In addition to ensuring that the battery pack is not overcharged or over-discharged, the battery management system BMS must have a balancing function to maintain the consistency of the battery pack. Currently, almost all BMS on the market have a balancing function, and balancing is mainly divided into passive balancing and active balancing.
1. What is a battery balancer?
A battery balancer, also known as a battery balancer, is an electronic protection device that prevents the voltage imbalance between the battery cells inside the battery from causing a decrease in service life. When the voltage between the battery cells is unbalanced, different battery cells charge and discharge at different speeds. While one battery cell is fully charged, other battery cells are in an overcharged or undercharged state. Overcharging or undercharging will damage the battery, causing the battery capacity to decrease and the life to be shortened. The battery balancer can ensure the balance of power between the battery cells in real time, thereby extending the battery life by 5-30%.
1.1 Main factors affecting the voltage difference
1. Cell capacity: We use a capacity divider to screen the cell capacity, and try to keep the capacity consistent as much as possible, so that the static voltage difference can be reduced; specifically, compared with two batteries with very similar internal resistances connected in series and parallel, a 20% difference in battery internal resistance between two batteries in series and parallel cycles may shorten the cycle life by about 40%.
2. Inconsistent cell internal resistance: The main impact is the dynamic voltage difference, and we need to use an internal resistance meter to screen before assembly;
3. Inconsistent connection line internal resistance: The main impact is also the dynamic voltage difference, which can be solved by adjusting the connection resistance of the protection board.
4. Simple voltage inconsistency: Starting assembly without matching the voltage will also cause a voltage difference. If the cell is slightly worse, there will be uneven self-discharge, which will also cause a part of the voltage difference
The pressure difference caused by the first three types cannot be solved by balancing. Only the last type is simply due to voltage inconsistency, which can be solved by balancing. Because the voltage difference involves many factors, it is impossible to guarantee that the performance of the battery cells is completely consistent. The smaller the voltage difference, the better. However, as long as it is within the normal range, you don't have to worry too much about it. For battery systems with a voltage difference of ≥50 mV at the end of charging, balanced charging is required, and the qualified standard for balanced charging is a voltage difference of ≤30 mV at the end of charging.
If your full-charge static voltage difference is within 60 millivolts, as long as the automatic equalization function is turned on, it will drop quickly. If it is a small-capacity battery of 40 to 50 ampere hours, it can generally drop directly in one night; for large-capacity batteries such as 120 ampere hours, it may take two or three nights to drop.
2. Two main methods of battery cell charge balance: passive balance and active balance.
2.1 Active balance (lossless balance)
Active balance is balanced in the form of energy transfer, transferring the high-energy single cell to the low-energy single cell battery, thereby achieving voltage balance of the entire group, and almost no energy loss is involved in the transfer process.
Active balancing conditions: Regardless of whether the battery is charging, discharging, or static, as long as the voltage difference is greater than the set value, balancing will begin. Therefore, as long as there is a voltage difference, the active balancing time should be 24 hours a day until the voltage difference is less than the set range.
Active balancing current: Since active balancing is a way of energy transfer and does not generate heat, all balancing currents can be large without affecting heat dissipation. Generally, active balancing currents of 1~2A are common.
Since active balancing is not limited by charging time, the balancing time is long, and the balancing current is large, so it is more suitable for use in large-capacity battery packs.
2.2 Passive balancing
Passive balancing generally discharges batteries with higher voltages through resistor discharge, releases electricity in the form of heat, achieves voltage balancing of the entire group, and gains more charging time for other batteries.
Passive balancing conditions: Passive balancing can generally start discharge balancing when the battery is close to full voltage, so the starting balancing time of passive balancing is relatively short, from the battery being close to full to the end of full, it is generally a few hours of balancing time depending on the charger.
Passive balancing current: Since passive balancing is a resistance energy-consuming balancing method, the energy consumed is released as heat. Therefore, the BMS will heat up during the balancing process, which leads to the fact that the balancing current cannot be set too large, otherwise it will cause the battery pack temperature to be too high. Generally, the passive balancing current ranges from 35mA to 200mA. The larger the balancing current, the more serious the heating.
If the balancing current is small, the efficiency of the power balancing function in a large-capacity battery pack with a large power difference is very low. It takes a long time to achieve balance, which is like scratching an itch in the application. The passive balancing circuit is simple and low-cost, and is suitable for battery packs with lower capacity.
Note: Passive balancing of a series of batteries can only be used for lead-acid and nickel-based batteries. The active balancing method does not depend on the chemical properties of the battery and can be used for most types of modern batteries.
3. The role of the battery equalizer
1. Extend the service life
2. Reduce battery replacement
3. Maintain charging balance
4. Save extra expenses
5. Insulation components
6. Sealed design
4. Common problems with equalizers: The balancing switch is turned on, but it is not balanced?
The trigger voltage difference value is set to 4V, which is incorrect. It should be changed to 0.003V or above. Or use the system default trigger value of 0.01V to restore the balancing function.
Special reminder:
There are several conditions for starting balancing, and they must be met to balance:
1. The actual number of strings is equal to the set number of strings, and there is no prompt "The number of monomers does not match the setting"
2. There is no warning of "balance line resistance is too large"
3. The actual voltage difference must be greater than the "trigger balancing voltage difference" in the parameters