Turning passive into active, accurate and robust battery management system is like this

Through passive and active cell balancing, each cell in the battery pack can be effectively monitored and maintained at a healthy state of charge (SoC), which not only increases the battery cycle life, but also provides additional protection against damage to the battery cells due to overcharge/deep discharge.

Figure 1 shows a typical battery pack at full capacity. In this example, full capacity refers to 90% charge, because keeping the battery at (or close to) 100% capacity for a long time will quickly reduce its service life. Fully discharged refers to discharging to 30%, which prevents the battery from entering a deep discharge state.

Figure 1. Full capacity.

Over time, some cells will have worse characteristics than others, resulting in a battery pack discharge characteristic as shown in Figure 2.

Figure 2. Mismatched discharge.

As you can see, even though some cells still have a lot of energy left, weak cells limit the system's operating time. A 5% mismatch in battery capacity will result in 5% of the energy not being used. For large-capacity batteries, this means a lot of energy is wasted, which is particularly critical for remote systems and systems that are difficult to maintain. Some of the energy is not used, which also leads to an increase in the number of battery charge and discharge cycles, reducing battery life and incurring higher costs due to frequent battery replacement.

Through active balancing, charge is redistributed from strong cells to weak cells, which can completely deplete the energy in the battery pack.

Figure 3. Complete energy depletion through active balancing.

If the battery pack is charged without balancing, the weak cells will reach full capacity before the stronger cells. Once again, the weak cells become the limiting factor; this time, they limit the total energy that can be contained in the system. Figure 4 demonstrates this limitation when charging.

Figure 4. Charging without equalization

Active balancing can achieve full capacity of the battery pack by redistributing the charge during charging. It should be noted that although this article does not discuss the time ratio required for balancing and the impact of balancing current on time, they still need to be considered.

ADI has a series of active battery balancing controllers, each targeting different system requirements. The LT8584 is a monolithic flyback converter with 2.5A discharge current, used in conjunction with the LTC680x series of multi-compound battery cell monitors; it can redistribute charge from one battery cell to the entire battery pack, or to another battery cell or a combination of several battery cells in the battery pack. Each battery cell needs to be equipped with an LT8584.

Figure 5. 12-cell battery pack module with active balancing

The LTC3300 is a standalone bidirectional flyback controller for lithium and LiFePO4 batteries that can provide up to 10A of balancing current. Because the control is bidirectional, the charge in any battery cell can be efficiently transferred back and forth with 12 or more series-connected battery cells. A single LTC3300 can balance up to six battery cells.

Figure 6. Efficient two-way balancing.

The LTC3305 is a standalone lead-acid battery balancing controller that can balance up to four cells simultaneously. It does this by continuously connecting the fifth storage cell (Aux) in parallel with each of the other cells (one at a time) to eventually balance all of the cells (lead-acid batteries are very durable, so this approach can be taken).

Figure 7. Four-cell battery-balancing controller with programmable high- and low-voltage cell fault thresholds.

Active and passive battery balancing can effectively promote the health of the battery system by monitoring and matching the SoC of each battery cell. Unlike passive battery balancing, which only consumes excess charge during charging, active battery balancing can redistribute the charge during charging and discharging. Therefore, active battery balancing can extend the system operation time and improve charging efficiency. Active battery balancing often requires more complex and larger solutions, while passive battery balancing is more cost-effective.

Whichever method better meets your application needs, ADI can provide the corresponding solution and provide you with accurate and robust battery management system by cooperating with our battery management ICs (such as LTC6803 and LTC6804) and other peripheral devices.

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