EV Charging Curve: Why Fast Charging Slows Down After 80%

DC fast charging slows down past about 80% because a lithium-ion battery cannot safely accept high current when it is nearly full. Charging starts fast and steady, then the current drops as the battery gets close to its voltage limit. That is why fast-charge times are usually quoted from 10% or 20% up to 80%. On a 75 kWh battery, 20% to 80% puts 45 kWh into the pack, and that is the fast part of the session.

The EV charging time calculator gives a steady-rate estimate. This page explains the shape that real fast charging follows instead.

What a charging curve is

A charging curve is a chart of charging power against state of charge. The flat part at the start is where the car takes close to its top power. The falling part near the top is where the battery management system cuts current back.

Lithium-ion batteries charge in two stages, known as constant current, constant voltage. Battery University describes it this way:

• In stage 1 the charger pushes a steady current and the battery voltage climbs.
• Once the voltage reaches its limit, stage 2 begins. The voltage holds steady and the current falls on its own as the battery saturates.
• The battery counts as full when the current drops to 3% to 5% of its rated amp-hours.

According to the same source, a high charging current “will quickly fill the battery to about 70 percent,” and the battery sits at about 85% state of charge at the end of stage 1. Pushing a higher current shortens stage 1 but makes stage 2 take longer. That is why more power does not save much time near the top.

Why the battery needs to slow down

Battery University’s page on fast and ultra-fast chargers says the stress comes “in the second half of the charge cycle towards full charge when acceptance becomes labored.” It says ultra-fast charging applies only during the first charge phase, and the current should be lowered after the battery reaches 70% state of charge.

Cutting back current as the battery fills is called step charging. Battery University notes that laptops have used it for years, and so do EVs.

Temperature matters too. The same page warns against fast charging a cold or hot battery, and against fast charging an aged one. When the pack is cold, the car will often hold power down until it warms.

Why 10% to 80% is the figure people quote

The low end of the curve is fast and the top end is slow. A 10% to 80% or 20% to 80% time shows what a car can do in the part of the curve that matters on a road trip. A 0% to 100% time mixes in the slow tail and makes cars hard to compare.

It also matches how people use fast chargers. You charge enough to reach the next stop, then go. Sitting at the charger to squeeze in the last 20% means paying for slow power with your own time.

Why there are no per-model curves here

Each car has its own curve, and it changes with battery temperature, the station’s maximum output, battery age and software updates. Figures that look precise would mislead more than they help, so the EV model pages stick to EPA data instead. Every EPA label on fueleconomy.gov lists a 240 volt full charge time and electricity use in kWh per 100 miles, both measured at the wall.

For DC fast charging, the Department of Energy’s Alternative Fuels Data Center gives a general range of about 100 to 200+ miles of range per 30 minutes, from stations rated up to 500 kW.

How to use the curve in practice

• Arrive low and leave around 80%. The battery takes high power when it is low, so you get the most range per minute.
• Plan two short stops instead of one long one if the route allows.
• Precondition the battery if your car offers it, so the pack is warm when you plug in.
• At home on Level 2, stop worrying about the curve. Home power is low enough that the car can take it for most of the session.

Does charging to 100% hurt the battery?

Charging to 100% now and then for a long trip is common, but sitting at full charge is hard on lithium-ion. Battery University’s guide to prolonging lithium batteries estimates that a battery stored a year at 25°C keeps 96% of its capacity at 40% charge, but only 80% at 100% charge. Follow your carmaker’s advice on daily charge limits. The FAQ on whether it is bad to charge an EV to 100% has more, and the FAQ on why charging slows at 80% gives the short answer.