Optimal Charging Amperage

[dhanson865]

higher amps always means higher power loss. For a given power, higher voltage means lower amperage and thus lower power loss/higher efficiency.

In general, there are 3 sources of losses in charging:

1. The power used by the car to stay awake
2. losses due to the resistance of the circuit
3. and energy used to cool the battery from number 2.

Number 1 increases linearly with time to charge. Number 3 is dependent on number 2 as well as the ambient temperature. Number 2 increases with the square of the current (doubling the current means 4x the resistive power loss.) This also includes resistive losses in the equipment that connects to the car. If you don’t believe number 2 is an issue, then ask yourself why Tesla built cooling lines into the battery? I also saw a post a while back from someone complaining that the cabin got to 90º F while they were supercharging. They didn’t understand that the A/C was being used to cool the battery temp and had limited capacity to cool the cabin on top of that.

At some point number 1 will be balance by numbers 2 and 3. Exactly where that point is is difficult for anyone here to say without more data.

I posted a link to the thread above if you can't be bothered to follow it I'll post a screenshot for you

@wk057 tested in that thread

If amps going up means more power loss how do you explain the highest efficiency measured at 40 amps instead of at 5, 10, 15, or 20? How do you explain 80 amp charging to be more efficient than 20 amp charging in those tests?

What's optimal for a specific car will vary as Tesla uses different model on board chargers but the laws of physics aren't being violated here. Higher amps just aren't as big of an issue as you are playing them out to be.

 

[MB11]

Power Conversion System (PCS) failure is one possibility (warranty will pay for the repair)

Charging Goes from 32A to 16A is another (Overheating at the plug, the adapter not fully inserted in the UMC, or a bad connection somewhere between the breaker and the outlet.)

best to follow up in one of those threads (whichever seems to apply)

Thanks, I think I have got to the bottom of this. The smart charger is programmed to only charge on off peak rate, but it tries to trickle charge at 6a until the time is reached (a legacy to try to kee the car awake). When you plug in, there’s some communication between the car and charger where the charger advertises it’s charge rate, which is low. The car was set to charge to a preset level (80%) during the of peak time but appeared to use the max rate advertised by the charger. When it all kicked in at 00:30 the car charged to 80% @ 32A, so all good. I’m going to have to play around a little to try to fully understand the best way forward, but it looks like it’s working.

 

[davewill]

Thanks, I think I have got to the bottom of this. The smart charger is programmed to only charge on off peak rate, but it tries to trickle charge at 6a until the time is reached (a legacy to try to kee the car awake). When you plug in, there’s some communication between the car and charger where the charger advertises it’s charge rate, which is low. The car was set to charge to a preset level (80%) during the of peak time but appeared to use the max rate advertised by the charger. When it all kicked in at 00:30 the car charged to 80% @ 32A, so all good. I’m going to have to play around a little to try to fully understand the best way forward, but it looks like it’s working.

That behavior of trying to keep the car awake is no longer necessary. Software updates to the car have fixed this bug. If there's a way to configure your EVSE to not do it, I would.

 

[Corndart]

higher amps always means higher power loss. For a given power, higher voltage means lower amperage and thus lower power loss/higher efficiency.

In general, there are 3 sources of losses in charging:

1. The power used by the car to stay awake
2. losses due to the resistance of the circuit
3. and energy used to cool the battery from number 2.

Number 1 increases linearly with time to charge. Number 3 is dependent on number 2 as well as the ambient temperature. Number 2 increases with the square of the current (doubling the current means 4x the resistive power loss.) This also includes resistive losses in the equipment that connects to the car. If you don’t believe number 2 is an issue, then ask yourself why Tesla built cooling lines into the battery? I also saw a post a while back from someone complaining that the cabin got to 90º F while they were supercharging. They didn’t understand that the A/C was being used to cool the battery temp and had limited capacity to cool the cabin on top of that.

At some point number 1 will be balance by numbers 2 and 3. Exactly where that point is is difficult for anyone here to say without more data.

Having supercharged in 100f weather on multiple occasions and enjoyed a nice cool cabin temp of ~70f, the fact that a single person was "complaining that the cabin got to 90f while they were supercharging" , seems both an anomaly and quite unrelated to the topic of charging efficiency by amperage.

 

[sleepydoc]

Having supercharged in 100f weather on multiple occasions and enjoyed a nice cool cabin temp of ~70f, the fact that a single person was "complaining that the cabin got to 90f while they were supercharging" , seems both an anomaly and quite unrelated to the topic of charging efficiency by amperage.

I can only go by the report - it may have been that their heat pump was having issues, too (I had to have mine replaced and I know they had a bad batch of them) As temperature goes up there’s a truple-hit that occurs with cooling the battery. There’s a limit to the amount the heat pump can drop the temp so for every degree the temp goes up not only does it need to pump more cool air to get the same temp in the cabin, but the battery temp will also rise more and take more energy to drop.

Either way, it’s relevant to the discussion as it demonstrates the heat generated by resistive power losses in the battery and charging circuitry.

 

[Mrbrock]

Thanks, I think I have got to the bottom of this. The smart charger is programmed to only charge on off peak rate, but it tries to trickle charge at 6a until the time is reached (a legacy to try to kee the car awake). When you plug in, there’s some communication between the car and charger where the charger advertises it’s charge rate, which is low. The car was set to charge to a preset level (80%) during the of peak time but appeared to use the max rate advertised by the charger. When it all kicked in at 00:30 the car charged to 80% @ 32A, so all good. I’m going to have to play around a little to try to fully understand the best way forward, but it looks like it’s working.

So you aren’t using a Tesla charger? I think that’s your problem in your first post.

Just trying to charge my new MY LR for the first time tonight, before plugging anything in I went to the charging screen and tried to schedule it for my off-peak time (1:30 - 8:30). The screen shows a maximum charge rate of 16a, although my home charger is 32a? There doesn’t appear to be any way to move it past 16a to 32, the + burton is grey out When you get to 16a. Any advise welcome?

Since you are using a smart charger, you don’t need to set anything in the car, it can only lead to problems with two timers. Let the smart charger handle when it sends current to the car. Are you sure there isn’t a way to disable the trickle charge? Seems very un smart to me. Also, are you sure it will trickle at 6A for the whole time and not just a burst to communicate charge settings then it goes silent until it needs to kick back on? Tesla wall connector does this when you plug in and have timing settings in the car (off peak or departure) but then stops until later.

The downside with Tesla Off peak scheduling is there’s no end time, just a start after time. So if your off peak is only 6 hours and you need 8 to charge, it will keep going until you hit your percentage so 2 hours of peak charging, at least this is how it worked when I last used the setting about a year ago.

You couldn’t change the amps on the charging screen because you didn’t have anything plugged in. You can only set amps when something is plugged in and it can determine how much current/voltage is available.

 

[THEbuz]

There is no proof that any of this that you are talking about from a home user charging point of view (worrying about the car sitting at 10% for a few HOURS before it charges, vs a few days, weeks etc) will matter even enough to measure over a 4-5 year period.

It sounds like you fall in the camp of wanting to micro manage this to the Nth degree, and some people do that, but I personally dont think sitting at 10% for a few hours will matter, in the context of keeping the car awake longer by charging slower.

Keeping the car awake uses more power than letting it sleep, and charging slower will keep the car awake longer, using more energy (thus putting more "miles" on the battery).

My daily commute is upwards of 35mi (round trip). I couldn’t find another TMC thread that addresses my specific question.

Just bought my first TESLA, a 2023 Model Y (LR/AWD). I live in a cold/er region. I have been using my TESLA Mobile Connector @ 12A (120V) while my TESLA Wall Connector (drawing 48A/60A) is being installed next week.

Wouldn’t it be ideal if I charged at the lower 12A for longer throughout the night than faster for less time at 48A—as long as I am getting the range needed and do not violate the
>10% but <90% rule…???

 

[Big Earl]

My daily commute is upwards of 35mi (round trip). I couldn’t find another TMC thread that addresses my specific question.

Just bought my first TESLA, a 2023 Model Y (LR/AWD). I live in a cold/er region. I have been using my TESLA Mobile Connector @ 12A (120V) while my TESLA Wall Connector (drawing 48A/60A) is being installed next week.

Wouldn’t it be ideal if I charged at the lower 12A for longer throughout the night than faster for less time at 48A—as long as I am getting the range needed and do not violate the
>10% but <90% rule…???

Ideal? No. The car consumes 250 watts anytime it’s awake. The more time it spends charging, the more energy it consumes to stay awake, lowering your charging efficiency.

My philosophy is to get it over with quickly and let it sleep. This will reduce run time on various systems including the computers and coolant pumps.

 

[jjrandorin]

Wouldn’t it be ideal if I charged at the lower 12A for longer throughout the night than faster for less time at 48A—as long as I am getting the range needed and do not violate the
>10% but <90% rule…???

Nope.

Charging keeps the car awake, and car awake longer = more energy lost. If you have to charge at 12v then thats one thing, but there is no reason at all from the car side to lower the charging rate for any home charging. If you want to slow it down for some personal reason, you can, but know that you will waste more energy because the car is awake longer.

Whether its enough more for you to worry about or not is another thing, but there is no benefit to charging slower on home L2 charging as it relates to "the car" or "the battery".

 

[THEbuz]

Ideal? No. The car consumes 250 watts anytime it’s awake. The more time it spends charging, the more energy it consumes to stay awake, lowering your charging efficiency.

My philosophy is to get it over with quickly and let it sleep. This will reduce run time on various systems including the computers and coolant pumps.

It consumes 250W every what??? Every hour that it’s awake? Every 10hrs that it’s awake???

Thanks for weighing in.

 

[jcanoe]

Powered on, parked your Tesla Model Y will consume approximately 230W (perhaps up to 250W) every minute it is awake. This works out to 1kWh consumed every 4 hours; 6kWh every 24 hours (that is a lot of energy for just sitting). When parked and in sleep mode the power consumption is ~25W (almost 90% less.)

You can charge immediately as soon as you arrive home, plug in but a better strategy (especially in winter) is to set the end of the off-peak rate window (found under charging settings) so that the Tesla Model Y charges during the early A.M. hours and completes just prior to your morning departure. This way the battery will be warm from charging at no additional power will be needed to warm the battery and less power may be used to warm the passenger cabin. (Even if your electric utility does not offer an off-peak rate time of use (TOU) rate plan you can set the Tesla Model Y as if there is an off-peak window end time, i.e. 0700 and this will ensure charging is always completed by the time you plan to leave.

If you also set Scheduled Departure (found under Climate Control settings and also under Charging Settings) to precondition the Tesla Model Y for your A.M. commute the Tesla Model Y will warm the passenger cabin prior to your departure. (You can also precondition the Tesla Model Y using the Tesla app before you head out.)

 

[Mrbrock]

If a 60 W light bulb is on for one hour, then that light bulb will have used 60 Wh of energy. If left on for two hours, then the 60 W light bulb will have used 120Wh of energy. Your car is a a 250w light bulb. Watts are speed and watt-hours are distance.

 

[Big Earl]

It consumes 250W every what??? Every hour that it’s awake? Every 10hrs that it’s awake???

Thanks for weighing in.

Watts are a measure of instantaneous power. 250 watts for four hours would add up to 1,000 Wh (watt hours) 1 kWh (kilowatt hour) of energy.

 

[ucmndd]

It consumes 250W every what??? Every hour that it’s awake? Every 10hrs that it’s awake???

Yes.

 

[THEbuz]

Powered on, parked your Tesla Model Y will consume approximately 230W (perhaps up to 250W) every minute it is awake. This works out to 1kWh consumed every 4 hours; 6kWh every 24 hours (that is a lot of energy for just sitting). When parked and in sleep mode the power consumption is ~25W (almost 90% less.)

You can charge immediately as soon as you arrive home, plug in but a better strategy (especially in winter) is to set the end of the off-peak rate window (found under charging settings) so that the Tesla Model Y charges during the early A.M. hours and completes just prior to your morning departure. This way the battery will be warm from charging at no additional power will be needed to warm the battery and less power may be used to warm the passenger cabin. (Even if your electric utility does not offer an off-peak rate time of use (TOU) rate plan you can set the Tesla Model Y as if there is an off-peak window end time, i.e. 0700 and this will ensure charging is always completed by the time you plan to leave.

If you also set Scheduled Departure (found under Climate Control settings and also under Charging Settings) to precondition the Tesla Model Y for your A.M. commute the Tesla Model Y will warm the passenger cabin prior to your departure. (You can also precondition the Tesla Model Y using the Tesla app before you head out.)

I’m sure you meant to say 230W - 250W consumed every hour, not every minute. Otherwise, your math does not add up. This was what I was trying to clarify before. 230W - 250W consumed every minute (not every hour) is 13.8kWh - 15kWh consumed every hour, not the 6kWh every 24hrs you’ve suggested. Thank you very much for sharing!

 

[davewill]

I’m sure you meant to say 230W - 250W consumed every hour, not every minute. Otherwise, your math does not add up. This was what I was trying to clarify before. 230W - 250W consumed every minute (not every hour) is 13.8kWh - 15kWh consumed every hour, not the 6kWh every 24hrs you’ve suggested. Thank you very much for sharing!

You still don't get it. Watts is an instantaneous rate. saying watts per minute or per hour has no meaning. 250 Watts consumed for an hour is 250 Watt-hours. in 4 hours, that adds up to 1kWh (1000Wh) consumed.

People get confused because most rates ARE expressed as something per time, like miles per hour or gallons per minute, so to get quantity, you multiply by the time and get miles or gallons. Watts works the same way, to get the quantity of power (energy) used, you multiply by the time and get Wh or kWh.

 

[ucmndd]

I’m sure you meant to say 230W - 250W consumed every hour, not every minute. Otherwise, your math does not add up. This was what I was trying to clarify before. 230W - 250W consumed every minute (not every hour) is 13.8kWh - 15kWh consumed every hour, not the 6kWh every 24hrs you’ve suggested.

No.

A watt is a measure of power (work over time).

A watt-hour is a measure of energy (the capacity to do work).

250 watts of continuous power for an hour is 250 watt-hours of energy expended.

250 watts of continuous power for a minute is 250 watt-minutes of energy expended (if we used such a unit of measure).

500 watts of continuous power for 30 minutes is 250 watt-hours of energy expended.

900,000 watts of continuous power for 1 second is 250 watt-hours of energy expended.

…and so on.

 

[cgell]

I’m sure you meant to say 230W - 250W consumed every hour, not every minute. Otherwise, your math does not add up. This was what I was trying to clarify before. 230W - 250W consumed every minute (not every hour) is 13.8kWh - 15kWh consumed every hour, not the 6kWh every 24hrs you’ve suggested. Thank you very much for sharing!

You are incorrectly calculating because you aren’t understanding the units of measurement. At any moment that the vehicle is awake it will consume the 250w constantly. If you were to run 250w for an hour it would be .25kWh (250Wh) of total energy used.

Just like telling you a flow rate (let’s say 2gpm) of water doesn’t tell you how much total water you used unless you know how long it was used at that rate. You would measure total gallons, not the gpm.

 

[Big Earl]

Think of it another way: horsepower is to watts, as gallons of fuel are to kilowatt-hours.

1 kilowatt is 1.34 horsepower. 1 gallon of gasoline is 33.7 kilowatt-hours, or 45.16 horsepower-hours.

Another way to flip it: 1 kW = 1 kWh per hour. Cancel hours from the numerator and the denominator, and you're left with kW, the measure of instantaneous power.

Maybe that didn't help.

 

[THEbuz]

You still don't get it. Watts is an instantaneous rate. saying watts per minute or per hour has no meaning. 250 Watts consumed for an hour is 250 Watt-hours. in 4 hours, that adds up to 1kWh (1000Wh) consumed.

People get confused because most rates ARE expressed as something per time, like miles per hour or gallons per minute, so to get quantity, you multiply by the time and get miles or gallons. Watts works the same way, to get the quantity of power (energy) used, you multiply by the time and get Wh or kWh.

I do get it. However, you stated: “Powered on, parked your Tesla Model Y will consume approximately 230W (perhaps up to 250W) every minute it is awake.”

That calculates to 13.8kWh - 15kWh consumed when my TESLA is just awake, as you said. Again, this is a ludicrous claim, and I think you meant to say that my TESLA consumes 230Wh - 250Wh just by being awake. I understand the difference between a watt (W) of energy and a watt hour (Wh). I am just responding to what you claimed.