Understanding Battery Metrics
Unlike measuring a gas tank in a gas car, where you can simply measure the amount of liquid, electric vehicles lack a similar mechanism for batteries. Consequently, battery metrics are complex and educated estimates.
Battery level, range and capacity are estimated by the vehicle's battery management system, often called the BMS.
This article explains how different battery metrics are calculated and some of the counterintuitive quirks that result.
Battery level
Battery level is the percentage shown by the vehicle, from 0% to 100%.
This number represents the vehicle's estimate of how much usable energy remains in the battery. It does not mean the battery is physically empty at 0%, or that the battery has reached its absolute physical limit at 100%.
This is because EV batteries have built-in safety buffers. These buffers help protect the battery from damage, because lithium-ion batteries can be harmed if they are discharged too low.
Range
Battery range, also called rated range, is the vehicle's estimate of how far it can drive on the current battery charge.
Range is based on a fixed efficiency assumption, determined by the vehicle's official rating set by the country's regulatory agency. It answers the question:
"How far can the car drive if it uses energy at its officially rated efficiency?"
Range usually changes in a predictable way with battery percentage. If the vehicle shows 300 miles at 100%, it typically shows 150 miles at 50%.
Rated range is useful for comparing the battery's performance over time, but it's not a great prediction of your exact driving range, because real-world driving is often not 100% efficient.
Capacity
Battery capacity is the battery management system's estimate of how much energy the battery can store, usually measured in kilowatt-hours (kWh).
There are two primary measurements of capacity:
Total capacity: The battery pack's current capacity, including energy that may not be available for normal driving because the vehicle reserves a buffer to protect the battery.
Usable capacity: The portion of the battery that the vehicle allows you to use for driving.
The vehicle estimates capacity using battery voltage, current flow, temperature, charge and discharge history, cell behavior and other battery management data.
Because it's an estimate, capacity can fluctuate slightly over time. Small day-to-day changes are normal and do not mean the battery has suddenly degraded or improved.
Capacity degradation occurs over time, as a pattern.
Battery level and capacity don't align perfectly
Battery percentage and kWh capacity are related, but they don't always line up.
For example, a vehicle with an estimated usable capacity of 75 kWh does not necessarily have exactly 37.5 kWh available at 50%.
That's because the battery percentage is based on the vehicle's estimate of usable capacity, which takes safety buffers into account.
0% battery level doesn't mean the battery is actually empty
A common misconception is that 0% battery means the battery is empty. It doesn't.
If an EV battery were truly discharged to 0 kWh, the battery could be permanently damaged. Lithium-ion batteries need to stay within a safe voltage range, so the vehicle protects the pack by reserving energy below 0% battery level.
That doesn't mean it's good to drive to 0%. The bottom buffer is there as protection, not as normal usable range. Repeatedly running the battery very low can still increase stress on the battery and may leave the vehicle unable to drive or charge normally.
Effective range changes over time
The range you actually get while driving can change for many reasons, including:
- Battery age
- Battery usage
- Temperature
- Recent driving efficiency
- Tire pressure
- Wheel and tire size
- Software updates
- Battery calibration
- Climate control usage
- Driving speed
- Elevation changes
- Weather conditions
This is completely normal. Your vehicle is constantly estimating, learning and adjusting based on the battery and the conditions around it.
Why battery metrics change after charging and calibration
The battery management system is constantly learning.
Battery metrics often become more accurate after the vehicle has time to observe the battery across a wide range of charging states. This is why calibrating the BMS increases the accuracy of the vehicle's battery metrics.