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battery protection board

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 is higher than the battery charge voltage 50-150mV. But the power battery is different. If you want to extend the battery life, choose the battery's full voltage as protection voltage, and even lower than this voltage. For example, for 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 lightly charged and lightly discharged every time, so the natural attenuation is much slower. In order to make the small-capacity battery charge and discharge lightly, 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 protect the battery.

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, after the lithium battery voltage is lower than 3.3V, the discharge characteristics of each battery are completely different, so the battery is protected in advance, this is a good protection for the battery life.

The general point is to try to make every battery work in lightly charge and lightly discharge, it must be a help to the battery life.

The over-discharge protection delay time, which varies according to the load, such as the power tool type, because the starting current is generally above 10A, so the battery will be pulled to the over-discharge voltage point for protection in a short time. The battery cannot work at this time. This is a noteworthy place.

2. Current protection: It mainly reflects the working current and over current to disconnect the switch MOS to protect the battery pack or load.

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 for the high current, we should deal with it. When the circuit is rated current, the small current is below 10A, we can directly drive the MOS tube with voltage. For large currents, it must be added to drive the MOS with a large enough drive current.

Working current, when designing, there should be no more than 0.3W of 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 is exceeded, MOS will produce a temperature rise above 25℃, 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, it has no place to dissipate heat. Of course, there is no problem with the MOS tube. The problem is that it generates heat that affects 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 magnitude of the protection current is closely related to the power of the MOS. Therefore, when designing, 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 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 to make corresponding adjustments according to different products.

3. Short circuit protection: Strictly speaking, it is a voltage comparison type protection, that is to say, it is directly turned off or driven according to the comparison of voltages, and no unnecessary processing is required.

The setting of the short-circuit delay is also critical, because in our products, the input filter capacitor is very large, charging the capacitor as soon as it is shorted, which is equivalent to short-circuiting the battery to charge the capacitor.

4. Temperature protection: generally be used on the battery, 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, there will be no problem. Since the working environment of the battery is uncontrollable, too many complicated changes, it is not easy to choice.

5. MOS protection: mainly MOS voltage, current and temperature. Of course, it involves the selection of MOS tubes. The withstand voltage of MOS must exceed the voltage of the battery pack, which is necessary. The current is said to be the temperature rise on the MOS tube body when the rated current is passed. The temperature rise generally does not exceed 25℃. 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 high temperature? This is the MOS tube drive part is not done well, and to drive MOS should have a large enough current, the specific drive current, according to the input capacitance of the power MOS tube. Therefore, the general overcurrent and short-circuit drive can't 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. The MOS tube has an input capacitor, the larger the MOS tube power and 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, the opening and closing times are the same. This should start with the drawing board. Their input and output must be symmetrical. It is necessary to ensure that the current through each tube is consistent.

6. Self-consumption power: this parameter is as small as possible, the most ideal state is 0, but it is impossible to do this. Self-consumption power should be considered in the case of reliable performance.

For the overall self-consumption, there is no problem at 100-500uA, because the capacity of the power battery itself is very large. Of course, it needs another analysis for 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 be 0. Of course, it is also possible when the performance is completely feasible. However, there is a point that the self-consumption of each string must be consistent. Generally, the difference between each string can't exceed 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.

7. Equilibrium: Equilibrium is the focus of this article. At present, the most common methods of equalization are divided into two types, one is energy-consuming and the other is trans-power.

A. Energy-consuming equalization, is mainly to consume the power of a certain battery in a series of batteries or use a resistor to consume excess power for the high voltage battery. It is also divided into the following three.

First, equalization during charging, it is mainly when the voltage of any battery is higher than the average voltage of all the batteries when charging, it starts to equalize, no matter what range of the battery voltage, it is mainly applied in the intelligent software program. Of course, how to define can be arbitrarily adjusted by software. The advantage of this solution is that it gives more time to do the voltage equalization of the battery.

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

Third, static automatic equalization, it can also be carried out during the charging process, or can be carried out during discharge. What is more characteristic is that when the battery is in static state, if the voltage is inconsistent, it is also balanced until the battery voltage achieves consistency. But some people think that the battery is not working, why is the protection board still hot?

All of the above three methods are balanced by the reference voltage. However, a high voltage battery does not necessarily mean that the battery capacity is high, perhaps the opposite.

The advantage is that the cost is low, the design is simple, and it 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 shelve. In theory, there is a weak feasibility.

Disadvantages, complicated circuit, many components, high temperature, poor anti-static, high failure rate.

When the new single cells are separated by capacitance, voltage and internal resistance, and then formed into a pack, there will always be a lowest capacity cell, and the cell with the lowest capacity tends to rise the fastest during charging. It is also the first to reach the starting equalization voltage. At this time, the large-capacity monomer has not reached the voltage point and has not started to equalize. The small capacity does start to be equalized, so that every time the cycle works, this small-capacity cell has been working in a full charge and full discharge state, and it is also the fastest aging, while the internal resistance will naturally increase slowly and bigger than other cells, thus forming a vicious circle. This is a huge drawback.

The more components, the higher the failure rate for the battery.

Temperature, as you can imagine, energy-consuming, is to consume excess power in the form of resistance heating, and it has become a veritable source of heat. The high temperature is a very fatal factor for the cell itself. It may cause the battery to burn and may cause the battery to explode. Originally, we are trying every means to reduce the temperature of the entire battery pack. In general, it is a heating element, and heat is the deadly natural enemy of the battery.

Static electricity, when I personally design the battery protection board, I never use a low-power MOS tube. It is the static problem of the MOS tube. Not to mention the environment in which small MOS is working, when the PCBA patch is produced and processed, if the humidity of the workshop is less than 60%, the defect rate of the produced small MOS will exceed 10%, and then the humidity will be adjusted to 80%. The small MOS has a low defect rate of zero. What does this mean? If our products are in the winter, whether the small MOS can pass, it will take time to verify. Moreover, the damage of the MOS tube is only a short circuit. If it is short-circuited, it means that the battery will be damaged immediately.

B. Energy transfer equalization, which is to transfer a large-capacity battery to a small-capacity battery in the form of energy storage, which sounds very smart and practical. It is also divided into real-time capacity equalization and capacity fixed point balance. It is measured by detecting the capacity of the battery, but it does not take into account the voltage of the battery. Take the 10AH battery pack as an example. If the battery pack has a 10.1AH cell, a small capacity cell is 9.8AH, the charging current is 2A, and the energy equalization current is 0.5A. At this time, 10.1AH cell should charge a small capacity cell of 9.8AH, and the charging current of 9.8AH cell is 2A+0.5A=2.5A. At this time, the charging current of 9.8AH battery is 2.5A, then the capacity of 9.8AH cell is added, but what is the voltage of the 9.8AH battery? Obviously, it will rise faster than other cells. If it reaches the end of charging, 9.8AH cell will reach overcharge protection greatly in advance. In each charge and discharge cycle, the small-capacity battery is always in deep charge and deep discharge. And if other batteries are full, there are too many uncertainties.

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