How a battery works out its percentage

The battery can't see the energy inside it. It can only measure a few things from outside, then calculate. The state of charge can't be measured directly, but it can be estimated.¹

The part that does the measuring is the battery management system, or BMS. It's the electronics that watch over the cells. One of its jobs is to report the charge level.²

What the BMS measures

A BMS can measure voltage, current and temperature.³ The percentage comes from those readings plus the maker's own rules.

Method one: add up the current

This is called current integration, or coulomb counting. The electronics measure the current and total it over time.¹ In plain terms, the BMS keeps a running tally of what flowed in and what flowed out.

A tally has a weak spot. Errors. No measurement is perfect, so small ones pile up, one on top of another, until the total no longer matches what's really in the cells. The method suffers from long-term drift and has no reference point of its own.¹

The fix is a fresh starting point. One example is resetting the charge to 100% when a charger decides the battery is full.¹

Method two: read the voltage

Voltage changes with charge, so it can act as a clue. The catch is that current affects voltage far more strongly, and temperature does too.¹

For some chemistries the clue is weak. The reference names lithium iron phosphate as a type whose voltage doesn't depend on charge. For those, a voltage reading at rest can't give a reliable estimate.¹ A second reference calls cell voltage a poor indicator of charge and, for certain lithium types, no indicator at all.²

Method three: combine them

Many designs blend the two. A Kalman filter is one way. It shifts its trust between the voltage and the tally as conditions change.¹

Your maker may use one of these, a blend, or its own variation. Check your own manual for what yours does.

Why this matters for what you see

Because the figure is calculated, it can be corrected later. One maker says the estimator may revise the percentage using voltage, current, temperature and model-specific logic.³

That's why a reading sometimes steps up or down with no change in how you use the battery. A separate page on reading drift covers those cases.

A plain picture of it

Think of a bank balance kept by hand. You note every deposit and every withdrawal. Over months, small slips add up. Now and then you check the statement and fix the total. A battery's electronics do the same job with current in place of money, and a full charge acts as the statement. That's an analogy, not a claim about any one product. The sources describe the tally and the reset, but they don't say how often a given maker resets.

For what the number means in the first place, start with the basics of state of charge.