DC Fast Charging: battery SOC vs power absorbed

[ChrisC]

[I really wish I could post an image to this forum, rather than having to jump through an image server hoop. Oh well, I guess it makes the site easier to admin.]

For the last couple months, I have been collecting data each time I do a DC Fast Charging session. As an electrical engineer, I was interested in where the rampdown thresholds were in the i3's charging curve. This data can be used to calculate how long it will take to charge, from any starting SOC and with precision. This curve is basically a fundamental characteristic of the i3's battery system; other cars like the Leaf and Model S will have similar (but different!) curves.

Here is the graph:

https://www.anony.ws/image/DxsQ

Observations:

- the 7 "higher" runs were done on an Efacec cabinet that delivers 50 kW peak; as you can see, it hits that reliably

- the 2 "lower" runs were done on a BMW-branded cabinet that says it delivers 24 kW but actually delivers 21 kW

- during charging, the i3 starts to throttle down from 50 kW when in the 50-55% SOC range (this is the 1st threshold of interest)

- during charging, the i3 starts to throttle down from 21 kW when in the 75-80% SOC range (this is the 2nd threshold of interest)

- both chargers were on pretty much the same curve below 21 kW, although the Efacec might have been faster; this may have been due to test conditions; that one "hot" run in green was very short and the battery may have been cooler

- charging power was greater than Level 2's 6-7 kW all the way up to 92-95% SOC! (this is the 3rd threshold of interest)

Ignore any big zig-zags in the data, those are just outliers or measurement error.

 

[jadnashuanh]

It might be interesting to chart this based on ambient temperatures. This would also take into account the CCS unit's ability to keep itself cool along with the starting temperature of the i3's battery pack. My guess is that come the height of the summer, the curves' slopes may get shallower. I wonder what the power input to the CCS unit is for its rated output...that would dictate how robust its cooling system needs to be.

 

[RJSATLBA]

Your third threshold of interest tallies with my own amateur observation. Some while ago I posted that if on a UK motorway one is in the situation of two rapid chargers out of range of each other with a 7kWh (or 3 phase 22kWh) charger in between, one should stay on the first rapid charger until around 92% if nobody is waiting before moving on as that appeared to be the point at which the charge rate went below 7kWh.