Lithium ion battery BMS balance function
From the previous article, we know the basics of lithium-ion battery BMS. In this article, we will mainly explain the BMS balance function.
- Why we need a balance function: Due to the limitations of the production process, it is impossible to achieve complete consistency in the voltage and internal resistance of each cell. When used in series, the cell with high internal resistance is first discharged and then fully charged. For a long time, the difference between the capacity and voltage of each cell becomes obvious.
The balance function: to keep each cell voltage consistent, maximize the discharge capacity, and extend the service life of the battery pack.
At present, there are two general types of balance methods. One is energy consumption, and the other is energy transfer.
So what is the principle of BMS balance?
- Multiple cells are connected in parallel to form a module (Module), and multiple modules are connected in series to form a battery pack (Battery). Each module has an independent parallel balance circuit.
- The lithium ion battery protection IC measures the voltage of each battery (module). When the battery voltage is lower than the balanced starting voltage, the balance circuit does not start.
The IC starts the balance, closes the switch, and discharges the battery through the balance resistance. Then the rising speed of the battery voltage is relatively slower than others. Can achieve the purpose of battery balance.
Balanced current: I=U/R.
U: the battery voltage.
R: balance resistance, generally taking the resistance between 20 Ω and 100 Ω.
It is determined by the balance ability.
principle of BMS balance
During the charging process, the battery voltage increases gradually. When any battery voltage reaches the balanced starting voltage of 3.6V, the balance circuit of this series of batteries is open.
I1=U1/R. I2=U2/R.
Because U1 > U2, so I1 > I2
During the charging process, the battery voltage increases gradually. When any battery voltage reaches the balanced starting voltage of 3.6V, the balance circuit of this series of batteries is open.
I1=U1/R. I2=U2/R. I3=U3/R.
As U1 > U2 > U3, then I1 > I2 > I3. Even if each battery with the balance turned on, the balance current is different. It still plays a balanced role.
Charging balance is the charging of balanced battery characteristics. When using the battery, the voltage is unbalanced for individual differences, temperature differences, and other reasons. To avoid this imbalance trend, it is necessary to improve the charging voltage of the battery pack. Activate and charge the battery to achieve the maintenance method of balancing the characteristics of each battery. and prolonging the battery life.
Smart BMS
For a battery pack with high voltage and large capacity, it is impossible to monitor and manage the battery pack with a simple BMS.
For electric vehicles, it is not enough to manage their battery pack, but also needs to communicate with the vehicle controller, need to communicate with the charger.
The smart BMS is to realize the self-protection of the battery energy package and to combine with charger and vehicle controller at the same time.
If it is a high-voltage and high-current system (such as an energy storage system, electric vehicle system), or the simple BMS can not achieve the system, the smart BMS can generally be solved.
The BMS has many functions, such as overcharge protection, over-discharge protection, balance function, and overcurrent protection. and short-circuit protection. Each function is very important for a lithium-ion battery(LiFePO4 battery) and plays a key role in the performance of the battery. BMS quality also directly determines the battery quality.
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