Introduction to the basic knowledge of lithium battery protection board

2021-08-26 09:31:10 0

The purpose of this article is to let everyone understand the protection board, how to choose your own protection board, and understand the interfaces of several protection boards, and reduce the failure rate of the protection board during wiring.

What are the main functions of the protection board?

Because lithium batteries are more sensitive to voltage, a higher or lower voltage will affect the battery life and even damage the battery. Therefore, the lithium battery must be equipped with a protection board. The protection board mainly plays a role in charging and discharging the lithium battery pack. The main functions are as follows:

1. Overcharge protection to prevent battery damage caused by overcharge of lithium battery cells;

2. Over-discharge protection to prevent battery damage caused by over-discharge of lithium battery cells;

3. Current limiting and short-circuit protection to prevent the lithium battery pack from exceeding the rated current or short-circuit causing damage to the battery and the protection board itself;

4. Balance function, which can make each battery cell be fully charged;

5. Temperature control protection, let the lithium battery work within the normal temperature range, otherwise the input and output will be stopped.

The protection board can be simply understood as a switch, but this switch is connected in series to the negative electrode of the battery. The same port board is the charge and discharge shared output negative electrode, and the split port board is the charge and discharge negative electrode separated, and the positive input and output of the battery are from the total of the battery pack. Positive output.

The equalization cable of the protection board is to monitor the voltage of each string of batteries, and then the main control chip will switch the control MOS according to the voltage of the equalization cable to achieve the most important overcharge and overdischarge protection function for the lithium battery pack. , It also has other functions such as equalizing charging, current limiting, temperature control and so on.

What is MOS?

What is MOS? The full Chinese name is Metal-Oxide Semiconductor Field Effect Transistor. When used on a protection board, MOS plays the role of a switch. How to say it, you can understand that MOS is a circuit breaker that protects the space on the board. Open, but this air switch does not need to be broken by hand, it is opened or closed by the control and management IC main control chip + peripheral circuit.

In the application of MOS protection board, the two most critical indicators, I think one is the withstand voltage, the other is the on-resistance R (DSon), the former represents the voltage that can withstand the entire battery pack and the charger voltage, such as 72V For lithium batteries, when we make protection boards, we cannot use 75N75 and other MOS with a withstand voltage of 75V. Instead, we must use 4310 4110 with high withstand voltage; for the protection board, MOS will only have the phenomenon of normally open and normally closed, so it is not There is switching loss. Only in the application of controllers and power supplies, the switching loss will be taken into account based on PWM drive.

What is lithium battery balancing?

The purpose of balancing is to charge each string of cells to full charge. Why do lithium batteries need to be balanced? The main reason is that the internal resistance and self-discharge of each lithium-ion cell are different! If a battery cell in a battery pack cannot be fully charged, it will cause the entire battery pack to be not fully charged, which is the barrel effect.

In terms of balance, there are two ways: active balance and passive balance

Passive balance: By discharging the number of strings with high voltage and recharging repeatedly, the voltage of each string of the battery can reach the expected voltage over time (that is, the battery is basically fully charged). Passive balance is a dissipative balance, which is to pass electricity through heat The method is dissipated, so there will be a problem of heating of the protection board during equilibrium. This problem also leads to the fundamental reason why the equilibrium current of the protection board is not large.

Active balance: Use the peripheral circuit to transfer the power of the high voltage string to the low voltage. Over time, everyone's voltage will be the same. Active balance is also called lossless balance, because the power is transferred between internals, and the heat is only The problem of conversion efficiency

At present, most of the electric vehicles are passively balanced. Why? Low cost and high reliability

Many people like to use a protection board with a relatively large equalization current. In fact, in terms of protection board design, the passive equalization scheme is nothing more than a choice of resistance. However, if you think about it, most of the protection boards are wrapped in How does the heat dissipate in the battery pack? For example, a 100ma balanced current protection board, according to Ohm's theorem, the heat of each balanced resistor is I*I*R=0.1*0.1*35=0.35W, and the worst case of 24 series protection boards is 24 balanced lights ( Or 23) at the same time, the total power is 8W, you first try to put an 8W bulb in your battery to see the situation. . . So here is another point. It is better to have a certain gap between the protection board and the battery to prevent heat from being directly transferred to the battery and causing various problems.

Precautions for installation of protection board

No matter which manufacturer’s protection board, the wires that need to be connected are nothing more than B- (battery connection), P- (discharge negative electrode), C- (charging negative electrode), and a large number of cables. The above markings may be slightly different. For example, some manufacturers charge the negative electrode of CH-, discharge is D-, etc., in fact, these are the first two letters of the English word for battery and charging.

Wiring the protection board, the sequence should be:

1. Know a few B- P- C- and the order of the cable, especially to understand where the first one is connected. . .

2. Solder the B- P- C- wire in advance, don’t wait for the connection to be connected before soldering, the position is not good for soldering

3. At the time of series and parallel connection, the cable is connected as expected

4. At 100%, confirm that the cable is as expected (check the voltage and sequence under the multimeter to see if it is OK)

5. Insert the cable into the protection board, and measure the voltage of P+ (battery positive electrode) and B- P+ and P- P+ and C- to see if they are the same. Normally they should be the same.

6. Glue and reinforce the sockets to avoid loosening, and fix the protective plate (the protective plate and the battery should not be directly attached to the battery, otherwise the heat transfer will be balanced)

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