Electric cars charge more slowly the closer the battery is to full. This response is called the charging curve. I was confused about what I was seeing and was wondering where the power goes if it’s not charging the battery. Turns out it’s never consumed in the first place: the car requests less and less power as the battery gets full.
Level 3
Level 3 DC chargers dump a lot of power fast into the battery. This forum discussion from May 2022 has a graph of the charging curve for the C40 and the XC40.
This graph shows charging power going from 150kW down to 50kW as the car charges from 10% to 80% in more or less a straight line, then further down to 25kW or so at 90%.
Level 2
Level 2 AC charging at home is providing a lot less power, 10% or less the wattage of Level 3 (typically). Here’s the curve going from 80% to 90% as measured by my Emporia home charger.
This shows charging power going from 9.2kW to 4.4kW. The drop started when the car was about 95% full and is not instant but ramps down over the course of 5 minutes or so. (The graph looks identical for amps.)
What seemed weird to me is that the Level 2 power draw stays at 9.2kW (38 amps) pretty much all the way up to 95%. I was expecting it to ramp down like the Level 3 does. But I guess the Level 2 power is so much less than the Level 3 power that isn’t necessary.
What also seemed weird to me is I thought on Level 2 my car charged faster going from 40–50% than from 80%–90%. But it’s drawing the same 9.2kW, so where was that extra power going? But I realize now I don’t really have a good measurement of that. I’m just eyeballing the time. And those battery percentages the Volvo reports are not particularly accurate measurements of how much power is actually stored in the battery.
I still don’t understand the underlying physics and chemistry of what’s going on to make charging slower when it’s nearly full. My hand-wavy explanation is the resistance on the battery goes up, so if you try to force too much current through a nearly full battery it gets wasted as heat instead. (Or worse, starts a fire). So the charging circuitry only requests as much power as it can safely put into the battery without overheating it. I think this is all some fairly complicated digital control logic but maybe it’s as simple as a circuit that responds to resistance.
Anyway for Level 2 the charging curve effect is pretty negligible. And not visible at all if you only charge your car up to 90%, as recommended. It’s more important at a Level 3 charger but the systems take care of it automatically for you.
Nelson's log 