**Editor's Note**
A large battery pack consists of lithium polymer or lithium iron phosphate (LiFePO4) battery cells connected in series. These packs offer high energy density and peak power, making them widely used in pure electric vehicles (EV or BEV), hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (PHEV), and energy storage systems (ESS) across various applications.
For EVs and PHEVs, the battery performance directly translates to the range the vehicle can travel on a single charge. To stay competitive, manufacturers must not only deliver top-tier battery performance but also provide long-term warranties that guarantee a minimum driving range. As the number of electric vehicles grows and driving times increase, the aging of battery cells within the pack has become a major concern.
Large battery packs made of lithium polymer or LiFePO4 cells are commonly used in EVs, HEVs, PHEVs, and ESS. The electric vehicle market is expected to see a significant rise in demand for large series/parallel battery cell arrays.
In 2016, global PHEV sales reached 775,000 units, with an expected increase to 1.13 million in 2017. Despite growing demand for high-capacity batteries, their prices remain very high, often accounting for the most expensive component in EVs or PHEVs. A battery that supports hundreds of kilometers of driving typically costs around $10,000.
To reduce costs, some manufacturers use low-cost or refurbished battery cells. However, these may suffer from greater capacity mismatch, which reduces the available driving range after a full charge. Even higher-quality batteries will eventually age and lose balance over time.
There are two main approaches to increasing the capacity of a mismatched battery pack: either start with larger, more expensive batteries or implement active equalization technology. Active equalization is a modern solution that can restore battery capacity and enhance performance quickly.
**Fully connected battery cells need to be balanced**
When all battery cells in a pack have the same state of charge (SoC), the pack is considered balanced. SoC refers to the percentage of the battery’s current remaining capacity relative to its maximum capacity. For example, if a 10-amp-hour battery has 5 amps left, its SoC is 50%. Battery cells must operate within a specific SoC range to avoid damage and extend lifespan. The allowable SoC range varies by application—some require a range between 20% and 100%, while others limit it to 30% to 70%.
The primary function of a Battery Management System (BMS) is to monitor all cells and ensure they do not exceed these limits. In series/parallel battery configurations, parallel-connected cells tend to self-equalize over time due to conductive paths between terminals. However, series-connected cells may develop SoC differences due to factors like temperature variations, impedance, self-discharge rates, or uneven load distribution.
Although charging and discharging currents help reduce these differences, cumulative mismatches can still occur unless periodic equalization is performed. Passive equalization is a common method, but it is slow and generates heat. It also reduces the overall capacity of the pack to match the lowest cell.
Active equalization, on the other hand, redistributes charge between cells, helping to recover lost capacity. This approach allows for better performance and longer battery life.
**Mismatch between battery cells can significantly reduce run time**
Capacity or SoC mismatch between cells can drastically reduce the usable capacity of a battery pack unless equalized. To maximize performance, cells must be balanced during both charging and discharging.
In an example of a 10-cell series battery pack, if the SoC is limited to 30–70%, and no equalization is performed, the available capacity could drop by 25% after a full cycle. Passive equalization helps during charging, but not during discharge, leading to lost capacity. Only active equalization can redistribute charge and recover lost capacity.
**High efficiency, two-way equalization for maximum capacity recovery**
The LTC3300-2 is a new active equalization IC designed for high-performance BMS systems. It offers bidirectional, efficient equalization for up to six lithium-ion or LiFePO4 cells. Each IC can redistribute charge between selected cells and adjacent groups, improving capacity and extending battery life.
The LTC3300-2 uses a boundary mode synchronous flyback power stage for efficient charging and discharging. It supports up to 10 A of current and achieves over 90% efficiency in both directions. This makes it ideal for high-current battery systems where heat management is critical.
**Conclusion**
New applications like EVs, PHEVs, and ESS are growing rapidly. Consumers expect long-lasting performance, reliability, and no loss of power. Whether using batteries or gasoline, vehicles must operate reliably for over five years without significant degradation.
For EVs and PHEVs, battery performance equals the driving range supported by the battery. Manufacturers must not only deliver strong performance but also ensure long-term warranty coverage. As battery aging becomes a growing issue, active equalization solutions like the LTC3300-2 can help maintain consistent performance and prevent costly replacements.
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