I heard something today that makes me very skeptical. A dealership claimed that charging an EV with 11 kW has higher energy efficiency than charging with 6 kW. (And they didn’t mean 3-phase vs. 1-phase!)
Even if it is somehow related to battery temperature and receptiveness, that would still be heat loss. With higher currents, resistance increases, too, no? Or is it proportional and always the same percentage?
In any case, I find it foolish to follow such advice because there are other factors involved, mainly a battery suffering more regarding longevity if charged faster, and that is a lot more expensive of a loss, eventually, also because an older battery is generally less efficient in several ways. (internal resistance, self-discharge)
Plus more load spike on the grid, external and house. But I don’t want to rant too much about the lack of reason in the world that seems to be a self-amplifying spiral of doom. Apparently house charger impatience is a thing. Three hours of charging twice a month - absolutely intolerable to some.
The cars power electronics will tend to have it’s peak efficiency at close to its maximum output. This is how most such power electronics work out.
Additionally if it is cold the battery must be heated to be charged whic reduces efficiency more the slower a charge gets, but this is mostly an issue in freezing temperatures so it might not apply.
Rest assured that any charge speed less than 0.2-0.1C will be neglible wear due to charging speed. Essentially any charging on AC will be slow charging for most cars.