EV Charging losses and Vampire drain.

JKenH

michaels said:

JKenH said:

michaels said:

When granny charging I calculated miles per kwh using the reading from an energy meter on the plug socket.

I always calculate my mpg by dividing the miles covered by the number of gallons of petrol to refill. You just can’t rely on onboard fuel gauges. I am sure manufacturers gear them to overread and I suspect that some (if not all) EV manufacturers do the same. My Golf isn’t too bad, overeading mpg by 4-5% but my wife’s Kia is about 10% optimistic. 

My son in his Leaf arrived at our house, earlier this month, having used 79% of his battery (indicated 97 down to 18) covering 100 miles. The Leaf said it was doing 4.2 mpk. It took 31.6kWh from the Zappi to recharge to 100% (adding 82%) so would have needed about 30.4 kWh to put back the 79% used. 100miles/30.4 = 3.3mpk (compared to the 4.2 reported by the car as being used). 

It is possible the car may still be correct (or within a few%) when it says it has used 4.2mpk which would equate to around 24kWh and the extra 6 kWh may by accounted for by charging losses - around 80% efficiency. 

The only issue is that while the car  may only be using 4.2 mpk then, unless, like you do, you measure the actual energy coming from the plug you may get a false sense of the cost per mile. On a cheap overnight tariff it’s probably only costing a fraction of a penny per mile so neither here nor there compared to running an ICE car but, personally, I don’t like to see the headline mpk figure quoted from the car’s dashboard and then applying this to home charging tariffs. Fair enough if you are topping up at a much more efficient DC charger, it probably does give a reasonable indication of cost per mile but it’s not right to mix the two.

I imagine as this is a money saving forum, many of us do keep accurate records of energy in and energy out but, beyond your comment that you measure your mpk from the plug, I don’t recall many instances (if in fact any) of this basis of calculation being applied. It is more frequently, (not just on here but on many forums), stated that my car reports X mpk and my home tariff is Y pence per kWh so the cost per mile is Y/X without any further adjustment/comment. (And the same sort of thing, of course, happens with ICE drivers reporting running cost based on mpg indicated by the car.)

Bit of a ramble, sorry.

Or 'proper' charger reports throughput but I don't think it is very accurate (and it is unclear whether it records energy drawn from the grid or sent to the car (it is a DC charger so the conversion losses will be in the charger).  Do you know if your zappi is accurate - could you turn everything else off and compare against your smart meter?

I have always assumed with DC chargers you did actually just pay for the energy delivered after the AC/DC conversion so they are better value (assuming the price per kWh was the same - there was a time when you could DC charge at Tesco for 28p - cheaper than the SVT)

Presumably there will be some losses from heat in the charging cable to the car (I believe DC charging cables are liquid cooled) which are to the consumer’s account and I imagine there are some losses, perhaps minor from resistance in the electrolyte. (I understand that batteries get hot during charging (and also during discharge). I think the article I linked mentioned heating the battery to optimise the charging, presumably where there is thermal management. In cold weather you would expect charging losses to be higher. I might have some data from way back of DC charging sessions and LeafSpy records but it will be hard to find. 

No, unfortunately I don’t know whether my Zappi is accurate and I had never thought to try and check, although it should be possible, as you suggest, by switching off everything else and measuring the grid draw over say an hour (but it would have to be after dark).It’s an interesting idea and I wonder if anyone has tried that? I can’t do that at the moment as I am between EVs. You would also need an accurate measure of charge added to the battery to calculate the losses. LeafSpy would be capable of measuring that. 

One of the problems we all seem to face (whether talking about EVs, solar panel inverters, batteries or heat pumps) is having accurate data to work with (outside of smart meter and generation meter which we presume to be accurate). Just as with the mpg of cars manufacturers’ data can’t always be relied on. 

I think the real problem area for charging losses is with the 10A granny charging. I think with some cars you can set the charging rate as low as 6 amps (to, say, take advantage of solar panels) so you could get very significant losses

JSHarris

Many rapid chargers use the AC input power for billing - probably the majority of them.  They vary a bit in power conversion efficiency, plus they have additional losses like running the active cable cooling system (many use coolant pipes inside the charge cable to be able to increase the safe capacity of the cable).  They expect customers to pay for all the energy used, whether that goes into their car or not, which seems fair enough (it's much the the same at petrol stations - the customers pay for all the overheads).

Grumpy_chap

Thinking around this, how does one measure the kWh used in feeding the charger?

JKenH

Grumpy_chap said:

Thinking around this, how does one measure the kWh used in feeding the charger?

In the case of a Zappi charger the amount drawn from the grid appears on the screen. In this case 34.72kWh.

Edit: The previous day I had started out with a full charge in the battery (100%/37kWh from LeafSpy)  and drove 125 miles - the charge remaining was 7.4kWh so the consumption was 29.6 kWh. The car showed 4.3mpk whereas the actual consumption was 4.22mpk - the car read out proving reasonably accurate. 

The following day’s charge was also to 100% but LeafSpy recorded the charge in the battery as only 36.7 kWh. LeafSpy thus told me that 29.3kWh were added to the battery compared to 34.72 drawn from the grid.  (84.3% of the amount drawn from the grid). The charging loss was therefore 15.7%. 

So to replace the electricity used to cover the 125 miles the previous day required a draw of 34.72kWh from the grid. The effective mpk was therefore 3.6mpk. 

So do I say my car’s consumption is 4.3 mpk or 3.6 mpk? Both are correct in their own way.

Grumpy_chap

Thanks @JKenH
I guess I will have a Tesla charger, given they are including it with the car.
I think the Zappi is better.
I guess, until the charger is installed, I will need to use a Granny Charger 

Grandad2b

JKenH said:

michaels said:

When granny charging I calculated miles per kwh using the reading from an energy meter on the plug socket.

I always calculate my mpg by dividing the miles covered by the number of gallons of petrol to refill. You just can’t rely on onboard fuel gauges. I am sure manufacturers gear them to overread and I suspect that some (if not all) EV manufacturers do the same. My Golf isn’t too bad, overeading mpg by 4-5% but my wife’s Kia is about 10% optimistic. 

...

I imagine as this is a money saving forum, many of us do keep accurate records of energy in and energy out but, beyond your comment that you measure your mpk from the plug, I don’t recall many instances (if in fact any) of this basis of calculation being applied. It is more frequently, (not just on here but on many forums), stated that my car reports X mpk and my home tariff is Y pence per kWh so the cost per mile is Y/X without any further adjustment/comment. (And the same sort of thing, of course, happens with ICE drivers reporting running cost based on mpg indicated by the car.)

Bit of a ramble, sorry.

We can do this because the pump shows the amount of fuel delivered and we trust the odometer in the vehicle. I've never taken any notice of the car's claimed fuel consumption. 

Until leccy vehicles show the energy delivered we can't do the same.

(Going right off topic - I do hope that drivers who start the engine and move off or arrive at their destination and switch off get much better fuel consumption figures even allowing for the amount of time spent stationary in traffic on far too many journeys. Most drivers just don't care - they park up with the engine running to keep the aircon or heating going or maybe just because they don't pay for their own fuel).

Martyn1981

JSHarris said:

Many rapid chargers use the AC input power for billing - probably the majority of them.  They vary a bit in power conversion efficiency, plus they have additional losses like running the active cable cooling system (many use coolant pipes inside the charge cable to be able to increase the safe capacity of the cable).  They expect customers to pay for all the energy used, whether that goes into their car or not, which seems fair enough (it's much the the same at petrol stations - the customers pay for all the overheads).

That's true, but as ever, Tesla are a bit different. They show and charge for the energy that reaches the battery, or I assume, reaches the battery or is being consumed by any battery management heating/cooling.

This did lead a lot of people to wrongly thinking the Tesla superchargers were ~99% efficient, when in fact losses were before the billing count.

Of course it's irrelevant as regards the final bill, as those losses will still be reflected in the unit price, as you have to pay for it one way or another, but will mean that for cost comparisons against most other rapid chargers, the Tesla will be a little cheaper than the prices suggest.

Qyburn

JKenH said:

I have been away on holiday for a week and took battery readings using LeafSpy before and after. My 40kWh Leaf has lost 0.4kWh over 8 days or around 50wh per day which equates to a drain of around 4 watts per hour.
...

I’m quite happy with that and imagine it is comparable if not better than a lot of modern ICE cars. 

.

Assuming "4 watts per hour" just means 4W then thats not quite right. 0.4kWh in 8 days is equivalent to just over 2W for 192 hours. 

Not sure about the cost comparison but a Freelander 2 draws 26mA when shutdown, at 12V that's 0.3W. Quite a bit less electrical loss, but probably more expensive to regenerate.

1961Nick

The biggest factor in charging efficiency is the temperature of the battery when charging commences. In winter it can take 20 minutes to heat the battery sufficiently before charging will actually start - even then it's at a reduced rate with some of the energy still going to heating. If you're using Intelligent Octopus with breaks in the charging schedule, the battery may require heating several times during the 6 hour period resulting in losses of 25%.

JKenH

Qyburn said:

JKenH said:

I have been away on holiday for a week and took battery readings using LeafSpy before and after. My 40kWh Leaf has lost 0.4kWh over 8 days or around 50wh per day which equates to a drain of around 4 watts per hour.
...

I’m quite happy with that and imagine it is comparable if not better than a lot of modern ICE cars. 

.

Assuming "4 watts per hour" just means 4W then thats not quite right. 0.4kWh in 8 days is equivalent to just over 2W for 192 hours. 

Not sure about the cost comparison but a Freelander 2 draws 26mA when shutdown, at 12V that's 0.3W. Quite a bit less electrical loss, but probably more expensive to regenerate.

Yes, well spotted, 2w it should be.