What is Usable vs Total Battery Capacity for an EV?

Since it is bad to fully charge or fully use a lithium ion battery, they are built with safety buffers that prevent drivers from accessing the entire battery capacity.

Battery designers and scientists try to make EV batteries foolproof: while science knows that a lithium ion battery should never be fully charged or discharged, an EV driver shouldn’t have to worry about using 'too much' of their range. Instead, the amount of battery that a user is allowed to access is restricted. This reduced capacity is called the “usable capacity,” or “available kilowatt-hours (kWh)” and is usually 95%-99% of the total available capacity. For instance, a battery that can physically hold a total of 65 kWh may only make available 62 kWh for the car for use. What’s happening with that extra 3kWh of capacity?

Safety buffer: The most common battery chemistry, lithium ion, is most stable and will last the longest if it never charges fully. Car makers use software to keep the battery healthy by setting a safety buffer that prevents a car from charging all the way to 100%. Your car’s dashboard still reads 100%, but the physical battery cells don’t actually charge all the way to 100%. Depending on the manufacturer, the usable capacity is usually between 95% and 99% of the total physical battery capacity. 

Safety buffer, part deux: Lithium Ion batteries can also be damaged by fully discharging. There is a safety buffer at the bottom end, too, so that even when your car says 0%, there is still some inaccessible charge in reserve as a safeguard against damage. You won’t be able to drive with those invisible kilowatt-hours, but you also won’t ruin your battery if you drive your car down to “zero.”

To recap, some “unusable” energy is reserved for the top end of the charge curve, and some is reserved for the lower end of the charge curve.