Battery protection board, lithium battery management system.
The battery protection board is mainly an integrated circuit board that protects the rechargeable lithium battery. The reason why the lithium battery needs protection is determined by its own characteristics. Because the material of the lithium battery itself determines that it cannot be overcharged, overdischarged, overcurrent, short circuited, and ultra-high temperature charging and discharging, the lithium battery components of the lithium battery will always appear with a protection board with a sampling resistor and a current fuse.
The role of lithium battery protection circuit board and battery management system:
1. Voltage protection: overcharge and overdischarge, which should be changed according to the material of the battery.
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 prolong the battery life, your protection voltage should be the full charge voltage of the battery, which is even lower than this voltage. Such as 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 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, such as manganese-lithium batteries generally choose 2.8V~3.0V. Try to be slightly higher than the over-discharge voltage of a single battery. Because, when the battery voltage is lower than 3.3V, the discharge characteristics of each battery are completely different for the batteries produced in China. Therefore, 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 under light charge and light down work, it must be a help to the life of the battery.
The delay time of over-discharge protection varies according to the load. For example, for electric 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: It is mainly embodied 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 due to the sharp rise in temperature. Its 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. When passing the rated current, the small current is below 10A, we can directly use the voltage to drive the MOS tube. For high current, it must be driven to provide enough drive current for the MOS. The following is mentioned in the MOS tube driver
When designing the working current, the MOS tube cannot have more than 0.3W of power. 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 laying out the board, the current detection point must be selected. It can't just be switched on. This requires experience. 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.
Over-current protection delay, it also needs to be adjusted accordingly for different products. Not much to say here.
3. Short-circuit protection:
Strictly speaking, it is a voltage-comparative type of protection, that is, it is directly turned off or driven by the voltage comparison, and does not undergo redundant processing.
The setting of the short-circuit delay is also very important, because in our products, the input filter capacitor is very large, and the capacitor is charged at the first time when contact is made. At this time, it is equivalent to the short-circuit of the battery to charge the capacitor.
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 protection
mainly 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, which generally does not exceed 25 degrees. Personal experience values are for reference only.
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 which each MOS tube passes, and the turn-on and turn-off time are the same. This starts 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.
6. Self-power consumption
this parameter is as small as possible, the most ideal state is zero, but it is impossible to achieve this. It's because everyone wants to make this parameter smaller. Many people have lower requirements, or even outrageous. Let's think about it. There are chips on the protection board. They need to work and can be very low, but what about reliability? The self-consumption problem should be considered when the performance is reliable and completely OK. Some friends may have entered a misunderstanding, self-consumption is divided into overall self-consumption and each string of self-consumption.
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. This cannot be zero. Of course, it is also carried out when the performance is completely feasible. But 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.
The balance is the focus of this article. 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 resistors to consume the excess power of a certain battery in multiple strings of batteries. It is also divided into the following three types.
* Balance at all times during charging, it is mainly used when the voltage of any battery is higher than the average voltage of all batteries during charging, it will start the equalization, no matter what the range of the battery voltage, it is mainly used in smart software solutions. Of course, how to define can be arbitrarily adjusted by the software. The advantage of this scheme is that it can have more time to balance the battery voltage.
* 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 the equalization. This method is only performed at the end of battery charging, so the equalization time is shorter, and the usefulness can be imagined.
* Static automatic equalization, it can also be carried out during charging or 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 reaches Unanimous. 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 after being left for a long time. In theory, there is a weak feasibility.
Disadvantages: complex circuit, many components, high temperature, poor anti-static, high failure rate.