Simple – it’s not really a 50kW charger. Eh?
Today’s chargers such as Polar’s Ultracharger, and most other rapid chargers, in the UK at least, will deliver, at most, 125 Amps*. Whether that translates to 50kW, depends on the voltage of your car’s battery. (Power = Current x Voltage). Take into account some loss along the way, and 48kW is likely to be the maximum you’ll see.
*A few higher current chargers have been installed, eg by Alfa, and we are promised more…
Another key variable is the battery pack temperature. At low and high temperatures, the Battery Management System will reduce the charge current to protect the battery. (The same also applies to regen braking – as it’s also charging the battery).
So, the power delivered depends on the voltage of the battery pack. A typical 96 series cell, “360V” pack, will really vary between 330V and 400V depending on the state of charge. Generally when you want to charge your battery, the voltage will be low – hence at the start of a charge, as long as your car can accept the full 125 amps, you can expect less than the full 50kW. It’s just the math(s)- for example starting from 14% state of charge I’ve seen a pack voltage of 335V using the “Torque” app;
125A x 335V = 41.8kW.
What you end up with, is a slowly rising charge rate as the battery charges and the pack voltage rises. Then the BMS slows things down, maybe by switching to voltage-controlled charging, to stop any cells getting overcharged beyond 4.1V and possibly damaged. In the Ioniq this is typically at 77%; in the Leaf24 it was around 50%.
When we get 200A+ chargers to take advantage of the Ioniq’s 200A rapid charging capability, then the power will go up – 200 x 335 = 67KW.