Backgrounds
During the large-scale application of LiFePO4 batteries, the voltage inconsistency problem has become a key challenge that restricts their performance, safety and service life. With the rapid development of new energy storage systems and electric vehicles, Li-FePO4 batteries are widely used due to their high safety, long cycle life and cost advantages, but the problem of voltage inconsistency between single batteries after grouping is becoming more and more prominent. This inconsistency is mainly manifested in the discrete parameters such as single cell voltage, internal resistance and capacity, especially when connected in series or parallel to form a group, the difference will be gradually amplified with the charging and discharging cycle and environmental changes, which ultimately leads to a decline in the overall performance of the battery pack, and even triggers thermal runaway and other potential safety hazards.
Analysis of problems and causes
Differences in manufacturing process and material properties:
There are differences between the reaction rate of the positive electrode material and the negative electrode material of lithium iron phosphate batteries, with the positive electrode reacting at a faster rate than the negative electrode, resulting in the accumulation of the potential difference between the positive and negative electrodes in the process of charging and discharging. In addition, minor differences in the electrode thickness and electrolyte distribution of single batteries during the manufacturing process will further aggravate the inconsistency of internal resistance and capacity. For example, uneven distribution of electrolyte concentration will affect the ion migration rate, resulting in local voltage differences.
Effects of aging and use environment:
After long-term cyclic use, the evaporation of electrolyte and crystallization of cathode material inside the battery will accelerate aging, increase internal resistance, and accelerate the rate of voltage drop. At the same time, high or low temperature environment will exacerbate the unevenness of the electrochemical reaction, further amplifying the voltage difference.
Operating conditions and external connection problems:
During the charging and discharging process of the battery pack, uneven current distribution may lead to overcharging or overdischarging of some monomers. In addition, false soldering and inconsistent connection resistance. External factors can also trigger local temperature increase and voltage fluctuation, for example, abnormal resistance of the virtual welding joints can lead to significant amplification of the voltage difference between parallel modules.
Effect of voltage inconsistency
Performance degradation:
Excessive voltage difference will reduce the usable capacity of the battery pack. For example, at the end of charging and discharging, high-voltage cells may trigger the protection mechanism, forcing the whole pack to terminate operation prematurely, resulting in a decrease in energy utilization.
Shortened cycle life:
inconsistency will accelerate the overcharging or over-discharging of some monomers, causing them to age much faster than other monomers, thus shortening the overall life. Experiments show that the cycle life of battery packs with soldering problems may be reduced by more than 30%.
Safety Risks:
Voltage differences may trigger localized overheating, leading to thermal runaway in extreme cases. Studies have shown that the safety risk of the battery pack increases significantly when the voltage difference of a single cell exceeds 0.1V.
For energy storage systems, the barrel principle is that the worst cell affects the charging and discharging of the entire cluster, affecting its overall performance.
Barrel effect due to capacity difference
Generally, the split capacity test will be done when the battery cell is shipped out of the factory, but there are exceptions to everything, when there is a mixing of the factory capacity, or problems that occur in the late stage of the battery cell itself.
As shown in the figure below, when the capacity of one of the cells in the battery pack is abnormal with the others, we will find that in the actual operation of charging and discharging, the priority cut-off of the low capacity of the cell, and at this time, the overall capacity of the whole battery cluster will depend on this abnormal cell. We use Q to denote the rated capacity of the cell, Q1 to denote the total capacity, and Q2 to denote the capacity of the abnormal cell. Ideally Q1=Q, then the actual situation will be Q1=Q2,and Q is > Q2.
The barrel effect due to voltage uniformity
In general, voltage consistency differences normally occur during use, such as as a result of self-discharge of the cell.
In a battery cluster with good voltage consistency, the voltage of a single cell can reach the charging cutoff condition and the discharge cutoff condition almost simultaneously. However, in the case of a cluster with poor voltage consistency, in the discharge stage, the one with low residual capacity will reach the discharge cut-off condition first, causing the whole cluster to reach the discharge cut-off condition earlier. In the charging process, the one with high residual capacity reaches the charging cutoff condition first, causing the whole cluster to reach the charging cutoff condition earlier. At this time, the capacity of the cluster is jointly determined by the cells with high and low residual capacity, Q1=Q-(Q high – Q low).
Manifestations and Causal Effects of Differential Pressure Failures
| Differential pressure failure | Cause analysis | Affect |
|---|---|---|
| discrepancy in volume | Cell mixing | Impacted Capacity |
| Cell connection row soldering abnormality | Impacted Capacity | |
| Voltage Consistency Differences | BMU power consumption inconsistency | Abnormal Capacity Consumption |
| BMU sampling abnormality | Abnormal Capacity Consumption | |
| Module replacement pressure difference difference is large | Impacted Capacity | |
| Initial S0C is low | Impacted Capacity | |
| Self-discharge is large | Impacted Capacity | |
| misinformation | Voltage circuit false alarms | Abnormal Voltage Capture |
When the welded cells in a battery have, for example, a false weld, when the cells are connected in parallel, the capacity of the cells decreases due to the false weld that causes the abnormal cells to not be able to fully charge and discharge (polarization voltage). When the cells are connected in series, the false welding will lead to high polarization voltage of charging and discharging, and the capacity will be reduced. The differential pressure failures listed above are mainly caused by the production process and manufacturing process.
active equilibrium
When the voltage inconsistency of the cell, active equalization is the use of energy migration methods, active migration of energy from high-capacity cells to low-capacity cells, and ultimately make the capacity maximization, of course, if the active equalization of the scheme is not stable, in fact, on the contrary, it will increase the risk of the operation of the system and increase the failure rate.
passive equilibrium
Passive equalization is a way of energy consumption, mainly through the switch to discharge the high voltage of the core, thereby reducing the core voltage to maintain the consistency of the voltage, however, this can only discharge the high voltage of the core, but there is no way to deal with the original low voltage, this can not be a fundamental solution to the problem of voltage inconsistency, to reach a certain level, it is necessary to artificially replenish the battery core! Or directly replace the module. But passive equalization is relatively more stable than active equalization, and the failure rate is relatively low.
When choosing to replace the module, it is generally best to do it at the beginning of the operation, because after running time is too long, the original normal module will also appear capacity decay, at this time to replace the new module up, there may also be inconsistency in the module voltage. Replacement of modules, we prioritize in the battery fully charged, replace the fully charged module, to achieve the consistency of the system voltage.
