The state of charge (SOC) significantly influences battery charging efficiency in several key ways:
- Decreasing Charging Efficiency with Increasing SOC: As the SOC rises, especially beyond about 80%, the charging efficiency tends to decrease. This means that a larger fraction of the energy supplied during charging is lost, often as heat, rather than stored in the battery. This decrease in efficiency is due to the battery’s internal resistance increasing as it becomes more charged, causing energy dissipation during the charge process.
- Slower Charging Rates at Higher SOC: Along with reduced efficiency, the charging rate also slows down as SOC increases. When the battery is nearly full, the charging current is reduced to avoid overcharging and damage. This reduction in charge rate is particularly pronounced during fast DC charging but less so with AC charging, which maintains a steadier charge rate across various SOC levels.
- Energy Transfer Efficiency and Internal Resistance: At higher SOC levels, more energy is required simply to overcome the battery’s internal resistance, which leads to lower energy transfer efficiency. This results in some energy being converted to heat rather than stored electrochemically, reducing overall charging efficiency.
- Charging Process Adaptation: Charging protocols often adjust according to SOC, using a “tapered charging” or “constant voltage charging” approach at high SOC levels to protect battery health. This approach reduces the current and thus the charging speed, which also impacts the energy efficiency of the charging process.
In summary, the battery’s SOC influences charging efficiency by causing slower charging speeds and greater energy losses as SOC increases, particularly above 80%. This is mainly due to increased internal resistance and safety mechanisms that limit charging rates to protect battery longevity.
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