State of charge — the fuel gauge, and why it lies a little

The percentage on the dashboard is calculated, not measured. The management system infers it from voltage, current flow over time and a model of the cells, and the inference is harder for some chemistries than others.

Why it matters when buying

LFP packs have a famously flat voltage curve through the middle of their range, which makes the estimate drift. That is why manufacturers using LFP ask owners to charge to full regularly: the full-charge point is where the controller recalibrates. An owner who never charges to capacity on an LFP car will eventually see a gauge that jumps, and the fault is in the routine rather than the pack.

The second thing worth knowing is that the percentage refers to usable capacity, not to the whole pack. Zero on the display is not an empty battery, and full is not a full one. Both ends hold a reserve the driver never sees.

Related: usable capacity · SOH · charging curve