The cycle life of a battery is defined as the number of charge-discharge cycles a battery can perform before its performance has degraded beyond its useful level. This is a somewhat imprecise definition, dependent on the amount of degradation that’s deemed acceptable.
The cycle life can be more accurately represented as a plot with cycles on the X-axis and the percentage of battery capacity remaining on the Y-axis. Multiple lines representing the cycle life, dependent on the cycle’s depth of discharge (DOD), can often provide additional information. Battery cycle tests are used to obtain this data.
Figure 1. EV batteries typically discharge at higher rates for shorter durations but even a brief discharge at 1 or 2C significantly boosts power output and acceleration. Frequent high discharges, however, rapidly deplete the battery’s SOC.
During the initial cycles of a cell’s life, electrolyte wetting increases capacity. The battery then enters a phase of moderate capacity loss, as successive cycles cause a lithium compound layer to form on the anode — which consumes cyclable lithium ions. Later in the life of a cell, significant capacity loss occurs due to mechanical damage to electrode materials. Mechanisms include internal short circuits, thermal decomposition of the electrolyte, and oxidation of the cathode.
The charge and discharge rates of electric vehicle (EV) battery cells affect the vehicle’s range and performance. Measured in C-rates, these variables quantify how quickly batteries charge relative to their maximum capacity. For example, a one-ampere-hour (Ah) EV battery can charge from 0 to 100% in 60 minutes at a rate of 1C. A rate of 2C is equivalent to a 30-minute charge time.
The C-rate may also refer to the current required to discharge a battery at this rate. The cycle life of a battery depends to varying degrees on factors such as:
- The depth of charge-discharge cycles
- The C-rates used during charging and discharging
- The temperature reached by the battery during this cycling
Cycle tests for cells typically ignore this complexity and are carried out with deep charge-discharge cycles and relatively low C-rates, which in turn ensure relatively low temperatures. As a lithium-ion battery cell is charged from a fully discharged state, the charge current is initially constant while the cell voltage increases to its peak value.
Once the peak voltage has been reached, the saturation charge phase begins, with the voltage remaining constant while the current decreases.
Typically, charging is terminated when the current reaches 3% of the rated current, corresponding to approximately 85% state of charge (SOC). The peak voltage reached during charging is generally higher than 4 V, but the open circuit voltage will drop to between 3.7 and 3.9 V after charging.
Discharging the battery is a critical process that should be carefully managed. The voltage decreases almost linearly with capacity until a voltage of approximately 3.4 V is reached, at which point the voltage begins to drop more rapidly.
Discharging from 4.2 to 3 V releases about 95% of the energy stored. However, discharging below 2.5 V can lead to significant battery degradation and should be avoided to maintain the battery’s capacity. A DOD of more than 50% is considered a deep discharge, and 80% is a typical practical limit to prevent excessive degradation.
In summary, EV batteries typically discharge quickly for higher power output. However, frequent high discharges can rapidly deplete their state of charge (SOC). Charging and discharging rates, along with maintaining optimal voltage levels, are crucial for preserving battery longevity and performance, with deep discharges above 80% DOD leading to faster degradation.
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