Short answer: The total physical battery capacity versus the portion your car actually lets you use, with a buffer reserved to protect battery health.
Explanation
Every EV battery has two capacity numbers. The nominal (or gross) capacity is the total energy the physical cells can store. The useable (or net) capacity is the portion the car allows you to access. The difference is a hidden buffer at the top and bottom of the battery that the BMS reserves to protect cell health and longevity.
For example, a car might have an 82 kWh nominal battery but only 77 kWh of useable capacity. The remaining 5 kWh is split between a top buffer (preventing the cells from reaching maximum voltage) and a bottom buffer (preventing them from being fully drained). When your car shows 0% SoC, there is still some energy in reserve.
This buffer is why you can sometimes drive a few kilometers after the car shows 0%. It is also why the advertised range is based on useable capacity, not nominal. When comparing battery sizes across brands, check whether the number is nominal or useable. Some manufacturers advertise the nominal figure (which looks bigger), while others show the useable figure (which is more honest).
Where you'll see this
- In vehicle specifications
Common confusion
People sometimes think a 77 kWh battery from one brand is smaller than an 82 kWh battery from another, when in reality both might have the same useable capacity. Always compare useable capacity for accurate range comparisons.
Example
The Tesla Model 3 Long Range has about 82 kWh nominal capacity but approximately 75 kWh useable. The Hyundai Ioniq 5 Long Range has 77.4 kWh nominal and about 74 kWh useable. The real-world difference is smaller than the headline numbers suggest.
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