to start with the drawing board. Their input and output must be symmetrical, and the current passing through each tube must be consistent. This is the goal.
6. The smaller the parameter, the better. The ideal state is zero, but it is impossible to do this. It's because everyone wants to make this parameter small, and many people's requirements are lower or even outrageous. Let's think, there are chips on the protection board, which need to work and can be very low, but what about the reliability? The problem of self power consumption should be considered when the performance is reliable and completely OK. Some friends may have entered the wrong zone. Self consumption is divided into overall self consumption and each string of self consumption.
The overall self consumption is no problem at 100~500ua, because the capacity of the power battery itself is very large. Of course, another analysis of electric tools. For example, the battery of 5Ah is very weak for the whole battery pack because of how long it takes to discharge 500uA.
The self consumption of each string is the most critical. This cannot be zero. Of course, it is also carried out when the performance is completely feasible, but one thing is that the self consumption of each string must be consistent. Generally, the difference of each string cannot exceed 5ua. You should know this. If the self consumption of each string is not temporary, the capacity of the battery will change after being shelved for a long time.
7. Static electricity. When designing the protection board, do not use low-power MOS tubes, not even one of them. Because many predecessors have suffered too many losses here. It is the electrostatic problem of MOS tube. Not to mention the working environment of small MOS, that is, when producing and processing PCBA patches, if the humidity in the workshop is lower than 60%, the defect rate produced by small MOS will exceed 10%, and then the humidity will be adjusted to 80%. The defect rate of small MOS is zero. You can try it. What problem does this indicate? If our products are in northern winter, it takes time to verify whether the small MOS can pass. Moreover, the damage of MOS tube is only short circuit. If it is short circuit, it can be imagined that this group of batteries will be damaged soon. What's more, we still use a lot of small MOS in our equilibrium. At this time, someone will suddenly realize that it is no wonder that the returned goods are damaged by the single battery due to the broken balance, and the MOS is broken. At this time, the battery factory and the protective board factory began to quarrel. Whose fault is it?
8. Balance: balance is the focus of this article. At present, the most common equilibrium method is divided into two types, one is energy consumption type, and the other is energy conversion type.
A energy consumption equalization is mainly to use resistors to lose excess electric energy in a battery with high power or voltage in multiple strings of batteries. It is also divided into the following three types.
1、 Charging is always balanced. It is mainly that when the voltage of any battery is higher than the average voltage of all batteries during charging, it starts equalization. No matter what the voltage range of the battery is, it is mainly used in intelligent software solutions. Of course, how to define can be arbitrarily adjusted by software. The advantage of this scheme is that it can have more time to balance the battery voltage.
2、 Voltage fixed-point equalization is to set the equalization start at a voltage point, such as manganese lithium batteries, many of which are set at 4.2V to start equalization. This method is only carried out at the end of battery charging, so the equalization time is short, and its usefulness can be imagined.
3、 Static automatic equalization can also be carried out during charging or discharging. What's more, when the battery is in static storage, if the voltage is inconsistent, it is also balanced until the battery voltage is consistent. But some people think that the battery is out of work, why is the protective plate still heating?
The above three methods are balanced by reference voltage. However, high battery voltage does not necessarily mean high capacity, which may be the opposite.
Its advantages are low cost and simple design. It can play a certain role in the case of inconsistent battery voltage, which is mainly reflected in the voltage inconsistency caused by the self consumption of the battery for a long time. Theoretically, it has weak feasibility.
Disadvantages: complex circuit, many components, high temperature, poor anti-static, high failure rate.
The specific discussion is as follows. When a pack is formed after the internal resistance of the new cell is divided into capacity, voltage and voltage, there will always be different cells with low capacity. The cell with the lowest capacity must have the fastest voltage rise during the charging process, and it is also the first to reach the starting equilibrium voltage. At this time, the large capacity cell has not reached the voltage point and has not started equalization, and the small capacity does start equalization, so every cycle works, This small volume monomer has been working in a full state, and it is also the fastest aging. At the same time, the internal resistance will naturally slowly increase compared with other monomers, thus forming a vicious circle. This is a great drawback.
The more components, the higher the failure rate.
It is conceivable that the energy consumption type is to use the so-called excess electricity resistance to consume the excess electricity in the form of heating. It has indeed become a real heat source. High temperature is a very fatal factor for the battery itself. It may burn the battery or cause the battery to explode. Originally, we were trying every means to reduce the temperature generation of the whole battery pack, and the energy consumption is balanced? Its temperature is surprisingly high. You can test it, of course, in a fully enclosed environment. In general, it is a heating body, and heat is the fatal natural enemy of batteries.
B energy transfer equalization, which allows large capacity batteries to be transferred to small capacity batteries in the way of energy storage, sounds very intelligent and practical. It also divides the capacity into time equilibrium and fixed-point equilibrium. It is balanced by detecting the capacity of the battery, but it seems that the voltage of the battery is not taken into account. It can be imagined that taking a 10Ah battery pack as an example, if there is a battery pack with a capacity of 10.1ah and a smaller one with a capacity of 9.8ah, the charging current is 2a and the energy balance current is 0.5A. At this time, 10.1ah needs to charge the energy conversion of 9.8ah with a small capacity, and the charging current of 9.8ah battery is 2a+0.5a=2.5a. At this time, the charging current of 9.8ah battery is 2.5A. At this time, the capacity of 9.8ah is added, but what is the voltage of 9.8ah battery? Obviously, it will rise faster than other batteries. If it reaches the end of charging, the 9.8ah will be greatly advanced for overcharge protection. In each charge and discharge cycle, the small capacity battery has been in the state of deep charge and deep discharge. There are too many uncertainties about whether other batteries are fully charged. This is the weak and intuitive analysis. I'm afraid I don't know what to say if I analyze too much.

Balance summary, there is such a conclusion: if you insist on using the balance function, I can conclude that this person has no experience in mass production of power battery protection panels or packs. If there is mass production, he will certainly suffer a lot of losses in balance. Experience has shown that the balanced use of protective boards is a little funny. Because the protective board is protective, it only acts as a battery to effectively protect the battery at the most extreme times. It has no ability to improve the performance of the battery. The protective board is only a passive part. Can the protective wire or protective switch at home improve the power at home? Of course not. It only plays a protective role.
The cell is the active device. What we need to improve is the performance and technology of the cell, mainly the consistency. Besides, balancing on the protection board, whether in theory or in practical application, has disadvantages and advantages, but in theory, balancing has a certain role, but it is obvious how useful it is. Why? Because the charging current is generally 2~10a, and we can only achieve 200mA at most. There are too many differences. At the same time, some equalization schemes start at the end of the charging voltage, which is more useless. And it has the disadvantage of too many.