How does your phone know its battery percentage?

Your phone does not measure its battery percentage; it estimates it. State of charge is the remaining capacity of a battery, usually shown as a percentage where 0 percent is empty and 100 percent is full, and it usually cannot be measured directly. A small electronic system watches the battery and works the number out from other things it can measure, mainly the battery's voltage and the current flowing in and out.

The voltage method uses a known curve of voltage against charge. Read the voltage, look up the percentage. The trouble is that voltage is also affected by how much current is being drawn and by temperature. Worse, designers deliberately aim for a voltage that stays as constant as possible whatever the charge. For some chemistries, such as lithium iron phosphate, the voltage hardly changes with state of charge at all, so it cannot give a reliable estimate.

The second method is coulomb counting. It measures the current and adds it up over time, like a bank statement that records every withdrawal and deposit. But no measurement is perfect, so over the long term the tally drifts. The fix is regular recalibration, such as resetting to 100 percent when the charger determines the battery is full.

Good gauges combine both. A tool called a Kalman filter adjusts, in real time, how much it trusts the voltage and how much it trusts the count. Some fuel-gauge chips in phones, such as Maxim Integrated's, use a combined voltage and charge approach.

The result is a good estimate, not a truth. Cars show the same limitation: on the Mitsubishi Outlander plug-in hybrid the dashboard shows zero while the real charge is still 20 to 22 percent.