Charging (In)Efficiency

I've been surprised how much power it takes to charge as well. We put in solar panels around this time last year, before we had the SE, and I got them to add a couple panels to the original quote. I made a WAG based on the battery size and my commute, but I was way off. We may add a couple more panels down the road and try to get back to a $0 electric bill. Though I need more data to figure out how many more panels to add.

Is Level 2 charging any more efficient? I've just been using Level 1 since I only drive about 20 a day.

 

I would not think that 60% efficient is terrible. To what are you comparing that “efficiency”. Recharging batteries on a laptop? An electric razor?

There are many aspects of energy consumption in any EV that need to be considered. There are losses in tire rolling resistance, weight and aerodynamics of the vehicle, heating and cooling of the HVAC and a dozen other aspects. Putting your foot down on the go pedal also results in energy losses and heat in the motor as well as the battery.

What is the efficiency of an ICE Cooper? How much of the actual energy in the gasoline is actually delivered to the driving wheels? Think about how much of that energy is simply waste heat that needs to be dumped through the radiator.

It would be lovely to have higher efficiency and BMW/MINI would most certainly be working on that - as are I am certain every EV maker is working to get there.
Just remember that there is no such thing as 100% conversion from potential energy form to kinetic energy - if there was, we would be looking at a perpetual engine.


Signed
Mike Wazowski
Using Frank’s iPad Inside EVs

 

This study based on 2 Leafs and 2 Volts showed an 83.8% charging efficiency for level 1 and 89.4% for level 2. A 60% charging efficiency seems dismal in comparison.

https://ieeexplore.ieee.org/document/7046253

 

Not super surprising if you’re on L1, I think there’s a static load of about 400W when you’re charging, and if you’re using the included charger it can only pull 1200W from the wall.

 

It’s not 60%.
I know it does seem that way if you use percentage of battery to figure out kWh used but those numbers don’t seem to add up.
You can see this when you compare to the cars reported miles per kWh on a trip vs the battery percentage loss, it just doesn’t line up. The Mini’s stats reporting is terrible.

AC to DC conversion at 120v seems to be about 80% which sounds reasonable to me as a basic 80+ based efficiency PC power supply is about the same.

Reports also indicate that 240v L2 is much closer to 88%.

DC fast charging is like 96% or something like that.

So the actual cost of charging your car on the included 120v cord is going to be higher than if you had a 240v L2 unit and personally, for the majority of people when they go EV, that’s what they should have. I feel Mini should have supplied the Turbocord as standard or frankly a better version of that can do 32A.

Bear in mind it’s so cheap to charge an EV at home vs paying for gas the loss doesn’t really matter financially. It’s more a matter of principle.

The rating for the car in terms of mpge etc is from battery to wheels. Otherwise the figure would be different for L1, L2 and DC fast charging and that would confuse people even more than this stuff already does.

 

Interesting - do you have a better way of accurately identifying kWh used? Lots of people are exporting the charge history and backing into mi / kWh by multiplying miles driven / (% used * 28.9)

 

What was the % state of charge range for your 60% efficiency calculation? I think those numbers could be possible if you are charging 98% to 100%.

Battery temperature does also play a role and I had better results when I charged immediately after getting home rather than leave it in the -10F garage overnight.

 

I've also had this thought. I do basically all my charging in the 80-100% and I'm wondering if it would be more efficient at lower charge levels.

I'm tempted to get a Level 2 charger (I have a handful of 220V outlets in the garage for my power tools), but I also think that low and slow is better for long term battery health.

 

I myself go by battery percentage based on 28.9 usable but I know in my heart it's not correct.

However...

If you care about what comes out of the wall, whether you have a smart EVSE that tells you, or you use the charging history (which btw do line up with each other) then yes, miles per kWh from the wall as a socket to wheels metric is perfectly fine. You're going to struggle to compare that with other vehicles though.

And the state of charge that you start from does come into it, because you are charging to 100% (it does not come into it if you are charging up to 80% like we do on our Tesla).

Basically when you charge up to 100% or close, the car starts balancing the charge across the individual cells.
What happens is, each individual cell is at a slightly different state of charge, so when you charge them up some will hit 100% while others will be stuck at 98% (for the sake of example). As I understand it the car tries to pull power out of those full cells (so yes it is wasted) to bring them down to the level of the lower cells. Then it charges them all up together hoping to hit 100% across all of them (it's actually voltage based but that's being pedantic). It repeats this process until it gets them all as close as it can. This is wasteful but necessary.

So if you charge up from say 90% to 100% this balancing will represent a bigger portion of your electricity session usage than if you charge from 20% to 100%.

Case in point...

Last night we charged from 79% to 100%. This should be 6kWh according to the percentage of 28.9 method.
The charging history and my EVSE say we used 8.0kWh. This would leave us to believe the charger is 75% efficient.

A previous session has us charging from 26% to 100%. This should be 21.4kWh. The charging history says 27kWh. 79% efficient by our method.

So the deeper we go, the most efficient the number appears to be.
At very small amounts of use like 8% the rounding error own the percentage value (ie is 7.4% use, or 8.4? Or does it tick over to 9 at 8.9? It's hard to say) and the rounding up to closest kWh figure in charging history are too inaccurate to calculate meaningfully.

I am using an L2 EVSE.

I'd suggest charging to 80% then unplugging and then check your charging history. Then plug it back in and continue to 100%. I suspect the next session, to 100%, will be less efficient than the one up to 80%.

 

Speak for yourself, I pay nothing to charge our cars.

 

No such thing of course.
But for us being on a free electricity plan from 8pm to 5am is cheaper than our old flat rate plan even though during the day the cost per kWh is higher of course as part of the stick to make us use less during peak times.

To be clear we were on this plan before we even had EV's as for us it was 25% cheaper that way. But that took a lot of analyzing our bills based on hourly usage with our smart meter and also using the house as a thermal battery ie running the AC to chill it down an extra amount by 5am and then it doesn't need to come on until 2pm even in Texas high summer or doesn't even need to come on at all until 8pm at either end of the summer as the house doesn't warm up fast enough. Especially as we set the target temp like 3 degrees warmer than we would normally and just use the ceiling fans which make it feel like it's 3degrees cooler so it feels no different. This reduces the temperature differential between the inside and outside which slows heat transfer and thus the house loses the cold air slower and the AC runs less frequently. This also reduces the amount of times the AC starts and stops which prolongs its life. It's complicated to explain this method but it works for us as we both work so don't even use the ceiling fans unless it's days we are home.

Anyway, adding two EV's which charge overnight means our consumption has increased but our bill has not. So for us, they are literally free.

 

It may have been asked, but temperature matters a lot. Level 1 efficiency is severely reduced below freezing. Much less losses (20-25% less) since we switched to L2 in winter.

 

I think it's an interesting discussion, but, for me, the bottom line is that a full charge on my SE is costing me around $4 per. The equivalent mile range for our Honda CR-V would be around 3.1 full charges of the SE. At current gas prices, that means it's costing me $12.40 to drive the SE compared to $42 on the CR-V. The charging efficiency rating would be nice to know, but I'm very happy with the cost savings regardless.