The DC charging curve explained: why everyone quotes 10–80% and why power drops after 80%

Every electric car spec sheet has a figure like "10–80% in 18 minutes." Why not 0–100%? And why does the power on the charger's screen drop so visibly toward the end? The answer is the charging curve.

What a charging curve is

The curve shows how much power the battery takes at each state of charge. It isn't a straight line: power rises quickly at first, stays near the maximum for a while, then tapers off gradually. The maximum charging power on the spec sheet is just the peak of the curve, reached over a short stretch.

Why power drops

• Cell voltage. As the battery fills up, its voltage rises. As it nears the maximum limit, the system has to cut the current, and therefore the power.
• The risk of lithium plating. At high current, high state of charge or low temperature, lithium can deposit as metal on the anode instead of entering the graphite. That reduces capacity and, in extreme cases, can affect safety. Avoiding it is the main reason fast charging usually tapers before 80%.
• Heat. Losses grow with the square of the current: double the current and you quadruple the heat. The battery has to stay within a temperature window.
• The BMS. The battery management system sets the limit based on the "weakest" cell, meaning the coldest or the hottest one, not the average.

Why 10–80% is the standard benchmark

That's the range where the battery accepts the most power, and manufacturers use it so they can be compared. The last 20% takes disproportionately long. An official example: Hyundai quotes 10–80% in 18 minutes for the IONIQ 5 with the 84 kWh battery on a 350 kW charger. On a 50 kW station, the same window takes 1 hour and 16 minutes. So the charger's power matters as well as the car's: you get at most what the slower of the two can deliver.

Preconditioning and cold weather

A cold battery charges slowly. Tesla recommends setting the Supercharger as your navigation destination at least 30–45 minutes before you arrive, so the car warms the battery on the way. A blue snowflake next to the battery indicator means the battery is too cold and charging may be limited. Many other brands have similar features, started from navigation or manually; check your car's manual.

LFP vs NMC

LFP batteries have a very flat voltage curve, so the car has a harder time estimating the exact percentage. That's why Tesla recommends charging LFP cars to 100% at least once a week. In a test of an LFP Model Y Standard, power stayed near its peak only up to about 25%. In the same article, an electrive test shows the car can report the battery as too cold even after preconditioning, which keeps charging limited.

Tips for long trips in Romania

• Arrive low (10–20%) and leave when the curve starts to drop, usually around 70–80%, if the next charger is within range.
• Watch for chargers that share power. Many fast chargers can feed two cars at once from the same total output. An alpitronic HYC300, for example, is built from 75 kW modules, up to 300 kW, and can charge in parallel. If the site has a free post on a different cabinet, pick that one.
• Set up the apps in advance. They're listed in our guide to EV charging apps and cards in Romania, and prices are in what charging costs at home, on AC and on DC.