Professional analysis of changsen lithium battery protection board PCBAfunction
1. Voltage protection
Overcharge and overdischarge, this will vary according to the battery material. Overcharge protection, in our past, the protection voltage of a single cell battery will be 50~150mV higher than the full battery voltage. But the power battery is different. If you want to extend the battery life, the protection voltage should be the full charge voltage of the battery, even lower than this voltage. For example, for ternary batteries, you can choose from 4.18V to 4.25V; for iron-lithium batteries, you can choose from 3.65-3.9. Because it has multiple strings, the life capacity of the entire battery pack is mainly based on the battery with the lowest capacity. The smaller capacity always works at high current and high voltage, so the attenuation speeds up. The large capacity is lightly charged and discharged every time, and the natural attenuation is much slower. In order to make small-capacity batteries be lightly charged and discharged, do not choose too high an overcharge protection voltage point. This protection delay can be 1S to prevent the influence of pulse and protect.
Over-discharge protection is also related to the battery material. For example, ternary batteries are generally selected at 2.8V~3.0V; lithium iron phosphate batteries are generally selected at 2.3-2.5V. Try to be slightly higher than the over-discharge voltage of a single battery. Because the battery voltage of domestically produced batteries is lower than 3.3V (ternary), 2.8V (iron lithium), the discharge characteristics of each battery are completely different, so it is to protect the battery in advance, so that the life of the battery is affected. Very well protected.
In short, try to make every battery work under light charge and light work, it must be a help to the battery life.
The delay time of over-discharge protection varies according to the load. For example, for power tools, its starting current is generally above 10C, so it will pull the battery voltage to the over-discharge voltage point in a short time. protection. The battery cannot be operated at this time. This is something worth noting.
2. Current protection
Mainly reflected in the operating current and overcurrent to disconnect the switch MOS to protect the battery pack or load. The damage of the MOS tube is mainly caused by the sharp rise in temperature. The heating is also determined by the size of the current and its own internal resistance. Of course, a small current has no effect on the MOS, but for a large current, this must be dealt with properly. At the rated current, the small current is below 10A, and the voltage can be used to drive the MOS tube directly. The high current must be driven to provide a large enough drive current for the MOS. The function and internal resistance of the MOS tube are discussed below.
The working current should not exceed 0.3W of power on the MOS tube when designing. Calculation method: I2*R/N. R is the internal resistance of MOS, and N is the number of MOS. If the power exceeds, the MOS will produce a temperature rise of more than 25 degrees, and because they are all sealed, even if there is a heat sink, the temperature will still go up when working for a long time, because it has no place to dissipate 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 protection board is placed with the battery.
Overcurrent protection (maximum current), this item is indispensable for the protection board, a very critical protection parameter. The size of the protection current is closely related to the power of the MOS, so in the design, the margin of the MOS capability should be given as much as possible. When arranging the board, the current detection point must be selected, and it can't just be switched on. It is generally recommended to connect to the middle end of the detection resistor. Also pay attention to the interference problem of the current detection terminal, because its signal is easily interfered.
Overcurrent protection delay time, it is necessary to make corresponding adjustments for different products.
3. Short circuit protection
It is a voltage comparison type of protection, that is, it is directly turned off or driven by voltage comparison, and does not go through extra processing.
The setting of short-circuit delay is also very important, because in our products, the input filter capacitors are very large, and the capacitors are charged as soon as they are in contact. At this time, it is equivalent to short-circuiting the battery to charge the capacitors.
4. Temperature protection
Generally used in smart batteries, it is also indispensable. But often its perfection always brings another shortcoming. 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. Since the working environment of the battery is beyond our control, there are too many and complicated changes, so it is not easy to choose. For example, in the winter in the north, how much is appropriate for us? As in the southern region in the summer, how much is appropriate? Obviously, the range is too wide and there are too many uncontrollable factors.
5. MOS function and internal resistance
It is the voltage, current and temperature of MOS. Of course it involves the selection of the MOS tube. Of course, the withstand voltage of MOS must exceed the voltage of the battery pack, which is a must. Current refers to the temperature rise on the MOS tube body when the rated current is passed. Generally, the temperature rise of not more than 25 degrees is the best. For the MOS drive, some people may say that I use MOS transistors with low internal resistance and high current, but why is there still a very high temperature? This is because the driving part of the MOS tube is not done well, and the driving MOS must be large enough The current, the specific driving current, depends on the input capacitance of the power MOS tube. Therefore, the general over-current and short-circuit drive cannot be directly driven by the chip, and must be added. When working with high current (more than 50A), multi-level and multi-channel driving must be done to ensure that the MOS is normally turned on and off at the same time and the same current. Because the MOS tube has an input capacitance, the greater the power and current of the MOS tube, the larger the input capacitance. If there is not enough current, complete control will not be made in a short time. Especially when the current exceeds 50A, the current design must be refined, and multi-level and multi-channel drive control must be done. In this way, the normal over-current and short-circuit protection of the MOS can be guaranteed.
MOS current balance mainly refers to when multiple MOSs are used together, the current through each MOS tube, and the opening and closing time are the same. This has to start with the drawing board, their input and output must be symmetrical, and it must be ensured that the current through each tube is consistent. This is the goal.
This parameter is as small as possible. The ideal state is zero, but it is impossible to achieve this. The self-consumption problem should be considered when the performance is reliable and completely OK. The self-consumption is divided into the overall self-consumption and the self-consumption of each string.
The overall self-consumption, if it is 100~500uA, there is no problem, because the capacity of the power battery itself is very large. Of course another analysis of power tools. For example, a 5AH battery, how long does it take to discharge 500uA, so it is very weak for the entire battery pack. The self-consumption of each string is the most critical, and this cannot be zero. Of course, it is also carried out when the performance is completely feasible. However, there is one point. The self-consumption of each string must be the same. Generally, the difference between each string cannot be More than 5uA. At this point, everyone should know that if the self-consumption of each string is not the same, the capacity of the battery will definitely change if it is left for a long time.
7. Equalization function
Balancing this one is the key point. At present, the most common equalization methods are divided into two types, one is the energy consumption type, and the other is the energy conversion type.
A energy-consumption equalization is mainly to use a resistor to consume the excess energy of a certain battery in a multi-string battery or a high voltage. It is also divided into the following three types.
One, equalization at all times during charging, it mainly starts equalization when the voltage of any battery is higher than the average voltage of all batteries during charging. No matter what range the battery voltage is in, it is mainly applied to smart software solutions. Of course, how to define can be adjusted arbitrarily by the software. The advantage of this scheme is that it can have more time to balance the battery voltage.
Second, the voltage fixed-point equalization is to set the equalization start at a voltage point, such as manganese-lithium batteries, in many cases, the equalization starts at 4.2V. This method is only performed at the end of battery charging, so the equalization time is shorter, and the usefulness can be imagined.
Third, static automatic equalization, it can also be carried out during the charging process or during discharging. What's more characteristic is that when the battery is left statically, if the voltage is inconsistent, it is also equalizing until the battery voltage Achieve unanimity. But some people think that the battery is out of work, why is the protection board still heating?
The above three methods are all based on the reference voltage to achieve equilibrium. However, high battery voltage does not necessarily mean high capacity, it may be the opposite. The following discussion.
Its advantages are low cost, simple design, and can play a certain role when the battery voltage is inconsistent, which is mainly reflected in the voltage inconsistency caused by the self-consumption of the battery for a long time. In theory, there is a weak feasibility.
Disadvantages, complex circuit, many components, high temperature, poor anti-static, high failure rate.
8. The specific discussion is as follows
When a new single battery is formed into a PACK after the partial pressure and internal resistance of the new single battery, there will always be a low capacity of each monomer, and often the lowest capacity of the monomer, the voltage must rise the fastest during the charging process It is also the first to reach the start-up equalization voltage. At this time, the large-capacity monomer has not reached the voltage point and has not started the equalization. The small-capacity has indeed begun to be equalized, so that each cycle of work, this small-capacity monomer It has been working in a fully charged state, and it is also the fastest aging. At the same time, the internal resistance will naturally increase slowly compared with other monomers, thus forming a vicious circle. This is a great disadvantage. The more components, the higher the failure rate. Temperature, one can imagine, energy-consuming, is to use the so-called excess electricity to use resistance to consume the excess electricity in the form of heat, and it has indeed become a veritable heat source. The high temperature is a very fatal factor for the battery itself. It may cause the battery to burn or cause the battery to explode. Originally, we were trying every means to reduce the temperature of the entire battery pack, and the energy consumption is balanced? At the same time, its temperature is astonishingly high, you can test it, of course, in a fully enclosed environment. In general, it is a heating element, and heat is the deadly enemy of the battery.
Static electricity, low-power MOS tube, anti-static ability is too poor. Someone once did an experiment. When the small MOS is in the working environment, if the humidity in the workshop is lower than 60% during the production and processing of PCBA patches, the defective rate produced by the small MOS will exceed 10%, and then the humidity will be adjusted to 80%. . The defective rate of small MOS is zero.
What problem does this indicate? If our products are in the northern winter, whether the small MOS can pass, it will take time to verify. What's more, we still use a lot of small MOS on the balance. At this time, some people will be surprised. No wonder the goods returned by our company are mostly caused by the damage of the single battery due to the broken balance, and the MOS is broken.
B energy transfer type equalization, which allows large-capacity batteries to be transferred to small-capacity batteries by means of energy storage. It sounds very smart and practical. It also divides capacity time balance and capacity fixed-point balance. It is balanced by detecting the capacity of the battery, but it does not seem to take the battery voltage into consideration. Think about it, take a 10AH battery pack as an example. If there is a battery pack with a capacity of 10.1AH and a smaller capacity of 9.8AH, the charging current is 2A, and the energy balance current is 0.5A. At this time, the 10.1AH battery needs to be charged with a small capacity of 9.8AH. The charging current of the 9.8AH battery is 2A+0.5A=2.5A. At this time, the charging current of the 9.8AH battery is 2.5A, and the capacity is 9.8AH. It is made up, but what is the voltage of the 9.8AH battery? Obviously it will rise faster than other batteries. If it reaches the end of the charge, the 9.8AH battery will definitely be overcharged greatly in advance, in every charge and discharge cycle. , The small-capacity battery has been in a deep charge and deep discharge state. There are too many uncertain factors whether other batteries are fully charged.
Balance summary: I personally think that using a protection board to achieve balance is a bit more harmful than beneficial. Because the protection board is for protection, it only serves as an effective protection for the battery at the most extreme time. It has no ability to improve the performance of the battery. The protection board is only a passive part, and it only plays a protective role.
The battery cell is the active device. What we want to improve is the performance and technology on the battery cell, mainly consistency. Besides, if balance is used on the protection board, it has disadvantages and advantages in theory or practical application, but in theory, balance has a certain effect, but how useful it is, it is obvious. Why? Because charging is generally at a current of 2~10A, and we can only achieve a maximum of 200mA for equilibrium. This difference is too much, and some equalization schemes are started at the end of the charging voltage, which is even more useless. And it has a downside, too much.