Fast charging in electric cars: why the maximum fast-charging power that manufacturers advertise doesn't tell the whole story

Not so long ago, when evaluating an electric car, people looked almost exclusively at its range: the more kilometres it promised, the better it seemed prepared for long journeys. But the reality is that, when covering long distances, the speed at which the car can recharge during a stop is just as important as the amount of energy the battery can hold. This is where DC fast charging comes into play, capable of adding hundreds of kilometres of range while we have a coffee or rest for a few minutes.

The problem is that the maximum DC charging power quoted by the manufacturer, the so-called “fast charging” figure, does not on its own explain how a car actually charges. A model that reaches a peak charging power of 250 kW does not always need less time to recharge its battery than another limited to 200 kW. Everything depends on how long it sustains that power, the size of the battery, its temperature, its state of charge when plugged in, and the energy the vehicle consumes to cover each kilometre.

That is why, to know which electric car travels best, you need to look well beyond the big number that appears on the spec sheet.

What exactly is fast charging?

An electric car’s battery stores energy as direct current. When we charge at home or at an AC point, the electricity must pass through the vehicle’s onboard charger, which converts it into DC before sending it to the battery. In a fast charger, that conversion takes place outside the car. The station delivers DC straight to the battery, bypassing the power limitation of the onboard charger.

That is why the same vehicle may accept, for example, 11 kW on AC and 150, 250 or even more than 300 kW on DC. It is worth distinguishing two units that are often confused:

• Kilowatts —kW— indicate power, that is, the instantaneous speed at which the car is charging.

Kilowatt-hours —kWh—

• indicate energy, that is, the amount the battery can store or that has been added during charging.
  

This matters, because a battery with 100 kWh of usable capacity will not necessarily charge in one hour at a 100 kW station. That simple division would only work if the car received a constant 100 kW from start to finish, which does not happen in practice because of the charging curve and process losses.

Actual charging time depends on the battery’s starting state, its capacity, the power the vehicle can accept and the power the station is able to deliver

.

What is the difference between a 400-volt and an 800-volt platform?

Most early modern electric cars used systems of around 400 volts. That architecture is still common and can reach high charging powers, so it should not be considered obsolete technology. However, more and more manufacturers are turning to 800-volt systems for their higher-performance models.

Electrical power, in simplified form, is the product of voltage and current:

Power = voltage × current.

To deliver 200 kW with a 400-volt system you theoretically need about 500 amps. At 800 volts, the same power can be reached using roughly half the current.

The advantages? Lower current reduces losses from resistance and heat generation in cables, connectors and electronic components. It also allows lighter conductors or higher power without a sharp rise in current.

Under suitable conditions, an 800-volt architecture makes it easier to reach and sustain very high powers.

As an example, the smart #5 can reach 400 kW and go from 10 to 80% in 18 minutes with its 100 kWh battery.
But beware: this does not mean that every 800-volt car always charges better than any car with 400-volt technology.

Having 800 volts does not guarantee good fast charging

The electrical architecture is only one piece of the whole.

Chemistry and cell design, their ability to accept current, the cooling system, the programming of the battery management electronics and the power of the charger all play a part.

There are 400-volt cars with very competitive charging curves, and 800-volt vehicles that do not fully exploit the theoretical potential of that architecture.

You also need to check what happens when an 800-volt car connects to a station whose output voltage is lower. Some vehicles incorporate a boost converter to adapt the voltage; others use certain components of the propulsion system to perform that function. The power available in those conditions may be lower than that obtained at a charger compatible with 800 volts.

Among the most recent systems, batteries are also being developed that can internally reconfigure themselves to charge efficiently at both 400- and 800-volt stations, such as the

bank charging technology debuted by the

Audi

Q6 e-tron, which splits the 800-volt battery into two partial batteries of equal voltage so they can charge in parallel without an additional voltage converter. The aim is for the driver to be able to use a mixed network without a significant penalty. Therefore, voltage gives an idea of the vehicle’s potential, but it does not replace the truly useful figure: how long it takes to recover energy under real conditions. And that is what you should focus on.

What is the charging curve?
When a manufacturer announces that a car charges at 250 kW, it is indicating the maximum power it can reach under specific conditions. It does not mean it receives 250 kW for the entire session. That peak may be held for several minutes, appear only for a moment, or not be reached at all if the battery is not in the right state.

Let us imagine two cars: the first reaches 250 kW but quickly reduces power to below 150 kW; the second has a maximum of 200 kW but stays close to that figure for a much larger part of the process. Although the first advertises higher power, the second could end up completing the 10-to-80% charge sooner. That is why the total time spent in that range and the average charging power are far more representative than the maximum peak.

The charging curve represents how the power accepted by the battery evolves as its state of charge, or SOC

, rises. Normally, a car connected with a low battery starts charging at a moderate power, reaches its maximum once its temperature has stabilised, and maintains a more or less wide zone of high power. It then begins to reduce it progressively. The exact shape varies greatly between vehicles. Some curves show a stable plateau; others have steps; and in certain models the power drops almost continuously from the initial peak.

The battery can accept more energy when its cells are relatively depleted. As they approach their maximum voltage, the system must reduce the current to control temperature, avoid damage and balance the charge level between the different cells.

In many vehicles, the fastest charging zone lies roughly between 10 and 60%. From there, power begins to decline, and near 80% the slowdown is usually much more evident. That is why many manufacturers use 80% as the reference point for resuming the journey when stopping to recharge, because beyond that level the process loses considerable speed. This explains why

manufacturers usually advertise the time from 10 to 80% rather than the time needed to complete a charge from 0 to 100%.

Why does the car not always charge at its maximum power?
The vehicle and the charger are continuously negotiating the voltage and current the battery can receive. The car requests the power it considers safe at each moment, and the station delivers the maximum possible within its own limits. The main factors involved include the following.

State of charge:

A battery at 10% can usually accept far more power than one at 80 or 90%. The closer it gets to full, the slower the process must be. It is similar to filling a container without spilling: at first you can open the tap wide, but when it is nearly full you have to reduce the flow.

Battery temperature:
• Lithium-ion batteries have a thermal window in which they accept fast charging best. If they are too cold, their internal resistance rises and the system limits the current to avoid harmful phenomena such as lithium plating on the anode. If they are too hot, power is also reduced to protect the cells and prevent accelerated degradation.
  

Preconditioning:

• The cars best prepared for travel can heat or cool the battery before arriving at the charger. In many cases it is enough to select a fast-charging station as the destination in the built-in navigation. The vehicle calculates when conditioning should start in order to reach the optimal temperature at the moment of plugging in. This feature is especially important in winter. A car capable of reaching 250 kW can fall well short of that figure if it is plugged in after spending the night at low temperature without preconditioning. Not all models activate it in the same way. Some do so automatically when a charger is entered into the navigation; others allow manual activation; and on certain cars it is not available. It is worth checking before buying.
  

Battery capacity and chemistry:

• A large battery can absorb more energy over the same percentage range than a small one. Going from 10 to 80% on a usable 100 kWh battery means putting in roughly 70 kWh; on a 60 kWh battery, around 42 kWh. That is why comparing times alone is not enough without taking capacity into account. Two cars may take 20 minutes, but the first will have added far more potential kilometres. Chemistry matters too. NMC and LFP batteries have different characteristics in energy density, thermal response, longevity and permissible power. But it cannot be said in general that one chemistry always charges better than another: it depends on the specific cell design and the complete system.
  

Charger limitations:

• A 350 kW station does not guarantee that the car will receive that power. It may be limited by its maximum voltage, by the current the cable can carry or by the total power available at the installation. Some stations share power between two or more connectors. If another vehicle arrives, the energy available for each one may decrease. Chargers also have thermal protection systems. On very hot days or after several consecutive high-power sessions, the station may temporarily reduce the current to prevent overheating. Logically, the final speed is always limited by the slowest element: a car that accepts 250 kW will charge at 100 kW at most if it connects to a 100 kW station.
  

What to look at when buying an electric car for long trips

• Maximum charging power is still a relevant figure, but it must be analysed alongside other parameters.

Real 10-to-80% time

: This is the simplest reference for comparing cars. Even better if it comes from independent tests carried out at different temperatures rather than only the manufacturer’s homologated figure.

Average charging power:
• This tells you whether the peak is sustained or purely symbolic. It is obtained by dividing the energy added by the time taken. For example, if a car adds 56 kWh in 20 minutes, its average over that interval will have been about 168 kW, even if at some point it may have reached 250 kW.
  

Range recovered per minute:

• For travelling, this figure can be more useful than kW. A very efficient car consuming 17 kWh/100 km will make better use of each kilowatt-hour added than an SUV needing 25 kWh/100 km. Although the second charges at higher power, it could recover kilometres at a similar or even lower rate. That is why fast charging and efficiency must be analysed together

.

• Real motorway consumption: WLTP range mixes different scenarios and does not necessarily represent what will happen at 120 km/h. For trip planning, real motorway range is worth knowing, especially in winter. A car may advertise 600 km WLTP and need to stop sooner than another rated at 520 km if its aerodynamics or high-speed consumption are worse.
  The existence of preconditioning

: It should be automatic, simple to activate and compatible with the navigation’s charger database. It is also useful to have manual activation, because it allows the battery to be prepared when an external app is used to plan the trip.

• Route planner:

A good planner calculates where to stop, what percentage you will arrive with, how long to charge and what level you will have at the next stop or destination. The most advanced systems also take into account the power, availability and operator of the chargers, and adapt the route if consumption changes during the journey.

• Performance over consecutive charges:

On a long trip it may be necessary to carry out several fast charges on the same day. An insufficient thermal system can make the second or third session slower because of the battery’s accumulated heating.

• Infrastructure compatibility

: It is also worth checking the car’s maximum accepted voltage, its behaviour at 400-volt chargers, the connector type and whether it offers Plug & Charge, a function that automatically identifies the vehicle and handles payment without extra apps or cards.

• 
  Is it better to arrive at the charger with a low battery?
• Generally, yes, because a car usually charges faster when it arrives with a low state of charge, provided the battery is at the right temperature. Entering the station at 8 or 10% lets you make use of the most powerful part of the curve. Arriving at 35 or 40% reduces the margin available before reaching the section where power starts to fall. The stop may be shorter, but less energy will be added and you will probably have to stop again sooner.

Of course, you should not run it down without a margin. The arrival percentage should be adapted to the reliability of the station, the availability of alternative chargers, the weather and the driver’s experience. A correctly configured planner can work with a safety margin of 8 or 10%, but in areas with little infrastructure it may be prudent to keep more in reserve.

Is it better to charge up to 100% or to make two shorter stops?

On most journeys, it is faster to make short stops within the favourable zone of the curve than to stay connected until reaching 100%.

Suppose a car charges very quickly from 10 to 70%, begins to reduce power from that point and becomes especially slow above 80%. The last 20 percentage points could take almost as long as adding the first 50 or 60. In that case it is more efficient to:

Arrive with a low battery.

Charge only the energy needed to reach the next station with a margin.

Resume the journey once the power has clearly dropped.

Repeat the process at the next stop.
2. Two sessions from 10 to 60 or 70% may allow you to cover the same distance in less time than one prolonged charge up to 100%. Moreover, disconnecting earlier frees the charger for another user and avoids staying in a range where the price per recovered kilometre may be less favourable if the operator applies a time-based tariff or occupancy penalties.
3. When can charging to 100% make sense?
4. There is no universal rule. Completing the charge may be reasonable in certain circumstances:

Before starting the journey, by charging slowly at home.

When the next leg exceeds the available range.

If the following charging network is sparse or unreliable.

During a long stop that would have happened anyway.

On arrival at the destination, if the car will be used little afterwards.

When the charger offers an especially favourable price and there are no other users waiting.

• The key is to distinguish between reaching 100% before setting off and doing so in the middle of the route. At home it matters little if the final stretch is slow, because the car can stay plugged in for several hours. At a roadside charger, every additional minute counts, and waiting for a full battery usually isn’t worth it.
• It should also be borne in mind that

keeping a lithium-ion battery at a very high state of charge for long periods can accelerate its degradation.

• For everyday use, many manufacturers recommend setting a limit close to 80%, reserving 100% for trips or situations where full range is needed. The exact effects depend on the chemistry and the management developed by each brand.

The best strategy is not to charge more, but to charge what is necessary

On a journey with an electric car,

the optimal stop is not always the one that leaves the battery at 80%. Sometimes it is enough to arrive at 55 or 60% to reach the next charger, where the car will once again use the fast zone of its curve. Route planners usually optimise precisely this balance. A ten-minute stop may be enough to carry on to a better-located station, with more power or more connectors available.

The fastest strategy usually consists of leaving home with a fully charged battery, arriving at the first station with a low level and making short top-ups, taking advantage of the most favourable part of the curve. But the human factor also comes into play. A family may prefer a 30-minute stop for lunch to two of 15 minutes. Another driver will appreciate stopping every two hours, even if the car could carry on. And some will prioritise stations with services, toilets and restaurants over absolute travel time.

The best fast charging is not necessarily the one that records the highest power peak, but the one that fits into the route without forcing changes to normal rest breaks.

The five key figures for comparing fast charging

When choosing an electric car for long trips, review the following, in this order:

Real motorway consumption and usable range between stops.

Real-world 10-to-80% charging time.

1. Shape of the charging curve and average power, not just the maximum peak.
2. Preconditioning and the quality of the route planner.
3. Behaviour in cold, heat and consecutive fast charges.
4. The 800-volt architecture is an important technical advantage, especially in vehicles with large batteries and very high powers. But it does not automatically make a car a good long-distance traveller. An efficient electric car, with a battery of reasonable size, a stable curve and a well-calibrated thermal system can be faster on the road than another with a higher homologated range and a higher peak power.

The conclusion is simple:

5. when buying an electric car, do not just ask how many kW it charges at, but how many kilometres it lets you recover during the minutes you are willing to spend stopped. That is the figure that ends up making the difference when it comes to travelling.