When someone chooses an EV charger for the first time, the question is almost always the same: “How many kilowatts does it have?” The logic is understandable – more power sounds like faster charging. Yet the charger’s rated power is only one link in the chain, and often not the one that sets the limit. To understand what actually determines how long charging takes, we need to start with the difference between two approaches: alternating current (AC) and direct current (DC).
Two Paths to the Same Battery
A vehicle battery stores energy in electrochemical form, and it can only be charged and discharged with direct current. This means that somewhere along the path from the distribution grid, which supplies alternating voltage, to the battery terminals, rectification must take place – the conversion of alternating current into direct current. The entire difference between AC and DC charging comes down to a single question: where this rectification happens.
With AC charging, alternating current reaches the vehicle, and the rectification is handled by a converter built into the vehicle itself – the onboard charger (OBC). With DC charging, the rectification is performed within the station, so the battery receives current that is already direct, regulated according to the vehicle’s demand. This seemingly technical detail has a decisive impact on the power at which the battery can actually be charged.
Where the Bottleneck Is
Suppose a 22 kW AC charger is installed, with the expectation of exactly that speed. If the vehicle has an onboard charger rated at 11 kW – which is the case with a large number of models – charging will be limited to 11 kW, regardless of the fact that the infrastructure can deliver more. The limit is set by the weakest link in the chain, and with AC charging that is almost always the vehicle’s OBC.
This is why an AC charger should be sized according to the fleet that will use it, rather than the highest figure in the specification. DC charging bypasses this limitation: because the rectification is performed in the station, it delivers considerably higher power directly to the battery terminals, circumventing the modest OBC. Hence the figures of 50, 150, and even over 350 kW found on fast DC stations – levels unattainable with AC charging.
Power Is Not the Only Factor
Even when the charger and the vehicle are matched, the actual speed is determined by the behavior of the battery itself. Charging follows what is known as the CC-CV profile: in the first phase an approximately constant current flows, and as the cell voltage approaches its upper limit, the system transitions to a constant-voltage phase in which the current gradually tapers off. For this reason, charging slows down noticeably after roughly 80 percent of capacity, which is why a figure such as “10 to 80 percent in 30 minutes” says far more than the rated power alone.
Temperature is another factor that is easily overlooked. At low temperatures the charge acceptance of the cells decreases, so the battery management system (BMS) limits the charging current to prevent damage – which is felt in practice as slower charging during winter. The real charging speed is therefore almost never equal to the maximum value declared on the device.
When AC, and When DC
The choice depends above all on how long the vehicle remains in one place. AC chargers rated between 7 and 22 kW are optimal where the vehicle is parked for hours anyway – in residential and commercial buildings, hotels, or parking facilities. Charging overnight or during the working day easily covers daily needs, with a lower investment and simpler installation.
DC charging comes into its own where dwell time is short: roadside stations along traffic corridors, fleets with an intensive operating rhythm, and locations where vehicles turn over quickly. There, the higher investment is justified by the throughput such a solution enables.
Conclusion
Charging speed is not a property of a single device, but the result of an entire chain – the charger, the onboard charger, the battery’s characteristics, and the operating conditions. The highest rated power means little if another link in the chain does not support it.
That is why the right question is not “how many kilowatts,” but “how much power can the vehicle and the specific scenario actually use.” The answer to that question is the starting point for selecting a solution that works as expected – without unnecessary slowdown and without over-investment.
