In theory, the power multi-string battery protection board has not much electronic technology content, such as circuit and software processing, there are too many choices. The main reason is how to make the protection part stable, reliable, safer and more practical. Of course, the price is one of them. To really want to do it well, it is a very complicated and careful revolving work. If you want to compare the ratio of experience to technical value, the technology only accounts for 20%. Experience accounts for 80%. It is still difficult to do a good job in the power battery protection board without three or five years of experience. Of course, doing well and being able to do it is two different things. Why is there such a conclusion? This is valid. To be honest, the circuit of the protection board is not complicated. As long as it has been working in the battery electronics industry for a year or two, it is not difficult to design a circuit and plagiarize a circuit. For example: a series of power batteries, he is mainly high voltage, high current, high internal resistance work (micro current), battery pack working environment, etc., which involve many years of comprehensive experience in electronics. It is so big to understand the whole PACK, as small as a resistor, capacitor or transistor selection, or attention to detail when laying the board. In a word, the protection board is mainly to stabilize, reliable, and safely protect the battery pack, to ensure the normal safe use or use of the battery pack, and other unique technologies and functions are floating clouds. Let's discuss it below.
The power battery protection board, as its name suggests, is used to protect the battery from damage and extend the life of the battery. And it only provides the most stable and effective protection against accidents in the event of extreme battery problems. Normally, it should not be operated. Of course, surveillance work is necessary, just like the fuses or cutouts in our household appliances. This is the purpose of this article's discussion and analysis.
Protection items and precautions
1. Voltage protection: Overcharge, over discharge, this should be changed according to the material of the battery, this seems simple, but in terms of details, there is still experience to learn.
Overcharge protection, in our previous single-cell battery protection voltage will be higher than the battery charge voltage 50~150mV. But the power battery is different. If you want to extend the battery life, your protection voltage will choose the battery's full voltage, and even lower than this voltage. For example, manganese lithium battery, you can choose 4.18V~4.2V. Because it is multi-string, the life capacity of the entire battery pack is mainly based on the battery with the lowest capacity, and the small capacity always works at high current and high voltage, so the attenuation is accelerated. The large capacity is light and light, and the natural attenuation is much slower. In order to make the small-capacity battery light and light, the overcharge protection voltage point should not be too high. This protection delay can be done 1S to prevent the influence of the pulse 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 voltage of its single battery over discharge. Because, in 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 battery life.
The general point is to try to make every battery work in light and light work, it must be a help to the battery life.
Over-discharge protection delay time, it has to change according to the load, such as power tools, his starting current is generally above 10C, so the battery voltage will be pulled to the over-discharge voltage point in a short time protection. The battery cannot be operated at this time. This is a noteworthy place.
The damage of the MOS tube is mainly caused by a sharp rise in temperature. The heat of the MOS tube is also determined by the magnitude of the current and its internal resistance. Of course, the small current has no effect on the MOS, but the large current, this must be done some well. When passing the rated current, the small current is below 10A, we can directly drive the MOS tube with voltage. The large current must be added to drive the MOS to a large enough drive current. The following is the operating current in the MOS tube driver. At the time of design, there must be no more than 0.3W of power on the MOS tube. Calculation method: I2*R/N. R is the internal resistance of MOS, and N is the number of MOS. If the power is exceeded, 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 he has no place to dissipate heat. Of course, the MOS tube is no problem, the problem is that his heat will affect the battery, after all, the protection board is placed with the battery.
2. Overcurrent protection (maximum current), this is an essential and very critical protection parameter for the protection board. The magnitude of the protection current is closely related to the power of the MOS, so in design, try to give the margin of MOS capability. When laying out the board, the current detection point must be selected, not just on, which requires experience. It is generally recommended to connect to the middle of the sense resistor. Also pay attention to the interference problem at the current detection terminal because its signal is susceptible to interference.
Overcurrent protection delay, it is also necessary to make corresponding adjustments for different products. Not much to say here.
3. Short-circuit protection: Strictly speaking, he is a voltage-comparative type of protection, that is to say, it is directly shut down or driven by voltage comparison, and does not undergo unnecessary processing.
The setting of the short-circuit delay is also critical, because in our products, the input filter capacitor is very large, and the capacitor is charged for the first time during contact. This is equivalent to short-circuiting the battery to charge the capacitor.
4. Temperature protection: It is generally used on smart batteries, and it is also indispensable. But often its perfection always brings about another deficiency. 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 uncontrollable, too many complicated changes, it is not a good choice. For example, in the winter in the north, how much we are suitable? Another example is the southern part of the summer. Obviously, there are too many factors that are too wide and uncontrollable. The benevolent sees benevolence, and the wise sees wisdom to choose.
5. MOS protection: mainly MOS voltage, current and temperature. Of course, it involves the selection of MOS tubes. The withstand voltage of MOS must of course exceed the voltage of the battery pack, which is necessary. The current is said that the temperature rise on the MOS tube body when passing the rated current is generally not more than 25 degrees, and the personal experience value is for reference only.
MOS driver, maybe someone will say, I use MOS tube with low internal resistance and high current, but why is there a very high temperature? This is the driving part of the MOS tube is not done well, the driving MOS should have a large enough current, the specific driving current, according to the input capacitance of the power MOS tube. Therefore, the general overcurrent and short-circuit drive can not be directly driven by the chip, it must be added. When working at high current (more than 50A), multi-stage multi-channel driving must be done to ensure that the same current is normally turned on and off at the same time of MOS. Because the MOS tube has an input capacitor, the MOS tube power, the larger the current, the larger the input capacitance. If there is not enough current, it will not make complete control in a short time. Especially when the current exceeds 50A, the current design must be more refined, and multi-stage multi-drive control must be performed. This will ensure normal overcurrent and short circuit protection of MOS.
MOS current balance, mainly when multiple MOS are used together, the current through which each MOS transistor passes is the same as the turn-on and turn-off times. This is to start with the drawing board. Their input and output must be symmetrical. It is necessary to ensure that the current through each tube is the same.
6. Self-consumption power, this parameter is as small as possible, the ideal state is zero, but it is impossible to do so. It is because everyone wants to make this parameter small. Many people have lower requirements and even outrageous. We think that there are chips on the protection board. They have to work. It can be done very low, but is it reliable? It should be to consider the problem of self-consumption when the performance is reliable and completely OK. Some friends may have entered a misunderstanding, and the self-consumption is divided into the overall self-consumption and the self-consumption of each string.
Overall self-consumption, if there is no problem in 100~500uA, because the capacity of the power battery itself is very large. Of course, another analysis of power tools. For example, the 5AH battery, discharging 500uA, how long it will take, so it is very weak for the entire battery pack.
Each string of self-consumption is the most critical, this is not possible to 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 consistent, generally the difference between each string can not be More than 5uA. At this point, everyone should know that if the power consumption of each string is different, the battery capacity will change when it is left for a long time.
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