Well, the Chargepoint app says “Based on 2024 bZ4X”, so it is adjusting the amount in some way.
At peak rate, the 6.6 kW L2 EVSE in 7 hours and 27 minutes could deliver 49.17 kWh at most.
Their number of 44.3518 kWh is 90.02% of 49.17 kWh. That’s about the correct factor to adjust for the efficiency of the on-board L1/L2 AC charger on the 2023-2025 bZ4X/Solterra/RZ.
So that’s where that number comes from. It’s pulled out of thin air, based upon how much (at peak rate) that EVSE
could supply.
Now, for what the car says…
Net capacity is around 66 kWh, maybe between the so-called 0% and 100% levels shown (not actual state of charge of the pack, but what the car typically shows) there’s 64 kWh. So 100% - 44% (displayed) is 56%. That’s 35.84 kWh (I shouldn’t show that many digits because the resolution of the displayed charge level is 1% units.
The car says it added 37.296 kWh to the battery pack. Well, within the rough estimates we have a number that’s pretty close.
The on-board AC charger is about 90% efficient, so let’s divide the 37.296 by .9. That’s 41.44 kWh. I suspect that is roughly the actual amount of AC energy used to get the 37.296 kWh increase in battery charge.
But the difference between 49.17 kWh (my back-calculated amount of energy that Chargepoint app probably thinks was actually, possibly, maybe consumed) and 41.44 kWh… well, let me address
that difference.
When the car is at 44% net state of charge, on a 6.6 kW L2 charger, it will pretty much pull the full 6.6 kW, unless you tell the car to limit AC charging current. But after about 90%, the actual amount it consumes ramps down as 100% net state of charge is approached.
Here’s a historical L2 charging session (at home, also 44% to 100%!) on a 40A EVSE that does the full 7.2 kW gross rate the car is capable of pulling (6.6 kW net rate after multiplying by 90% for the on-board charger losses):
16:59 is the disconnect time, not the charge suspension time. That charge session ran from 09:41 to 15:50 (369 minutes or 6 hours and 9 minutes). This is obviously less time than the Chargepoint app which also shows the EVSE as limited to 6.6 kW, not 7.2 kW. But, the most important thing is how long the charge takes as it ramps down, as Chargepoint is likely considering that whole time the car is consuming 6.6 kW.
My ramp-down started at 14:51 and ran until 15:50. That’s a full hour that was
not at 7.2 kW. So if your 7:27 session was 6:27 at 6.6 kW (gross) and an additional hour billed at 6.6 kW… it’s cheating you.
Here’s a 90% to 100% (net) charge session on my L2 EVSE:
Actual time 12:10 to 13:41. So 8.0524 kWh (gross) in 89 minutes. Obviously not 7.2 x 1.5 hours.
I can‘t tell from my records what the net state of charge is when the ramp-down starts.
Let’s see if the extra .5 hours of charging can account for the difference between 49.17 and 41.44 kWh. Let’s go at it from the opposite direction and assume that the 7.73 kWh difference is at a 6.6 kW rate. That would be an extra 1.17 hours, not the extra .5 hours spent in the ramp-down portion.
Oh, well, I’m not able to account for all of the difference, only about half.
Anyone have any better ideas? Did I miss something?
