EV Charging at Home minimising cost
Discussion
I have recently purchased an EV and have a home charger. I have noticed like my phone and laptop it takes longer to charge the last 90% of charge. I have not measured how much extra time it takes but if it was to take an extra 20% to charge the last 10% then that would mean I am using and extra 2Kw of power to charge the last 10% (the charger supplies 7Kw per Hour). I use the car for commuting and use about 30% of power on one days commuting so instead of topping the battery up every day I could charge it every other day which would potentially save me paying for 4Kw a week, over a year that could be a saving of 200Kw which represents a saving of £20 a year.
Does anyone know if this is correct?
I plan to monitor amount of power used for charging over a week, charging daily one week and then charging on alternative days the next week. The problem with this is driving conditions can affect the power consumption, when it is cold more battery power is used and the length of time of commute is affected by traffic. so to accurately determine which is best and by power much power is saved I will need to repeat the two week test a number of times. So I hope to return and report the difference in a few months.
Does anyone know if this is correct?
I plan to monitor amount of power used for charging over a week, charging daily one week and then charging on alternative days the next week. The problem with this is driving conditions can affect the power consumption, when it is cold more battery power is used and the length of time of commute is affected by traffic. so to accurately determine which is best and by power much power is saved I will need to repeat the two week test a number of times. So I hope to return and report the difference in a few months.
I'd be surprised if your charger is drawing the full 7Kw for the last 10% of the charge.
We have a smart meter, and for the first 90% of the charge, you can see it drawing full power, however when the battery gets to just over 90% the amount of electricity used is gradually reduced until it reaches 100%.
So I pretty sure you are using the same amount of electricity to fill the last 10% of the battery as the rest of the battery, except that the charger is delivering a much lower rate for the final 10%.
I think it designed like this to avoid damaging the battery.
My Tesla owning friend compared the process to filling a glass of water from the tap. When the glass is empty, you open the tap to full to fill the glass as quickly as possible, but as the water nears the top of the glass you gradually reduce the water pressure and deliver water more slowly and accurately to ensure it's properly filled to the top and doesn't make a mess.
We have a smart meter, and for the first 90% of the charge, you can see it drawing full power, however when the battery gets to just over 90% the amount of electricity used is gradually reduced until it reaches 100%.
So I pretty sure you are using the same amount of electricity to fill the last 10% of the battery as the rest of the battery, except that the charger is delivering a much lower rate for the final 10%.
I think it designed like this to avoid damaging the battery.
My Tesla owning friend compared the process to filling a glass of water from the tap. When the glass is empty, you open the tap to full to fill the glass as quickly as possible, but as the water nears the top of the glass you gradually reduce the water pressure and deliver water more slowly and accurately to ensure it's properly filled to the top and doesn't make a mess.
er, your "7kW" charger is the rating of the charger. It has NOTHING to so with the power actually pulled by the vehicle when charging!
For AC charging, all the "Charger" actually is a bit switch, that connects the cars on-board rectifier to the 240VAC grid supply. That rectifier then converts the AC mains voltage to a suitable DC voltage to charge the battery.. When the charge rate tails towards the end of charging, the power pulled from the mains also tails, there is no "saving" to be had, sorry!
The 7kW maximum rating is really just a 30 amp maximum current limit, because 240V x 30A = 7,200 watts, the vehicle can pull any current it requires up to that limit.
Also, because 7kW is nothing for a large EV battery (the electric motor is likely to be over 100 kW and a DC fast charger at least 50 kW) unless it's really cold or hot, then the car will pretty much pull the full power all the time. Usually the last 10% is slowed simply to be able to get a better cell voltage balance in the pack and hence a greater overall State of Charge (SoC), but for a nicely balanced pack, chances are it can run the full 7kW all the way to 100% SoC
For AC charging, all the "Charger" actually is a bit switch, that connects the cars on-board rectifier to the 240VAC grid supply. That rectifier then converts the AC mains voltage to a suitable DC voltage to charge the battery.. When the charge rate tails towards the end of charging, the power pulled from the mains also tails, there is no "saving" to be had, sorry!
The 7kW maximum rating is really just a 30 amp maximum current limit, because 240V x 30A = 7,200 watts, the vehicle can pull any current it requires up to that limit.
Also, because 7kW is nothing for a large EV battery (the electric motor is likely to be over 100 kW and a DC fast charger at least 50 kW) unless it's really cold or hot, then the car will pretty much pull the full power all the time. Usually the last 10% is slowed simply to be able to get a better cell voltage balance in the pack and hence a greater overall State of Charge (SoC), but for a nicely balanced pack, chances are it can run the full 7kW all the way to 100% SoC
Edited by anonymous-user on Saturday 2nd March 17:06
Mr E said:
Well covered above. I only charge to 80% as it appears to be better for the battery long term.
80% of what? All EV batteries are automatically prevented from actually reaching a true 100% SoC by there battery management systems. As more and more battery life info becomes available from real world useage, the manufacturers are tending to allow a greater and greater proportion of the true battery capacity to be used. In the early days, around 50% of the capacity was out of bounds, today, that is closer to 20%Max_Torque said:
Usually the last 10% is slowed simply to be able to get a better cell voltage balance in the pack and hence a greater overall State of Charge (SoC), but for a nicely balanced pack, chances are it can run the full 7kW all the way to 100% SoC
I've read many people stating they don't charge all the way to 100%, often stopping around 80%. This confused me, as along the lines of what Max_Torque has written, i was under the impression that the last phase of charging was cell balancing. Not allowing the cell pack to balance seems madness. I kind of see the OP’s point, but without data it’s hard to know if it’s worth doing.
Does the balancing process take the same time for a 70->100% charge as it does for a 30->100% charge? Or is the pack “more unbalanced” and so requires more energy to complete the process?
Preheating might require the car being connected to AC.
What if the day you don’t charge you end up needing to do a longer trip and blow your yearly savings at an Ecotricity charger..
Does the balancing process take the same time for a 70->100% charge as it does for a 30->100% charge? Or is the pack “more unbalanced” and so requires more energy to complete the process?
Preheating might require the car being connected to AC.
What if the day you don’t charge you end up needing to do a longer trip and blow your yearly savings at an Ecotricity charger..
I can only speak for Nissan 24kWhr batteries. (Leaf and eNV200).
There are settings so the battery can be charged to 80% only.
It is NOT recommended to leave the battery for any length of time above 80% or below 20%, keeping between these two figures undoubtedly prolongs battery life.
The Leafs maximum rate of charge at a 50kW charger is around 25kW, and even this tails off the higher you try and go - cutting off entirely at 90%. It makes rapid chargers that are priced by the minute expensive.
Virtually all my charging is done with "night rate" power - about 7p per kWhr.
Works out around £1 per 60 miles.
There are settings so the battery can be charged to 80% only.
It is NOT recommended to leave the battery for any length of time above 80% or below 20%, keeping between these two figures undoubtedly prolongs battery life.
The Leafs maximum rate of charge at a 50kW charger is around 25kW, and even this tails off the higher you try and go - cutting off entirely at 90%. It makes rapid chargers that are priced by the minute expensive.
Virtually all my charging is done with "night rate" power - about 7p per kWhr.
Works out around £1 per 60 miles.
hab1966 said:
I've read many people stating they don't charge all the way to 100%, often stopping around 80%. This confused me, as along the lines of what Max_Torque has written, i was under the impression that the last phase of charging was cell balancing. Not allowing the cell pack to balance seems madness.
You can trust the BMS and charge to 100%, but just dont expect good results.https://insideevs.com/nissan-leaf-lost-over-half-b...
The science says the less charge status you keep the battery at the better. This is perusmably why Toyota Hybrids only ever use 30-70% charge range of their batteries, and seems to last forever.
All current EVs only have a very small buffer, certainly most are at 90% true SOC at 100% BMS SOC. The exception is the Audi eTron, Audi appears to have put in much bigger upper buffer, which am pretty sure will mean fantastic degredation performance.
There is a US study funded by the military that showed using the battery from 0-50% is better than from 50-100%, but I cannot find it at the moment, though paper essentially suggests the same.
https://www.sciencedirect.com/science/article/pii/...
Edited by gangzoom on Sunday 3rd March 08:44
gangzoom said:
You can trust the BMS and charge to 100%, but just dont expect good results.
https://insideevs.com/nissan-leaf-lost-over-half-b...
The science says the less charge status you keep the battery at the better. This is perusmably why Toyota Hybrids only ever use 30-70% charge range of their batteries, and seems to last forever.
All current EVs only have a very small buffer, certainly most are at 90% true SOC at 100% BMS SOC. The exception is the Audi eTron, Audi appears to have put in much bigger upper buffer, which am pretty sure will mean fantastic degredation performance.
There is a US study funded by the military that showed using the battery from 0-50% is better than from 50-100%, but I cannot find it at the moment, though paper essentially suggests the same.
https://www.sciencedirect.com/science/article/pii/...
To give you another random example.. I had my i3 for 3 years and 25k miles. Use was mainly a 16 mile each way commute and charged to 100% every night. the result? Range was the same at the end of the lease as at the beginning. Just because one car on the internet has a problem does not mean it is actually an issue in the real world. If it really was a problem it would be plastered all over the owners handbook, or the software would stop you doing it regularly.https://insideevs.com/nissan-leaf-lost-over-half-b...
The science says the less charge status you keep the battery at the better. This is perusmably why Toyota Hybrids only ever use 30-70% charge range of their batteries, and seems to last forever.
All current EVs only have a very small buffer, certainly most are at 90% true SOC at 100% BMS SOC. The exception is the Audi eTron, Audi appears to have put in much bigger upper buffer, which am pretty sure will mean fantastic degredation performance.
There is a US study funded by the military that showed using the battery from 0-50% is better than from 50-100%, but I cannot find it at the moment, though paper essentially suggests the same.
https://www.sciencedirect.com/science/article/pii/...
Edited by gangzoom on Sunday 3rd March 08:44
Ryder35 said:
To give you another random example.. I had my i3 for 3 years and 25k miles.
Hence why we have peer reviewed scientific publications. All the published literature says the same thing, the less you charge the battery the better. Who would you believe more, multi national companies who's main aim is profit or scientific literature?
BMS systems work, but take two identical cells, charge one to 90% and one to 50% repeatedly, and the 50% charge cell will last longer.
BrettCollins said:
I have recently purchased an EV and have a home charger. I have noticed like my phone and laptop it takes longer to charge the last 90% of charge. I have not measured how much extra time it takes but if it was to take an extra 20% to charge the last 10% then that would mean I am using and extra 2Kw of power to charge the last 10% (the charger supplies 7Kw per Hour). I use the car for commuting and use about 30% of power on one days commuting so instead of topping the battery up every day I could charge it every other day which would potentially save me paying for 4Kw a week, over a year that could be a saving of 200Kw which represents a saving of £20 a year.
Does anyone know if this is correct?
I plan to monitor amount of power used for charging over a week, charging daily one week and then charging on alternative days the next week. The problem with this is driving conditions can affect the power consumption, when it is cold more battery power is used and the length of time of commute is affected by traffic. so to accurately determine which is best and by power much power is saved I will need to repeat the two week test a number of times. So I hope to return and report the difference in a few months.
What car did you get?Does anyone know if this is correct?
I plan to monitor amount of power used for charging over a week, charging daily one week and then charging on alternative days the next week. The problem with this is driving conditions can affect the power consumption, when it is cold more battery power is used and the length of time of commute is affected by traffic. so to accurately determine which is best and by power much power is saved I will need to repeat the two week test a number of times. So I hope to return and report the difference in a few months.
The simple answer is that the power supplied will drop as your battery reaches somewhere between 80% and 90% of capacity.
Although technically correct I wish EV users would stop with all the BMS stuff and charging limit stuff that just puts potential buyers off and makes using and EV sound really complex.
Topping up daily, if you don't have high daily mileage, is a personal choice. Personally I say it's worth doing during the colder weather and setting the climate or departure timer to preheat the car and the cabin, so you're not having to use your battery to heat a freezing car up to 22 to 23 degrees as you set off in the morning and then creating the good old range anxiety. This really helps for that day you actually need to make a slightly longer trip.
I can send you a PM if you like showing a graph with the drop and and the subsequent spike when the heating kicks in to show the principle.
Hardly complex.
The car offers the option to stop charging at 80%. The manual recommends using this for best battery life if you don’t need full range.
The chemistry behind it is complex, but the end result doesn’t need to be.
Manufacturers may choose to mask cell degredation in software by supplying more capacity than advertised.
(Interestingly, the 30kw first generation leaf appears not to have the limit to 80% option. I don’t know why, possibly different cells?)
The car offers the option to stop charging at 80%. The manual recommends using this for best battery life if you don’t need full range.
The chemistry behind it is complex, but the end result doesn’t need to be.
Manufacturers may choose to mask cell degredation in software by supplying more capacity than advertised.
(Interestingly, the 30kw first generation leaf appears not to have the limit to 80% option. I don’t know why, possibly different cells?)
gangzoom said:
Hence why we have peer reviewed scientific publications. All the published literature says the same thing, the less you charge the battery the better.
Who would you believe more, multi national companies who's main aim is profit or scientific literature?
BMS systems work, but take two identical cells, charge one to 90% and one to 50% repeatedly, and the 50% charge cell will last longer.
Maybe it varies by car but I will take the advice of the manufacturer who designed the BMS...Who would you believe more, multi national companies who's main aim is profit or scientific literature?
BMS systems work, but take two identical cells, charge one to 90% and one to 50% repeatedly, and the 50% charge cell will last longer.
Quote from my current i3 manual
"To optimally operate the high-voltage battery charge the vehicle regularly and fully on a suitable charging fixture"
Ryder35 said:
Maybe it varies by car but I will take the advice of the manufacturer who designed the BMS...
Quote from my current i3 manual
"To optimally operate the high-voltage battery charge the vehicle regularly and fully on a suitable charging fixture"
As I've already said, car manufactures have proven record of putting customers interests first don't they Quote from my current i3 manual
"To optimally operate the high-voltage battery charge the vehicle regularly and fully on a suitable charging fixture"
.gangzoom said:
Ryder35 said:
Maybe it varies by car but I will take the advice of the manufacturer who designed the BMS...
Quote from my current i3 manual
"To optimally operate the high-voltage battery charge the vehicle regularly and fully on a suitable charging fixture"
As I've already said, car manufactures have proven record of putting customers interests first don't they Quote from my current i3 manual
"To optimally operate the high-voltage battery charge the vehicle regularly and fully on a suitable charging fixture"
.Tinfoil hat anyone.............
Max_Torque said:
you think the engineers responsibly for the battery at BMW are deliberately putting in a calibration that wears out their customers battery faster?
Tinfoil hat anyone.............
Nope, but charging daily to 100% compared to 50% will increase degredation. If your car has 200 miles range at 100% and you only use 50 miles a day, why would you charge to 100%??Tinfoil hat anyone.............
But if you car only has 100 mile range and you only charge to 50% than 50 mile trips will be a struggle.
Am keeping our EV for at least 8 years, ours charges to 70% on the BMS daily, which is around 60% total capacity. Gives me enough range for my daily usage with no worries.
Degredation at is affected by SOC, that's true for every single type of lithium ion cell. I really dont see why you would charge to 100% daily if you don't need the range?
Actually for me this is the biggest 'unknown' for buying used EVs is how previous owners charged their car. I couldn't careless about servicing stamps on EVs, but I personally wouldn't touch one that's been charged to 100% daily.
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