Analysis of the concept and method of equalizing charge of electric vehicles

Let's take you to understand the equalizing charge of the battery pack, and under what circumstances we need to equalizing charge.

1. The meaning of equalizing charge

In short, it is a charging method that balances the characteristics of the batteries, and the power battery pack used in the car is not a large single battery, but a battery pack composed of many cells, a number of modules, and there is already a structure that skips the module and groups the cells directly (blade batteries).

Some vehicles that use lead acid battery or lifepo4 battery will have hundreds or even thousands of batteries in the battery pack. These batteries will be combined in series, increasing the voltage in series and expanding capacity in parallel. Then in the process of use, it is impossible to ensure that each battery or module does not have differences in voltage, capacity and temperature, which will cause imbalance or imbalance trend.

2. The principle of equalizing charge

The principle is simple. After the battery pack is charged, even if the power is shown to be off, it is actually trickle charging (the current is very small). In the process, you can find cells with differences, activate and charge the batteries, and the charging voltage will be increased.

After a slow equalizing charge, the battery cells can basically reach the equilibrium state, and then there will be no more jumping problems, and the cruising range can also be seen to increase significantly. In fact, after the battery is charged at a constant voltage, the control unit will detect the battery pack until the charging current lasts for three hours without fluctuation, and the equalizing charge is considered complete.

3. Why do EV need equalizing charge

In the use of the batteries, the customer may make the battery undercharged after discharging, not charging in time after discharging, etc. As a result, the active material of the battery cannot be restored in time, and the phenomenon of plate passivation will occur. This affects the capacity and life of the traction battery.

In addition, when batteries are used in groups, there will be differences between cells. In long-term use, the difference will become larger and larger, forming an imbalance between the battery cells and reducing the performance of the entire battery pack.

Regular equalizing charge can effectively prevent and reduce the probability of the above problems in the batteries. With cell balancing, each unit relying on the battery hookup in the battery pack is effectively monitored and maintains a healthy state of charge. This not only increases the number of battery cycles, but also provides additional protection against damage to the battery cell due to overcharge/deep discharge.

① What happens if there is an imbalance

Assuming that the capacity of some cells or modules is severely reduced, the voltage will naturally decrease. When charging, these batteries will be fully charged first, and then they will stop charging, but the other batteries are not fully charged at this time.

However, it is also necessary to stop charging, the battery pack is always not fully charged, and the cruising range will of course be affected. There may be power skipping during the use of the car, and the problem with the battery pack is very serious, and equalizing charge is required at this time.

② When equalizing charge is needed

During the use of car battery, if the batteries have one of the following situations, it should be charged:

• Under normal use, the battery should be charged evenly every two months.
• The condition that the battery is charged after three consecutive uses, need equalizing charge.
• When batteries are not used for more than two months, it should do equalizing charge.
• When the pressure difference between cells is large.
• When batteries are discharged and used for twenty hours before replenishing batteries.

4. Method of equalizing charge

• First charge the batteries normally, wait for the charging to complete, and let it stand for an hour.
• Continue charging the batteries using the current from the second stage of normal charging until strong bubbles are generated, stop charging and leave it again for another hour.
• Repeat the above steps until car battery voltage and density remain the same.

The equalization of the charging method of the car can be judged by the charging indicator. Finally, it is only necessary to understand that a balanced charge will always take a long time.

At this point, the concept and operation mode of equalizing charge should be understood, and it is recommended that vehicles that have been using fast charging regularly balance their own battery packs. The owner of the car will hang up the charging gun when he has time, and developing this habit will greatly increase the service life of the battery pack and ensure the cruising range of the vehicle.

5. Precautions for equalizing charging

● Maintain ventilation

Battery charging should be carried out in a cool, ventilated environment. Pay attention to changes in the temperature of the battery surface and electrolyte to prevent damage to the batteries caused by excessive temperature.

● Keep away from fire

A small amount of hydrogen is precipitated during charging. After the density of hydrogen reaches a certain level, in the case of a fire source, an explosion will occur, so batteries should be ventilated during the charging process and kept away from the fire source.

6. Active equalization and passive equalization

● Passive equalization

The excess charge is dissipated by the bleed resistor so that all cells have approximately equivalent SOCs. However, it does not extend the operating time of the system, and equilibrium using energy dissipated by resistance is usually called passive balancing.

● Active balancing

This is a more complex balancing technique. Since the charge inside the cell is redistributed during the charge and discharge cycle, the total available charge in the battery pack is also increased, extending the system run time.

● Active batteries balanced discharge

Keeping batteries at (or close to) 100% capacity for a long time will make its lifespan decrease quickly. Full discharge refers to discharge to 30%, which prevents batteries from entering a deep discharge state.

Over time, the characteristics of some batteries will become worse than others. Even though some battery cells still have a lot of capacity left, weak battery cells limit system uptime.

For large-capacity batteries, this means a lot of energy is wasted. Some of the energy is not used, which also leads to an increase in the number of battery charge and discharge cycles, reducing the life of the batteries and incurring higher costs due to frequent battery replacement. With active equalization, the charge is redistributed from the strong cells to the weak cells, and the energy in the battery pack can be completely depleted.

● Active batteries equalizing charge

If the battery pack is not equalized, the weak battery cell will reach full capacity before the strong battery cell. Active balancing enables the battery pack to reach full capacity by redistributing charge during charging.

● Comparison of the two equalization modes

By monitoring and matching the state of charge of each battery cell, it can effectively promote the health of the battery system. Unlike passive cell balancing, which only dissipates excess charge during charging, active cell balancing enables charge to be redistributed during charge and discharge.

As a result, active battery balancing can extend system runtime and improve charging efficiency, requiring more complex and larger solutions, while passive cell balancing is more cost-effective.

7. Conclusion

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