Charging (In)Efficiency

Public outdoor charging station, maybe the super cold air caused superconducting

My 40-mile trip this morning in -13 ºF weather came out to 3.11 mi/kWh for a total calculated range of 89.83 (using @GvilleGuy's MINI EV Range shortcut). On days like this I add about 10% SoC during the day to make it home since my total trip is 80-85 miles.

 

Here's my level 2 charge session at home overnight (inside unheated garage, Grizzl-E Classic set to 32 A output), I'm showing 86% efficiency: (1.00 - 0.23) * 28.9 / 26 = 0.85588.

 

I don't know exactly the number for fixed losses for EVSEs, but let's say 0.300 kilowatts for easy math. DC fast charging would probably be 0.15kWh for 30 minutes, Level 2 7kW would be ~1.2kWh for 4 hours and Level 1 could be 7.2kWh for 24 hours of charging. It's probably much lower!

 

My electricity company offers off-peak rate of $0.04/kWh for EV charging. Totally worth adding a second meter.

 

I have the Chargepoint Home Flex level 2 EVSE. I selected charging sessions where I used at least 20 kWh. I included some hot summer months along with recent cooler ones in South Carolina. I wish the Mini app could show a more precise value on kWh used.

 

Since those were all going to 100%, did you have cabin preconditioning on? If that power usage is included you'd be understating your actual efficiency

 

No - that was just plugging the car in to charge overnight and walking away. And no departure stuff scheduled, either. I rarely do that. Is that what you meant?

 

Yep the precondition for departure what what I was asking about. Seems weird to me that there is such a huge variation in efficiency. My thought was that there was something else using some of the power but if not the HVAC compressor I'm not sure what else it could be.

 

It's like $0.36 USD/kWh in Germany and $7 USD/gal. In the UK it's about $0.284 USD/kWh and $7.5 USD/gal plus BIK taxable benefits for company cars.

 

There's a really interesting article at https://chargedevs.com/features/a-closer-look-at-minimizing-ac-charging-losses-from-the-breaker-to-ev-part-1/

While every home is built differently (different gauge wire and wire lengths), a 3% voltage drop tolerance could mean line losses of Level 1 81W range while a Level 2 40A could top out at 288W. This is on the extreme side, but the Smart EVSEs and the MINI app won't factor in the line loss. As for auxiliary and parasitic losses for the coolant circulation pump and heat exchanger fan, the article cites a "few hundred watts". Maybe my 300 watt estimate was actually not too far off, but I'd imagine it the load would cycle on and off.

 

There seems to be discrepancies between MINI app and ChargePoint home-flex app on how much juice was used to charge 87% of battery.

Per MINI app ~ 31 kWh used => 81% efficiency: (1.00 - 0.13) * 28.9 / 31 = 0.8110

Per ChargePoint app 29.06 kWh used => 86% efficiency: (1.00 - 0.13) * 28.9 / 29.06 = 0.8652

Not sure which one to trust

 

I noticed that in my Chargepoint app, as well. The kWh values differed from Mini app by a bit.

 

The smart EVSE might not have a revenue grade consumption tracker (less than +/- 0.5% accuracy). You could alternatively put trackers on your main panel and then you can monitor each breaker.

 

I was inspired to make my own spreadsheet to track charging sessions. My estimated $$ is $0.125 per kWh.



Note that the charging on the 1st was at Electrify America. All others were at home using a Grizzl-E at 24A on a 30A 240V circuit.

Rob

 

Yes. It was DC fast charging. I think the peak I saw was 47 kW (50 kW max theoretical for the Mini). I posted the charging session afterwards.

Rob