State of Charge (SoC)

State of Charge (SoC) quantifies the remaining capacity available in a battery at a given time, expressed as a percentage of the battery's total capacity when fully charged. It indicates the proportion of charge left in the battery, where 0% signifies an empty battery and 100% signifies a fully charged one.

The SoC is a critical metric for understanding the current energy status of a battery, especially in electric vehicles (EVs), where it serves a role analogous to a fuel gauge in internal combustion engine vehicles. SoC informs users about the remaining driving range and helps manage the charging cycles to optimise battery health and performance. 

Determining SoC involves various methods due to the complex nature of battery discharge and charge processes. One common method is Coulomb Counting, which integrates the battery current over time to estimate the charge removed or added. This method, while effective, can accumulate errors over time due to sensor drift or inefficiencies in battery behaviour. To mitigate this, SoC measurements are often recalibrated using other methods like Open Circuit Voltage (OCV), which correlates the battery’s voltage with its state of charge. OCV, however, can be influenced by temperature and requires the battery to be in a rest state for accurate readings. 

Advanced techniques such as Kalman Filtering combine multiple measurements and model-based approaches to provide more precise SoC estimations. This method adjusts its estimates in real-time based on variations in battery performance and environmental conditions.

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