Passive balancing VS active balancing of battery 2022-06-23

It's common and liable to occur with cell imbalance in every battery system. If lithium batteries are overheated or overcharged, they can easily accelerate battery degradation. Even worse, they can catch fire or even explode from a thermal runaway condition. Almost every battery pack that OSN designs and manufactures has a built-in BMS with overvoltage protection and along with cell balancing to prevent such events from happening.

Just as there are no two identical leaves in the world, no two cells are exactly the same. There are always subtle differences in state of charge, self-discharge rate, capacity, impedance and temperature characteristics. This is the case even if the batteries are the same model, same manufacturer or even the same production batch. While OSN produces battery packs, the cells will be matched into groups as high consistency as possible in similar voltage, capacity and internal resistance etc., but there are still slight differences in the impedance, capacity and self-discharge rate of individual cells. If without balancing, they may eventually result in a large difference in voltage over time and hence decreases battery capacity.

Generally speaking, a "weak" cell in some sense will limit the performance of the battery pack and eventually render the pack unusable unless the cells are balanced. Without balancing, discharge stops when the "weak" cell with the lowest capacity is depleted, even though there may be considerable capacity left in the other cells, but the weak cell limits the energy delivery capability of the battery pack and the runtime of the system.

In applications with series cells, the impact of cell imbalance on battery runtime, performance and battery life is certainly undesirable. To avoid this, cells should be balanced frequently so that the differences between cells are as small as possible.

The basic methods of balancing batteries can be divided into active battery balancing and passive battery balancing: Passive balancing drains charge from a battery with excess charge and dissipates the drained energy as heat. Active balancing, on the other hand, transfers charge from a "high battery" to a "low battery" in an attempt to preserve as much energy as possible in the battery pack.

Passive balancing

The passive balancing method is relatively simple and straightforward. The battery is discharged through a dissipative bypass route. This bypass can be integrated externally or externally to the integrated circuit (IC). Its cost-effective. Thus this approach is beneficial for low-cost system applications. However, 100% of the excess energy from higher energy batteries is dissipated as heat, which makes passive balancing method less desirable during discharge as it has a noticeable effect on battery runtime. This technique limits balancing to high SOC platform and can only be performed during charging.

Passive balancing currents are relatively small, so multiple cycles may be required to equalize a battery pack. Usually the BMS comes with 200mA~500mA balancing current. If the current is too large, the heat generated may be too high. The battery pack is prone to overheating.

Active balancing

Active balancing, which utilizes capacitive or inductive charge shuttling to deliver energy where it is most needed with minimal loss. It is significantly more efficient cause energy is diverted to where it is needed, rather than being consumed. Of course, the trade-off for this improved efficiency is the need for additional components at higher cost.

Therefore, it is more preferable for efficiency-conscious designs and applications where providing maximum runtime is a priority.

Active balancing can happen during any battery operation - charging, discharging or rest. Compared to passive cell balancing, almost no energy is lost as heat. It extends battery life by maximizing the capacity of the battery pack, ensuring that all its energy is available.

OSN has a family of active cell balancers, with each device targeting different system requirements. Here we highly recommend our below Battery Active-Balancer 2 ~ 24 series 1A or 2A, and the max up to 5A, which uses ultracapacitors as the medium to balance the active energy transfer. Applicable to Li-ion, Lipo, Lifepo4, LTO and other battery on the market. The balancer is equipped with bluetooth communication function and supports mobile APP software. It can be connected to the phone via bluetooth to check the individual battery voltage, balance state, modify parameters and other operations.

In passive balancing, the practical goal is to achieve capacity balance at the end of charge; however, due to the low balancing current, if the battery pack itself is very unbalanced, it is almost impossible to correct the voltage imbalance at the end of discharge. In other words, passive balancing avoids overcharging weak cells, but may not improve the battery's runtime because the extra energy is wasted in shunt resistors as heat. With OSN active balancing, the two goals—achieving voltage balance at the end of charge, and minimizing voltage differences among cells at the end of discharge—can potentially be achieved at the same time. Energy is conserved and transferred to the weaker battery, which increases the discharge capacity.

In summary

Cell balancing is not only important for improving the performance and life cycle of the battery, it also improve safety to the battery. Both active and passive cell balancing are effective ways to improve system health by monitoring and matching each cell's SoC. Unlike passive cell balancing, which redistributes charge during charge and discharge cycles, passive cell balancing simply dissipates charge during charge cycles. Therefore, active cell balancing increases system runtime and can improve charging efficiency. Active balancing requires a more complex, larger footprint solution; passive balancing is more cost-effective. You can choose the most suitable solution according to your application.

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