What frequent fast charging costs a battery
Geotab, a company that collects data streamed automatically from vehicles, called telematics, studied 22,700 electric vehicles across 21 models. It grouped them by how often each one used direct current fast charging, a public charger that refills a battery in well under an hour instead of overnight, and by how much of that fast charging ran above 100 kilowatts, a measure of how fast power moves into the battery. Vehicles that rarely fast charged lost 1.5% of their original battery capacity a year, Geotab reports. Vehicles that fast charged often, with more than 40% of those sessions above 100 kilowatts, lost 3.0% a year, about twice as much. A third group, frequent fast chargers mostly under 100 kilowatts, lost 2.2% a year.
Geotab counts a vehicle as a frequent fast charger if direct current fast charging is more than 12% of its charging sessions, and as mostly above 100 kilowatts if more than 40% of those sessions ran above that power level. Geotab states it found and controlled for a link between fast charging frequency and time spent at a very low or very high charge level before reporting these figures.
Show the numbers
| Rarely fast charges | 1.5 |
| Frequent, under 100 kilowatts | 2.2 |
| Frequent, over 100 kilowatts | 3.0 |
What the gap becomes after 8 years
Projected forward, the lightest fast charging group keeps 88% of original battery capacity after 8 years, and the heaviest group keeps 76%. Geotab states it found that vehicles which fast charge more also spend less time at a very low or very high charge level, a separate pattern it says it controlled for before reporting these figures. It does not state whether it also accounted for how hard or how many miles a vehicle is driven, a separate factor that could push the same drivers toward both more fast charging and more wear.
What a controlled lab test shows
A laboratory study published in the Journal of Power Sources isolated the charging rate itself by cycling identical battery cells, meaning one full charge and one full discharge, at different fixed speeds. A number followed by the letter C states how many hours a full charge takes, so 2C fills a cell in about 30 minutes and 4C to 6C fills it in about 10 to 15 minutes. Cells charged at the moderate 2C rate lasted over 1,000 cycles. Cells charged at the aggressive 4C to 6C rate lasted around 500 cycles, roughly half as many. The study names the cause, a layer of material called the solid electrolyte interphase building up where lithium metal plates onto the electrode surface, which both uses up lithium the battery needs and raises internal resistance, the resistance that turns some of every charge into heat instead of stored energy.
Moderate is a 2C charge rate, about 30 minutes to fill. Aggressive is a 4C to 6C charge rate, about 10 to 15 minutes to fill. The study itself uses over and around for these 2 numbers, so both counts here are approximate rather than exact.
Show the numbers
| Moderate fast charge, 2C | 1,000 |
| Aggressive fast charge, 4C to 6C | 500 |
Fast charging speed is not the whole story
A separate laboratory study from Idaho National Laboratory cycled cells under a demanding 20 minute fast charge protocol and found that how a fast charge is managed matters as much as how fast it is. Cells kept to a lower top charging voltage, given a 2 minute rest right after charging, and even discharged more deeply between charges aged less. Managed this way, cells survived hundreds of fast charge cycles, said to equal more than 100,000 miles of driving, without significant added wear. For a driver, the Geotab numbers describe a link between heavy fast charging and faster wear, not proof that fast charging alone caused it. A well managed charging system can absorb far more fast charging than that link alone suggests.