The main function of the battery protection board

1. Voltage protection: overcharge and overdischarge, which should be changed according to the material of the battery. This seems simple, but in terms of details, there is still experience and knowledge.

Overcharge protection, in our previous single-cell battery protection voltage will be 50~150mV higher than the full battery charge voltage. However, 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, or even lower than this voltage. For example, manganese lithium battery, you can choose 4.18V~4.2V. Because it has multiple strings, the life capacity of the entire battery pack is mainly based on the battery with the lowest capacity. The small capacity always works at high current and high voltage, so the attenuation is accelerated. The large capacity is charged and discharged lightly every time, and the natural decay is much slower. In order to make the small-capacity battery charge and discharge lightly, the overcharge protection voltage point should not be selected too high. This protection delay can be achieved 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 batteries are generally selected at 2.8V~3.0V. Try to be slightly higher than the over-discharge voltage of its single battery. Because, for domestically produced batteries, after the battery voltage is lower than 3.3V, the discharge characteristics of each battery are completely different, so the battery is protected in advance, which is a good protection for the life of the battery.

The general point is to try to make each battery work in light charge and light down work, which must be a help to the life of the battery.

Over-discharge protection delay time, which should be changed according to different loads, such as electric tools, whose starting current is generally above 10C, so the voltage of the battery will be pulled to the over-discharge voltage point in a short period of time. Protect. The battery cannot be operated at this time. This is where it's worth noting.

2. Current protection: It is mainly reflected in the working current and overcurrent to disconnect the switch MOS to protect the battery pack or load.

The damage of the MOS tube is mainly due to the sharp rise in temperature, and its heat generation is also determined by the size of the current and its own internal resistance. Of course, small current has no effect on MOS, but for large current, this needs to be handled properly. When passing the rated current, the small current is below 10A, we can directly use the voltage to drive the MOS tube. For large current, it must be driven to give the MOS a large enough driving current. The following is mentioned in the MOS tube driver

Working current, when designing, the power of more than 0.3W cannot exist 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, the MOS will generate 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 rise when working for a long time, because he has no place to dissipate heat. Of course, there is no problem with the MOS tube. The problem is that the heat it generates will affect the battery. After all, the protection board is placed with the battery.

Overcurrent protection (maximum current), this is an essential and very critical protection parameter for the protection board. The size of the protection current is closely related to the power of the MOS, so when designing, try to give the margin of the MOS capability. When laying out the board, the current detection point must be located in a good position, not just connected, which requires experience. It is generally recommended to connect it to the middle end of the sense resistor. Also pay attention to the interference problem at the current sensing end, because its signal is easily disturbed.

Overcurrent protection delay, it also needs to be adjusted according to different products. Not much to say here.

3. Short circuit protection: Strictly speaking, it is a voltage comparison type of protection, that is to say, it is directly turned off or driven by voltage comparison, without unnecessary processing.

The setting of the short-circuit delay is also critical, because in our products, the input filter capacitors are very large, and the capacitors are charged as soon as they are in contact, which is equivalent to short-circuiting the battery to charge the capacitors.

4. Temperature protection: It is generally used in smart batteries and is also indispensable. But often its perfection will always bring the other side of the shortcomings. 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 out of our control, there are too many complicated changes, so it is not a good choice. For example, in the winter in the north, how much is appropriate for us? Another example is in the southern region in summer, how much is appropriate? Obviously, the scope is too wide and there are too many uncontrollable factors.

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 the MOS must exceed the voltage of the battery pack, which is a must. The current refers to the temperature rise of the MOS body when the rated current is passed, which generally does not exceed 25 degrees. The personal experience value is only for reference.

MOS drive, some people may say, I use a MOS tube with low internal resistance and high current, but why is the temperature still quite high? This is because the driving part of the MOS tube is not well done, 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 drivers cannot be directly driven by the chip, and must be added. When working with a large current (over 50A), multi-level and multi-channel driving must be done to ensure that the MOS can be turned on and off normally at the same time and the same current. Because the MOS tube has an input capacitor, the greater the power and current of the MOS tube, the greater 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 multi-channel drive control must be achieved. In this way, the normal overcurrent and short circuit protection of MOS can be guaranteed.

The MOS current balance mainly refers to the fact that when multiple MOSs are used in parallel, the current through each MOS tube must be the same as the turn-on and turn-off times. This needs to start with the drawing board. Their input and output must be symmetrical, and it must be ensured that the current passing through each tube is consistent. This is the purpose.

6. Self-consumption, the smaller the better, the ideal state is zero, but it is impossible to do this. It is because everyone wants to make this parameter small, and many people have lower requirements, which are even outrageous. Let's think about it, there are chips on the protection board, they have to work and can be very low, but what about reliability? It should be considered the problem of self-consumption when the performance is reliable and completely OK. Some friends may have entered a misunderstanding. Self-consumption is divided into the overall self-consumption and the self-consumption of each string.

The overall self-consumption power is no problem if it is 100~500uA, because the capacity of the power battery itself is very large. Of course additional analysis of power tools. Such as a 5AH battery, how long it takes 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 under the condition that the performance is completely feasible, but one point, the self-consumption of each string must be the same. Generally, the difference between each string cannot be more than 5uA. Everyone should know this. If the self-consumption of each string varies, the capacity of the battery will definitely change after a long period of shelving.

7. Equilibrium: Equilibrium is the focus of this article. At present, the most common balance methods are divided into two types, one is energy consumption type, and the other is energy conversion type.

A energy-consuming equalization, mainly to use a resistor to dissipate the excess power of a certain battery in a multi-string battery or with a high voltage. It is also divided into the following three types.

First, it is balanced during charging. It is mainly used in intelligent software solutions when the voltage of any battery is higher than the average voltage of all batteries during charging. Of course, how to define can be adjusted arbitrarily by software. The advantage of this scheme is that it has more time to do the voltage equalization of the battery.

Second, the voltage fixed-point equalization is to set the equalization start at a voltage point, such as manganese-lithium batteries, many start equalization at 4.2V. This method is only performed at the end of the battery charging, so the equalization time is short, and the usefulness can be imagined.

Three, static automatic equalization, it can also be carried out in the process of charging, or it can be carried out during discharging. What is more characteristic is that when the battery is in a static state, if the voltage is inconsistent, it is also equalizing until the voltage of the battery is equal. reach agreement. But some people think that the battery is not working, why is the protective plate still heating?

The above three methods are all based on the reference voltage to achieve balance. However, high battery voltage does not necessarily mean high capacity, perhaps the opposite. Discussed below.

Its advantages are low cost, simple design, and it can play a certain role when the battery voltage is inconsistent. Theoretically, there is a slight possibility.

Disadvantages, the circuit is complex, the components are many, the temperature is high, the anti-static is poor, and the failure rate is high.

The specific discussion is as follows.

When the new unit battery divides the capacity, voltage and internal resistance to form a PACK, there will always be a low capacity of each unit, and the voltage of the unit with the lowest capacity must rise the fastest during the charging process. , it is also the first to reach the start-up equilibrium voltage. At this time, the large-capacity monomer has not reached the voltage point and has not started to balance, and the small-capacity has indeed begun to balance, so that every cycle of work, this small-capacity monomer It has been working in a full and full state, and it is also the fastest aging, and the internal resistance will naturally increase slowly compared with other monomers, thus forming a vicious circle. This is a huge disadvantage.

The more components, the higher the failure rate.

Temperature, as can be imagined, is energy-consuming. It wants to use the so-called excess electricity to use resistance to consume excess electricity in the form of heat. It has indeed become a veritable heat source. High temperature is a very fatal factor for the battery itself, it may cause the battery to burn, or it may cause the battery to explode. Originally, we were trying to do everything possible to reduce the temperature of the entire battery pack, but how about balanced energy consumption? At the same time, its temperature is surprisingly high, you can test it, of course, in a fully enclosed environment. In general, it is a heat generating body, and heat is the deadly natural enemy of the battery.

Static electricity, when I personally design the protection board, I never use low-power MOS tubes, not even one. Because I have eaten too many losses in this one. It is the electrostatic problem of the MOS tube. Not to mention the working environment of the small MOS, it is said that during the production and processing of PCBA patches, if the humidity in the workshop is lower than 60%, the defective rate produced by the small MOS will exceed 10%, and then adjust the humidity to 80%. The defect rate of small MOS is zero. You can try. What problem does this indicate? If our product is in the northern winter, whether the small MOS can pass, it will take time to verify. In addition, the damage to the MOS tube is only a short circuit. If it is short circuited, it can be imagined that this group of batteries will be damaged soon. What's more, the small MOS on our balance is still used a lot. At this time, some people will suddenly realize that it is no wonder that the returned goods are all damaged due to the failure of the balance, and the MOS is damaged. At this time, the cell factory and the protection board factory began to argue. Whose fault is it?

B energy transfer balance, which is to transfer large-capacity batteries to small-capacity batteries in the form of energy storage, which sounds very smart and practical. It also divides capacity from time to time balance and capacity fixed point balance. It is balanced by detecting the capacity of the battery, but it seems that the voltage of the battery is not considered. You can think about it, taking 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 charge the small-capacity 9.8AH transfer energy, and the 9.8AH battery charging current is 2A+0.5A=2.5A. At this time, the 9.8AH battery charging current is 2.5A, and the 9.8AH capacity is at this time. It is added, but what is the voltage of the 9.8AH battery? Obviously, it will rise faster than other batteries. If it reaches the end of charging, the 9.8AH battery will definitely be overcharged in advance. Protection, in every charge-discharge cycle , the small-capacity battery has been in a state of deep charge and deep discharge. And whether other batteries are fully charged, there are too many uncertain factors. Weak and intuitive analysis is limited to this, too much analysis is afraid of being confused.