Speak EV - Electric Car Forums banner

1 - 17 of 17 Posts

·
Registered
Joined
·
704 Posts
Discussion Starter #1
There is a discussion elsewhere about average miles/kWh for EVs but it is important to recognise that this is battery power and not the power required to charge the battery (which is what you actually pay for). My guess would be about 85% efficiency in the charging process which means consumption is actually 15% worse than recorded in terms of cost of electricity. Does anybody have accurate figures? I assume the efficiency varies over the charging cycle.
 

·
Registered
2020 BMW i3S 120Ah BEV
Joined
·
439 Posts
This is true. I would guess however that calculating all the losses from power generation through distribution and charging to actual useful engine torque output the process is significantly more efficient than the extraction of fossil fuels, transport, and processing and delivery to a 30% efficient ICE. So at least you are comfortable it's still the right choice no matter what...
 

·
Registered
Joined
·
704 Posts
Discussion Starter #4
So, the answer seems to be that we don't actually know other than a supplier claiming better than 90% for their electronics. The efficiency must vary over the charging process.

The point that I am making is that if I buy a gallon of petrol and I know the consumption of my vehicle in mpg I can accurately work out the fuel cost per mile. For an EV I only have the average kWh used but don’t know what the metered kWh is to charge the battery so cannot compare costs. Best estimate seems to be around 90% efficiency so power in costs 10% more than power used so cost per mile is 10% higher than it may appear.
 

·
Registered
Joined
·
129 Posts
Yes it's often ignored. My 7kw charger works out around 85% efficient when I've done the calculations.
 

·
I'm not crazy, the attack has begun.
Joined
·
28,548 Posts
In the general case for BEVs, from my observations it is between 75% and 92%. You can see there is a wide range of performance, and it should be a prime area for efficiency improvements.

FWIW the 92% efficiency I have seen is from my Ampera. You'd think 10 years later designs would be more efficient but the new battery chemistry with higher capacity are less efficient to charge. My Fluence was around 87%, the Soul is 86%. @cah197 's recent postings suggest the new Zoe is only 80%.

It's been the drivetrain and power systems where manufacturers have put their efforts for efficiency, but overall 'from the wall' there has been little net improvement.

I am afraid I cannot tell you, from experience, where the i3 is in that scale.
 

·
Registered
2020 BMW i3S 120Ah BEV
Joined
·
439 Posts
You only know cost per mile on an ICE because there's a meter in the petrol pump.. put a meter in your charging "pump" and you've got an equally accurate data source. A clamp meter and raspberry pi solution can do this for less than £30.
 

·
Registered
Joined
·
704 Posts
Discussion Starter #8
It seems an anomaly to me that electrical appliances have energy ratings, homes have energy ratings, yet an EV that uses far more than the largest domestic appliance is categorised by a miles/kWh rating which is a meaningless figure without knowing the efficiency of the input power conversion (charging). Of course, the efficiency will vary depending on SOC at start and finish but it should be quite simple to establish a test for a standard situation, say, 20% to 100% charge at a standard ambient temperature etc. The test can then be used as a comparison between EVs and the buyer, motoring press etc. will have a far better set of figures.

There is a well known mantra in process improvement that if you can’t measure something you can’t improve it. At present there is no incentive for manufacturers to design a better charging system.
 

·
I'm not crazy, the attack has begun.
Joined
·
28,548 Posts
It seems an anomaly to me that electrical appliances have energy ratings, homes have energy ratings, yet an EV that uses far more than the largest domestic appliance is categorised by a miles/kWh rating which is a meaningless figure without knowing the efficiency of the input power conversion (charging). Of course, the efficiency will vary depending on SOC at start and finish but it should be quite simple to establish a test for a standard situation, say, 20% to 100% charge at a standard ambient temperature etc. The test can then be used as a comparison between EVs and the buyer, motoring press etc. will have a far better set of figures.

There is a well known mantra in process improvement that if you can’t measure something you can’t improve it. At present there is no incentive for manufacturers to design a better charging system.
They do, but they are bundling in traction and charge efficiency together. WLTP runs the car to flat then measures how much energy to recharge. It doesn't concern itself with what the battery is actually putting out during traction power output.

Have you been in a car dealer recently? Often you will see the placard next to the car showing the 'rainbow' A to G efficiency ratings, like any other appliance ... that's why VED is now classed A to G! The problem, perhaps, is that EVs are all in the 'A' class!! Once the scale expands a little as the bottom performers get pushed further down, we might start seeing some differentiation at the top.
 

·
Registered
Joined
·
704 Posts
Discussion Starter #12
Whilst the WLTP takes input power, the average consumption displayed on the car’s computer in miles/kWh is presumably output kWh so has to be increased by around 15% for charging losses.
 

·
I'm not crazy, the attack has begun.
Joined
·
28,548 Posts
Whilst the WLTP takes input power, the average consumption displayed on the car’s computer in miles/kWh is presumably output kWh so has to be increased by around 15% for charging losses.
It depends on your objective. If you want to know how much you are spending, yes, if you want to know how far you can get, no.
 

·
Registered
Joined
·
704 Posts
Discussion Starter #14
People ask me what it costs to run and since I pay 5 pence per unit (off peak) and my average efficiency over 5000 miles is 4.2 miles per kWh, it costs me 1.2 pence per mile for electricity - except that it doesn't. I need to add around 15% to allow for charging efficiency so around 1.4 pence per mile. Just seeking clarity.
 

·
I'm not crazy, the attack has begun.
Joined
·
28,548 Posts
Yes, that is correct.

But bear in mind your tyres probably double that cost!

I used to pay 3.9p/kWh with the Ampera, which was delivering ~5mi/kWh for me. ~0.8p/mile in electricity and ~1.2p/mile in tyres (£480 of tyres for ~40k miles).

It changes one's language. Example "I was caught in a diversion and wasted fuel driving the wrong way" becomes "I was caught in a diversion and wasted tyres...".
 

·
Registered
Joined
·
129 Posts
It's interesting how different electricity costs alter each persons version of 'mpg'. I use a 30x multiplier of miles/kWh to give me a comparable mpg (£1.25 per gallon at 16p per kWh and 85% efficiency gives me 30 kWh). So 4.0 miles per kWh is approximately 120mpg. However I use that because that's about the average rate for electricity. Obviously overnight rates can be much lower as noted above.
 

·
Registered
2020 BMW i3S 120Ah BEV
Joined
·
439 Posts
It seems an anomaly to me that electrical appliances have energy ratings, homes have energy ratings, yet an EV that uses far more than the largest domestic appliance is categorised by a miles/kWh rating

At present there is no incentive for manufacturers to design a better charging system.
The EV also tells you it will pull max 2.3kw on a normal charger and meets that commitment for the vast majority of the charging cycle.
I think that there is increasing focus on "miles per hour" charging rates tho, which will provide incentives to be more efficient overall, particularly as batteries get bigger, cars get heavier and it doesn't suddenly get any cheaper to have three phase fitted in consumer premises. It may not be too long before standardised reporting procedures come up for this.
 
1 - 17 of 17 Posts
Top