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How does lithium battery BMS work?
Date: 2022-11-23 17:40:47Downloads: 214
BMS is the charge and discharge protection of series lithium battery pack; When fully charged, it can ensure that the voltage difference between each single battery is less than the set value (small battery pack is generally ± 20mV),realize the uniform charging of each single battery of the battery pack, and effectively improve the charging effect under the series charging mode; At the same time, it can detect the overvoltage, undervoltage, overcurrent, short circuit and overtemperature of each single battery in the battery pack, protect and extend the service life of the battery; The undervoltage protection prevents each single cell from being damaged due to over discharge. The reason why a lithium battery needs a BMS is that the material of the lithium battery determines that it cannot be overcharged, over discharged, over-current, short circuit, and ultra-high temperature charging and discharging. Therefore, the lithium battery pack always follows the BMS and a current protector.
The protection function of lithium battery is usually completed by the cooperation of the protection circuit board and PTC. The protection board is composed of electronic circuits. Under the environment of - 40 ℃ to+85 ℃, the voltage of the battery cell and the current charged and discharged are monitored accurately at all times, and the current circuit is controlled on and off immediately; PTC can prevent battery damage under high temperature.
Main technical parameters of lithium battery BMS: equalizing current, over discharge delay, over discharge release, over current release, over temperature protection, working current, static power consumption, short circuit protection function, etc. Main functions: overcharge protection function, over discharge protection function, short circuit protection function, overcurrent protection function, over temperature protection function, and balanced protection function. Interface definition: the charging port and discharging port of the board are independent of each other.
BMS can be simply understood as a switch, but this switch is connected in series to the negative pole of the battery. The same port plate shares the output negative pole for charging and discharging, while the port plate separates the charging and discharging negative poles. The positive input and output of the battery are output from the total positive pole of the battery pack. The BMS equalizing wire is used to monitor the voltage of each string of batteries, and then the main control chip controls MOS to open and close according to the voltage of the balance wire, so as to achieve the main overcharge and overdischarge protection functions of the lithium battery pack, as well as other functions such as equalizing charging, current limiting, temperature control, etc.
The main functional components and input/output interfaces of the protection board are described as follows (taking 4 series as an example, the same for multiple strings)
Of course, the boards of different manufacturers may be slightly different. For example, in order to improve the monitoring and management of the entire battery pack, various management and control interfaces will be added
BMS has a master management chip unit, which is the most important component. At present, the mainstream lithium battery management chips have single chip management of 1, 2,... 20, and of course, the price ranges from a few cents to dozens of yuan.
What is MOS?
The full Chinese name is metal oxide semiconductor field-effect transistor. On the protection board, MOS acts as a switch. You can understand that MOS is an air switch,
but this air switch does not need to be broken by hand. It is opened or closed by the control management IC master chip+peripheral circuit.
In the application of MOS in the protection board, I think the two most critical indicators are voltage withstand and conduction impedance R (DSon).
The former represents the voltage that can withstand the entire battery pack and the charger voltage For the above two points,
we should choose MOS with appropriate withstand voltage and the lower the impedance, the better, because MOS with high voltage usually has large impedance.
What is balance?
The purpose of balance is to make each series battery cell fully charged, but what causes lithium batteries to be balanced is consistency, such as different internal resistances and self discharges
The balance is divided into active and passive modes
Passive balance: discharge the high voltage series, and cycle it. Each string of battery voltage can reach the expected voltage (that is, it is basically fully charged). Passive balance is a dissipative balance, which dissipates electricity through heat. Therefore, there is a problem of BMS heating. This problem also leads to why BMS balance can't be high.
Active balance: tank the power of high voltage series cell to low series cell. As time goes by, everyone has the same voltage.
Active balance is also called lossless balance, because electricity is transferred internally, and heat generation is just a matter of conversion efficiency. At present, the majority of electric vehicles are passive balances. Why? Low cost and high reliability.
Many people like to use a BMS with a large balance current. In fact, the passive balance scheme is a problem of selecting the resistance value in the design of the protection board.
But how does the BMS dissipate heat when it is wrapped in the battery pack? For example, according to the Ohm theorem, the heat value of each equalizing resistor is I * I * R=0.1 * 0.1 * 35=0.35W for a 100mabalance current BMS.
In the worst case of a 24series BMS, 24 balance lights (or 23 lights) are turned on at the same time, and the total power is 8W.
You can calculate the temperature of it.. Therefore, it is also important to note that there should be some isolation between the BMS and the battery to prevent the direct transfer of heat to the battery from causing various problems.