2019 Kona EV charger slows to 1kw on fast charger at around 90%

No idea if it's still there, I haven't gone past 80% on my new battery. It did look like this Dec 2020, 10 minutes at 82% SoC.

 

Don't all EVs require occassional charging to 100% so they can perform cell balancing, which supposedly extends battery life?

 

I suspect the BMS can read all 98 3-P cell group voltages in a matter of milliseconds. It would record those and check again several times over the 10 minutes to look for any unusual pattern of behaviour. With the addition of the few (4?) temperature sensors the BMS has no other tools available to understand battery status. Even worse, the voltage resolution appears to be only 0.02 V.

Correct cell balancing maximises range because all 98 3-P cell groups wired in series will contribute fully over the same voltage range from 0-100% SoC, that being 3.16 to 4.16 V for each group.
If one parallel group is significantly lower or higher than the others, the pack as a whole will not be able to either charge or discharge past when the worse-case group(s) hit the edges of that voltage range.
It's not necessarily harmful being out of balance but the presence of that situation in one group very likely indicates a problem.
There was an example of this on the Ioniq 5 group yesterday (screenshot of a few groups below). The pack could be disassembled and the rogue cell(s) brought into line manually but the large deviation certainly indicates that those cell(s) need replacement. The way that's handled today means replacing the entire pack.

Does balancing only take place at 100%? Hyundai haven't told us unfortunately and we don't know exactly what method it uses. The one thing to remember is that it can take some time, so it's unlikely to be happening during a gap in charging. It also generally needs to check open-circuit voltages to understand what’s needed which means the pack must be unloaded for some time, a portion of an hour at least.
The BMS runs off the 12V system and may take the opportunity while parked to assess the pack then apply small corrective measures (resistances) that take action while charging and/driving. When parked again it might reassess its determination and make corrections. So, it could be a process that takes place over hours.