Technology | Understanding the Dangers of Lithium Battery Inconsistencies and How to Address Them

Inconsistencies in battery cell performance develop during the production process and worsen with use. Within the same battery pack, weaker cells become weaker, and their degradation accelerates. The dispersion of parameters between individual cells increases with aging.

Power lithium batteries have firmly established themselves as the leading power source for electric vehicles. They offer long service life, high energy density, and significant potential for improvement. Safety can be improved, and energy density can continue to rise. In the foreseeable future (rumored to be around 2020), they will rival the range and cost-effectiveness of fuel-powered vehicles, ushering in the first stage of electric vehicle maturity. However, lithium batteries also have their challenges.

Why are most lithium batteries small?

The lithium batteries we see—cylindrical, pouch, and prismatic—are generally elegant in appearance, unlike the bulky, traditional lead-acid batteries. Why is this?

Because of their high energy density, lithium batteries are often not designed with large capacities. Lead-acid batteries have an energy density of around 40 Wh/kg, while lithium batteries exceed 150 Wh/kg. As energy concentration increases, safety requirements rise.

First, a single lithium battery with excessively high energy capacity can, in the event of an accident, trigger thermal runaway. This rapid reaction within the battery can quickly dissipate excessive energy, creating a dangerous situation. Especially when safety technology and control capabilities are not yet fully developed, the capacity of each battery should be limited.

Second, the energy trapped within the lithium battery casing is inaccessible to firefighters and fire extinguishers in the event of an accident. Firefighters are unable to reach the scene and are unable to extinguish the fire. They can only isolate the scene and allow the battery to react on its own until the energy is exhausted.

Of course, for safety reasons, current lithium batteries have multiple safety features designed in. Take cylindrical batteries as an example.

Safety valves: When the internal reaction of the battery exceeds the normal range, the temperature rises, and side reaction gases are generated. When the pressure reaches the designed value, the safety valve automatically opens to release the pressure. The moment the safety valve opens, the battery fails completely.

Thermistors: Some battery cells are equipped with thermistors. In the event of an overcurrent, the resistance increases sharply after reaching a certain temperature, reducing the current in the circuit and preventing further temperature rise. Fuse: The battery cell is equipped with a fuse with overcurrent melting function. Once an overcurrent risk occurs, the circuit is disconnected to avoid serious accidents.

Lithium Battery Consistency Issues

Lithium batteries cannot be made into large units, so they must be organized into numerous small cells. Working together and working in unison can also propel electric vehicles to great heights. This presents a challenge: consistency.

In our daily lives, if we connect the positive and negative terminals of two dry cell batteries, a flashlight will light up, and who cares if they're inconsistent? However, the large-scale application of lithium batteries presents a much more complex situation.

Lithium battery parameter inconsistencies primarily refer to inconsistencies in capacity, internal resistance, and open-circuit voltage. Using inconsistent cells in series can lead to the following problems:

1) Capacity loss. Cells form a battery pack, and their capacity follows the "barrel principle": the capacity of the weakest cell determines the capacity of the entire pack.

To prevent overcharging and over-discharging, the battery management system's logic is configured as follows: During discharge, when the lowest cell voltage reaches the discharge cutoff voltage, the entire pack stops discharging; during charging, when the highest cell voltage reaches the charge cutoff voltage, charging stops.

For example, consider two batteries connected in series. One has a capacity of 1C, while the other has a capacity of only 0.9C. In a series connection, the same current flows through both batteries.

During charging, the smaller battery will inevitably be fully charged first, reaching the charge cutoff condition, and the system will stop charging. During discharging, the smaller battery will inevitably use up all its available energy first, and the system will immediately stop discharging.

This way, the smaller cell is always fully charged and discharged, while the larger cell is always using a portion of its capacity. A portion of the battery pack's capacity is always unused.

2) Lifespan Loss: Similarly, the lifespan of a battery pack is determined by the cell with the shortest lifespan. This cell is most likely the smaller one. The smaller cell, constantly fully charged and discharged, is likely to reach the end of its lifespan first, pushing it too hard. When this cell reaches the end of its lifespan, the entire battery pack will also reach the end of its lifespan.

3) Internal resistance increases: Given different internal resistances and the same current, the cell with the larger internal resistance will generate more heat. Excessive battery temperature accelerates degradation, further increasing internal resistance. Internal resistance and temperature rise form a negative feedback loop, accelerating the degradation of high-resistance cells.

The three parameters above are not completely independent. Cells with advanced age have higher internal resistance and greater capacity decay. I explain them separately simply to clarify their respective impacts.

How to Address Inconsistencies

Inconsistencies in battery cell performance are formed during the production process and exacerbated during use. Within the same battery pack, weaker cells remain weaker and weaken more rapidly. The dispersion of parameters between individual cells increases with aging.

Currently, engineers address individual cell inconsistencies from three main perspectives: individual cell sorting, thermal management after grouping, and balancing functions provided by the battery management system when minor inconsistencies occur.

1) Sorting

Battery cells from different batches should theoretically not be used together. Even cells from the same batch require screening to place cells with relatively similar parameters in the same battery pack.

The purpose of sorting is to select cells with similar parameters. Sorting methods have been studied for many years and are mainly categorized into two types: static sorting and dynamic sorting.

Static sorting involves screening cells based on characteristic parameters such as open-circuit voltage, internal resistance, and capacity. Target parameters are selected, statistical algorithms are introduced, and screening criteria are set to ultimately divide cells from the same batch into several groups. Dynamic screening is based on the characteristics exhibited by battery cells during the charge and discharge process. Some methods utilize constant current and constant voltage charging, others pulse charge and discharge, and still others compare the relationship between their own charge and discharge curves.

Combined static and dynamic sorting uses static screening for initial grouping, by dynamic screening. This approach yields more groups and improves screening accuracy, but also comes with a corresponding increase in costs.

This demonstrates the importance of scale in power lithium battery production. Large-scale shipments allow manufacturers to conduct more detailed sorting, resulting in battery packs with more consistent performance. If production volumes are too low and there are too many groups, even a single batch cannot produce a single battery pack, rendering even the best methods ineffective.

2) Thermal Management

To address the problem of inconsistent heat generation in cells with varying internal resistance, a thermal management system can regulate temperature differences across the entire battery pack, keeping them within a narrow range. Cells that generate more heat will still experience a higher temperature rise, but not by a significant difference from other cells, resulting in less noticeable degradation. 3) Balancing

Inconsistency among individual cells can cause some cell terminal voltages to consistently exceed others, reaching the control threshold first and reducing overall system capacity. To address this issue, the battery management system (BMS) incorporates a balancing function.

If one cell reaches the charge cutoff voltage first, while the voltages of the remaining cells lag significantly behind, the BMS activates the charge balancing function, either inserting a resistor to partially discharge the high-voltage cell or transferring energy to the lower-voltage cell. This removes the charge cutoff condition, restarts the charging process, and allows the battery pack to charge more.

Cell inconsistency remains a key area of ​​research within the industry. Even the highest energy density cells can be significantly compromised by inconsistency, significantly reducing the battery pack's performance.