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How Does Active And Passive BMS Work?

Jun 20, 2018

In the article of power system design "active and passive balancing of battery management systems", Stefano Zanella described how a multi-battery system loses balance. Here, I would like to explore how the battery will become unusable if the battery is unbalanced and slightly enlarged to affect the battery capacity mismatch. I will focus on automotive Li-ion batteries, generally these principles apply to all batteries.

Multi-cell batteries are usually built as series or parallel battery arrays. Too series many cells will result in higher cell voltages, while too many parallel cells will result in higher capacity (expressed as ampere hour rating or Ahrs). The battery capacity will then indicate the number of parallel batteries, multiplying the battery capacity equal to the number of parallel batteries by the battery capacity needed for system operation. According to the battery type, cars tend to use 96 series lithium-ion batteries and 24 parallel batteries. For example, an electric vehicle traveling a 100-mile range would require 20-30 kWh of battery, depending on the weight of the vehicle, the intended mode of use, and the various system efficiencies in the vehicle. Several aspects of the system will determine the battery pack voltage, including the overall size and type of motor, cable size, and isolation requirements.

The multi-cell battery is charged by supplying current to the positive terminal of the battery at the top of the stack. (Assume that the battery includes n series cells). In other words, the battery unit is not charged separately. If you read Stefano's article, you will understand that at the end of charging, the amount of electricity remaining in each cell is different; and when you repeatedly charge and discharge the battery (if there is no balance), this difference increases. .

If you associate the two batteries in FIG. 1 into the same charging container, driving the electric vehicle will result in the extraction of energy from the battery, which will deplete the containers. The charging of the electric vehicle injects a charge into the battery to fill those containers. Not all cells are identical to each other, and they are not uniform; therefore, weaker cells will charge and discharge at slightly different rates. The voltage level of each battery will slowly rise and fall as the battery charges and discharges, respectively.

Let's start with a full battery. All the energy (usable energy) contained in the battery can power the car. In order not to over-discharge the battery (because over-discharging will reduce battery life and may affect safety), the discharge must be stopped when the first battery reaches the under-voltage threshold (plus usually depends on the safety margin of the protector). In order not to overcharge the lithium-ion battery, when the first battery reaches the over-voltage threshold, it must stop charging. However, a lagging battery is not yet fully charged, leaving some power in the battery that cannot be used for driving, because when the first battery is full, it must stop charging again.

In other words, some energy is retained in the battery pack after the first charge/discharge cycle. It can never be used to power a car.

As the battery is repeatedly charged and discharged, the amount of electricity that remains is increased, thereby reducing the available power. In addition, the loss of available power is twice the amount of retained power because the retained power is not available and the equivalent power cannot be injected into another battery.

After sufficient charge and discharge cycles are performed, the available energy begins to approach zero. How do you avoid this problem? Balance! You can balance the battery by dissipating excess power to the resistor to regain the ability to fully charge and fully charge the battery.

As long as all batteries have the same capacity, they do not need to be completely balanced at the end of each charging cycle - because the effect of charge imbalance is completely reversible. I have observed a case during development of battery electronics where the passive balance of the battery was achieved after many charge/discharge cycles. When the balance system is ready, the available power drops by more than 25%. However, after balancing all the batteries, the battery pack is fully charged with minimal loss of available energy.

You should choose the amount of balance current based on application and thermal considerations. For example, in a 24kWh system (96 cells in series), the 66Ah system would need to compensate for 660mAh assuming that the battery has a difference in charge time of less than 1% at the end of its life (the difference in charge time increases over time). With a 200mA balance, you can balance the system in 3.3 hours, but it takes twice as long to balance 100mA.