An electric vehicle's battery is a large number of individual cells governed by a control system. That system does considerably more than report a percentage on the dashboard.
State of charge is an estimate
There is no sensor that reads how much energy remains in a lithium cell. The figure shown to the driver is inferred from voltage, current flow over time and temperature.
Voltage alone is unreliable because the relationship between voltage and remaining charge is nearly flat across the middle of the usable range for many chemistries.
The system therefore combines methods, counting energy in and out while correcting against voltage readings when the pack is at rest, and the estimate drifts between corrections.
Cells drift apart and must be matched
Cells in a pack are manufactured to the same specification but age at slightly different rates, particularly those sitting in warmer positions.
Because cells are wired in series, the weakest one limits the whole string. Charging must stop when the highest cell is full and discharge when the lowest is empty.
Balancing circuits address this by bleeding a small amount of charge from the cells that run ahead, usually during charging, so the string stays matched over years.
Buffers are reserved at both ends
Manufacturers hold back capacity at the top and bottom of the pack that the driver cannot reach. The usable figure is smaller than the gross capacity of the cells.
Keeping cells away from full and empty extremes substantially slows degradation, and the reserve at the bottom prevents damage if a vehicle is left sitting at zero.
This is why a percentage reading is not a direct measure of the chemistry, and why a car showing zero still has enough charge to protect itself.
Thermal control runs constantly
The system decides when to heat or cool the pack, balancing the energy that costs against the performance and longevity gained.
It also anticipates. A vehicle told it is heading to a fast charger will warm the pack in advance so it can accept high current on arrival.
Cooling matters most during rapid charging and sustained high-speed driving, when the heat generated inside the pack rises sharply and must be removed to avoid throttling.
Safety limits override everything
The control system monitors for conditions that indicate a fault, including cells drifting outside expected voltage bands, unexpected temperature rises and insulation problems.
Where a fault is detected it can reduce available power, refuse to charge or disconnect the pack entirely, which the driver experiences as a warning and a limp mode.
These interventions look like malfunctions and are usually the system working as designed, which is why a sudden power limitation warrants inspection rather than a restart.