EV fast charging: why it slows down, and what actually wears

What is going on

On DC rapid charging the charger does the converting and feeds direct current straight to the pack, bypassing the car's onboard charger. The charger's rating is only the most it can supply.

What the pack accepts is decided moment by moment by the battery management system, and it falls as the pack fills: fast while the pack is low, slower as it fills, and slow near the top. That fall is the charging curve. Charging pushes lithium into the electrode against a rising voltage, so the fuller the cell, the less current it can take without harm.

Temperature is watched throughout: a warm pack charges faster than a cold one, and a hot one is throttled. The shape of the curve belongs to the car, not the charger.

What decides the speed

• The pack's state of charge: it accepts power fastest when low and slows sharply as it fills
• The pack's temperature: a cold pack is not allowed full power, and a hot one is throttled
• The car's own charging curve, which a more powerful charger cannot make it exceed
• The charger itself, and whether it is splitting its power with a second car
• The car's software

What helps

1. Arrive with the pack low rather than topping up from high.
2. Where your car supports it, set the rapid charger as the destination in the navigation so the pack arrives warm.
3. Leave at around 80 percent, where the curve has dropped toward the speed of an AC charge, unless the trip needs more.
4. If you need a full pack, let AC charging finish the job overnight.
5. Choose a quieter site where you can. Many rapid chargers split their power between two cars.
6. After a hard rapid charge, do not leave the pack full and warm for long.

Normal, or worth a look?

• Fast at first, then slower as the pack fills: normal, that is the charging curve.
• The last few percent take disproportionately long: normal, the battery management system is waiting for the weakest cells.
• Slow from the very start on a cold day: a cold pack. Slow rather than harmful, and preconditioning on the way fixes it.
• Two cars on identical chargers take very different times: the curve belongs to each car, and neither is faulty.
• Battery temperature too high lamp after a rapid charge: the pack is hot and protecting itself. Let it cool.
• Same charger, same starting charge, same weather, and still clearly slower a few weeks apart: the pattern worth a qualified diagnosis.

What people get wrong

That fast charging will ruin the battery, or that it is harmless. Rapid charging is intended use, and the battery management system and the thermal system cap the current, taper the curve and cool the pack. Its effect on long-term health depends on chemistry, temperature, state of charge, charging power and the car's thermal strategy, and the manufacturer's guidance, not avoidance, is the right response.

When to have it checked

A slow charge is not a diagnosis. State of charge, temperature, preconditioning, the curve, a shared site, the car's own maximum and the software all slow a charge, and the battery comes after all of them. If the same charger, the same starting charge and the same weather still give a clearly slower charge a few weeks apart, with no software change and no shared site, have it diagnosed. A battery temperature fault lamp means the heating or cooling is not working, and should be read.

Read next

• EV battery temperature warning: too hot, or a cooling fault?
• Charging an EV at home: the habits that help the battery
• EV charging fault: usually the charger, not the car

This page is general information and does not replace a professional assessment. Symbols, codes and behaviour vary between makes. Always check the handbook for your own car.