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Why do lithium-ion batteries need BMS?

With the development of the global new energy industry, lithium batteries, as part of new energy, have become a new hot spot for manufacturing investment. Lithium batteries are great, but need protection. To ensure the safety of lithium iron phosphate batteries, the BMS of lithium-ion batteries seems to be a top priority.

 

BMS features

1、Basic protection: overcharge protection, overdischarge protection.

2、Enhanced protection: overcurrent protection, high temperature protection, low temperature protection, short circuit protection, reverse connection protection.

3、Expansion requirements: good consistency, pressure difference and temperature difference is small.

What are the consequences of overcharge and over-discharge of lithium-ion batteries?

1、Overcharge: There will be a large amount of gas in the battery, resulting in a rapid increase in internal pressure, resulting in a battery explosion.

2、Over-discharge: Shorten the battery life, directly damage the battery scrap.​

Depending on the environment and application, lithium battery BMS requires more additional features to meet your needs.

  • Communication function – to help you understand the operating status information of battery packs, RS232/RS485/CAN, etc.
  • SOC (State of Charge) Calculation/SOH Estimation.
  • Alarm function – Notifies you when there is a problem with the battery.
  • Recording function – record the history of the battery pack, including alarm history data.
  • Troubleshooting function – Diagnostics can be performed in the event of a problem with the BMS.
  • Display function – you can read information and set parameters directly from the display.
  • Equalization function.

BMS1

 

Simple BMS

Hardware BMS; Software BMS (communication BMS, start-up power BMS, etc.)Hardware BMS: Use a dedicated lithium-ion battery to protect the chip. When the battery voltage reaches the upper or lower limit value, the switching device MOS tube will cut off the charging or discharging circuit. to protect the battery pack.

Simple BMS functionality

1、Only overcharge and over-discharge protection. Some are protected against overcurrent and reverse connection. But there are no other additional features.

2、Protection thresholds cannot be changed. (Typical protection points are 3.9V and 2.0V)

3、The balancing threshold cannot be changed. (Generally, the equilibrium current is less than 150mA)

Considerations for BMS usage 

  •  Not arbitrarily connected in series: The switching devices of the BMS use MOS tubes. The price of a MOS tube is proportional to its pressure resistance. When designing a BMS, the withstand voltage of the MOS tube will generally be higher than the battery pack voltage, but not too much.
  • For example, four-string BMS, the maximum voltage of the battery pack will not exceed 16V, generally will choose 20V or 25V MOS tube.
  • If four BMS in series are connected in series, when the MOS tube is disconnected, the voltage of the entire battery pack will be applied to the MOS tube, causing the MOS tube to overvoltage breakdown damage.

 

The BMS balancing function is also important.

Why the balance function is needed: Due to the limitations of the production process, it is impossible to achieve completely consistent voltage and internal resistance of each cell. When used in series, the cells with high internal resistance are discharged first and then fully charged. Over time, the difference in capacity and voltage of each cell becomes obvious.Balancing function: Keep the voltage of each cell consistent, maximize the discharge capacity, and extend the service life of the battery pack.Currently, there are generally two balanced approaches. One is energy consumption, and the other is energy transfer.Multiple cells are connected in parallel to form a module, and multiple modules are connected in series to form a battery. Each module has an independent parallel balancing circuit.Lithium-ion battery protection ICs measure the voltage of each battery (module). When the battery voltage is lower than the balanced start-up voltage, the balanced circuit does not start.

 

Smart BMS

1、For high-voltage, large-capacity battery packs, the BMS alone cannot monitor and manage thebattery packs.

2、For electric vehicles, it is not enough to just manage their own battery pack, they also need to communicate with the vehicle controller and the charger.

3、Smart BMS is the realization of self-protection of battery energy packs while combining with chargers and on-board controllers. If it is a high-voltage and high-current system (such as an energy storage system, an electric vehicle system), or a simple BMS cannot implement the system, the smart BMS can generally be solved.

BMS

BMS has a variety of functions such as overcharge protection, overdischarge protection, balancing function, and overcurrent protection. and short circuit protection. Each function is very important for lithium-ion batteries (LiFePO4 batteries) and plays a key role in the performance of the battery. The quality of the BMS also directly determines the quality of the battery.

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