EV charging calculation

Not sure I follow this. The price you pay at the charger is the kWh metered flowing from charger to car.

How much the car uses is usually given as miles per kWh, which like mpg varies with driving style and speed, as well as temperature.

The battery charging level is only really involved in calculation of range.

However, there is a wrinkle, which is that the car may be less efficient at charging as the battery gets fuller, as more energy may be wasted as heat. That means that you'd get slightly fewer miles per kWh metered going in if you regularly use the battery near full charge.

nib

I see where the confusion arose. I assumed the electricity would be metered entering the charging point from the grid. I wasn't looking for the car's range but at how to calculate the cost of charging the battery (as this was the discussion last night)

If I assume the battery to be 50 kWh, charging from 20% to 80%, and the unit cost 80 pence, this would be 0.6 x 50 kWh = 30 kWH x £0.8 / kWh = £24. Is this correct? I assume no heat loss as the battery is nowhere near full charge.

No, a full charge is 0% to 100%. For NMC batteries, it causes less wear to keep them in the 20%-80% region, for LFP batteries it doesn't matter.

Due to the way the battery chemistry works, the first %s and the last %s charge slower than the middle %s (at least for NMC, I think LFP is less so). So if you're a motorway driver you might only charge to 80% and be on your way, rather than hanging around while it more slowly climbs to 100%. It can arrive at the next charging station with a low state of charge and quickly take some charge there. 'Splash and dash' rather than brimming it each time.

The charging planner apps (eg Zapmap, A Better Route Planner) will calculate a route with optimal topups along the way - eg it might say just to stop for

If you're driving a long trip, it makes sense to charge to 100% before you set off - the extra wear caused by slow-charging overnight on a home charger isn't signficant if you don't do it very often.

Indeed. How much losses there are before the charger depends on where the energy comes from - if it's a solar panel on the roof of the motorway services stored in a local battery, perhaps very little.

I've not heard that one. The car will back off the charging current as the battery approaches full, but I wasn't aware there was more heat generated. I'd expect the cooling system would going into overdrive to dump the heat?

With other lithium ion batteries (eg phones) when it's fast charging at a high current (eg 0% to 60%) is when it's warmest, and once the charge rate has slowed down above 80% it's cool - it's the current that causes the heating.

Theo

I think that just saying you shouldn't aim to get close down to 0% every trip and you shouldn't charge to 100% every charge, but if you're on a long trip, just do what you need to do.

if you arrive with 20% and leave when it hits 80% then yes, but it'll be difficult to ensure you arrive with exactly 20% and it depends how far away your next planned stop is whether you're convinced you'll get there on 80%

You'll only be charged for what makes it through the transformer and charger, any losses are fr the charger operator to worry about

It's per kWh dispensed. There might be some small losses (<1%) in the cable, but a DC rapid charger skips the voltage converter inside the car and connects directly to the battery so the power dispensed almost all goes into the battery. An AC home charger might lose a few percent as it goes through the voltage converter in the car, perhaps a bit worse at lower currents.

(ie DC charging is billed on the *output* of the power conversion electronics, AC charging is billed on the *input*)

At rapid charging kind of powers (~100kW), anything that isn't very efficient would get very hot - even 1% is 1kW. So unless something has active cooling you can assume there are no significant losses.

Yes. If the car is capable of say 4 miles per kWh that 60% charge would get you 120 miles of range to the next charging station. If you charged the battery up to 100% overnight (or hung around the motorway services for perhaps an hour) the range would be 200 miles.

Theo

The cost can vary hugely depending on time of day, speed of charging, payment method, if you have a subscription, which app you use, its a minefield.

on a long run where you charge on the way you might relax these limits...

generally not significant.

The Octopus app says here its about 0.75p/kw hour which is expensive, so for my id.5 which has an 80kWh battery 0.6 x 80 = 48kWh = £36. This might not be 100% correct as there can be a connection charge per charge, plus a unit rate.

At home I am on a standard tariff so 24p kWh so £11.52

Dave

It's not so much the absolute heat produced, it's the proportion of the current from the mains that ends up as charge in the battery. Charging typically slows approaching 100% so it doesn't get even hotter but it's still wasting a higher proportion of the reduced power.

It's hard to be sure on my car as the battery % is not always linear, eg it takes more miles from 100% to 99% than any other 1% change.

nib

The battery capacity is not an exact number, the battery % calculation is not always linear and you always have to put more in that you get out, so that calculation will give you a rough idea of what you'd pay but no more.

nib

I think that actually loss of efficiency is more down to charge *rate*. High currents heat the battery rather than charge it, via the EyeSquaredR formula. As do high discharge rates.

Trickle charging overnight is probably the best esp. if you have cheap night rate leccy

For LIPO that is the case. no excessive heat. For lead acid, you start to get hydrogen produced. I cant remember the case for nickel batteries.

Its ridiculously cheap. Not even nuclear power is that cheap.

-- “It is not the truth of Marxism that explains the willingness of intellectuals to believe it, but the power that it confers on intellectuals, in their attempts to control the world. And since...it is futile to reason someone out of a thing that he was not reasoned into, we can conclude that Marxism owes its remarkable power to survive every criticism to the fact that it is not a truth-directed but a power-directed system of thought.” Sir Roger Scruton

Tebay was the subject of our discussion:

To follow up on that, I record the kWh per % battery added for each charge and it's always bigger on the few occasions when I charge to

nib

Absolutely, Octopus quoted me 7p per Unit. I think it is well recognised that ultra-fast (DC) charging points are very substantially more expensive than domestic chargers. I would say the 'business model' for an EV has to be premised on doing nearly all charging at home with occasional top-ups on occasional long trips.

I suspect they mean 3/4 of a GBP.

Even then its a challenge on which tariff to select. As we are both retired and are at home all day its hard to move a lot of usage without buying a battery so it seems the extra you pay for day-time usage outweighs the saving on off-peak.

I probably only charge once a week so still on a normal tariff.

Dave

I'm not retired but am at home most of most days.

I only ever charge the car overnight at home, at least one full charge and a couple of top-ups per week.

Washer/dryer can't necessarily fit a full cycle into 5 hour slot, I tend to start the wash on peak rate, so there will be some water heating, I presume the drum agitation and pumping is negligible, but the spin/drying is the bulk of the cost ... not much opportunity for line drying this time of year and I never dry on radiators.

Oven (not air fryer!) maybe one day in three, half an hour inc. pre-heat

I daresay fridge/freezer is a chunky consumer overall?

I'm quite lazy in terms of leaving lots of IT kit on 24x7, so kind-of evens out (20% of those units at 24% the price)

All lights (except garage, shed and loft fluoro tubes) are LED, so could stay on 24h for peanuts.

And what tells you how 'full' the battery is?

Well I was getting to that. Like back in the day all Ford cars had a really lively throttle response... ...for the first 1/4". After that not so much. Ford owners thought they were 'more powerful'...