Charging on NEMA 5-20 at 7mph!

[jdcollins5]

It seems to me it depends on each individual’s situation.

Being retired I typically drive say 15 miles per day. In my case, the manual and videos tell us it is better to keep the battery plugged in and topped off rather than to deplete the battery to say 50% and charge back to 80-90%.

I have a 120V, 15A outlet in my carport where I currently keep my car. I also have a 240V, 30A dryer outlet in my garage. For daily driving I plan to use the 120V outlet for topping off each night. At my off peak rate of $0.07/kWh and a charging rate of 5 mph, it will take about 3 hours to charge at about $0.30. Taking in to account the 12% efficiency loss in the table above, it will cost me $0.34. No big deal to me.

When I want to fully charge for a trip I will use the 240V dryer outlet.

Being a EE I could easily install a 14-50 outlet or HPWC but it is not worth it to me. YMMV!

 

[SigNC]

Here let me try to summarize. The higher wattage that you charge at the less efficiency is lost to the sunk cost of the car being on while charging. Less energy is lost in transfer using 240V than 120V but if you should find yourself in the strange scenario where you can provide more power to the car over 120V with a lot more amperage than you can with 240v you would likely have higher efficiency when you cross the point where the decreased transfer efficiency off 120V is more than offset by the lower duration of charging time.

Even shorter version:
240v transfer is more efficient than 120v but charging your car faster is the big key. Less time charging means less energy lost to the car being on.

 

[davewill]

Here let me try to summarize. The higher wattage that you charge at the less efficiency is lost to the sunk cost of the car being on while charging. Less energy is lost in transfer using 240V than 120V but if you should find yourself in the strange scenario where you can provide more power to the car over 120V with a lot more amperage than you can with 240v you would likely have higher efficiency when you cross the point where the decreased transfer efficiency off 120V is more than offset by the lower duration of charging time.

Even shorter version:
240v transfer is more efficient than 120v but charging your car faster is the big key. Less time charging means less energy lost to the car being on.

Exactly. That's why adding a mere 4a (from 120v 12a to 120v 16a) ends up close to doubling the effective charge rate, and going to 240v 16a nearly triples it. It has little to do with a mere 10% conversion efficiency improvement.

 

[ZeApelido]

So if I'm charging on a nema 10-30, it is more efficient to charge at 24 amps then dropping the amps down to say 20?

What % difference in efficiency would that be?

 

[TexasEV]

If you have an appropriately wired circuit there is no reason to lower the amps and have the car take longer to charge. If your circuit is suspect, better to get that fixed than to dial down the amps.

 

[EVDRVN]

Why do people continue to measure charging in miles per hour? UGH!!! And of course charging at a higher total wattage is more efficient at many levels. Pumps, contactor, etc. etc.

 

[Kermee]

Charging at 400V DC is the most efficient!

Sorry. Couldn't resist since this thread keeps popping up...

 

[aydyn]

Oh really? Try this one:
120V at 30A = 3,600W
240V at 5A = 1,200W
You are saying that the second one is 10% more efficient. Most would disagree with you, including, you know, science.

You have apparently no background in electrical science. Why is every datacenter in the us run at 208 (or higher). Power is LOST, evaporates poof (ok its heat I’m being dramatic on purpose) during transmission the lower the voltage the higher the percentage of this loss. 208/10a is better then 110/20a even though on paper it’s the same.

 

[GWord]

Oh really? Try this one:
120V at 30A = 3,600W
240V at 5A = 1,200W
You are saying that the second one is 10% more efficient. Most would disagree with you, including, you know, science.

I'm disagreeing with your statement about "science". If you really want to bring science into it, your Tesla charger is a reactive load and is more efficient at higher voltage levels. At a more basic level, at a given power level, higher voltage is marginally more efficient since you have smaller line losses (we'll assume household voltages so we don't have to have a discussion around RF and coronal losses). Sure, you can pretend that lower voltages are equal in efficiency if you ignore the "science", but that's like saying that street is pink as long as you ignore the fact that it's black.

 

[nwdiver]

Oh really? Try this one:
120V at 30A = 3,600W
240V at 5A = 1,200W
You are saying that the second one is 10% more efficient. Most would disagree with you, including, you know, science.

You're cherry picking your science. Power = (Current)(Voltage); Line Losses via heat = (Current)² (Resistance) => You can achieve the same power by doubling the voltage and halving your current => reduce line losses by ~75% because, you know, math.

 

[chilman408]

So here is my math (conclusion is 14-50 is more efficient):

240V at 32A (14-50) = 7680W (Tesla calls it 7.7kW) translate to 30 miles of range per hour
117V at 16A (5-20) = 1872W translate to 7 miles of range per hour

(7680W / 1872W) < (30mi / 7mi) or (4.10) < (4.28)

Charging at 240V is more efficient (not to mention faster)

There you have it kids.

 

[VT_EE]

You have apparently no background in electrical science. Why is every datacenter in the us run at 208 (or higher). Power is LOST, evaporates poof (ok its heat I’m being dramatic on purpose) during transmission the lower the voltage the higher the percentage of this loss. 208/10a is better then 110/20a even though on paper it’s the same.

I think the added cost of unnecessarily large wiring as well as pdu power density play a larger role in the choice of 208 at the data center.

 

[eprosenx]

I think the added cost of unnecessarily large wiring as well as pdu power density play a larger role in the choice of 208 at the data center.

Nearly all server PSU units are rated for 100v to 240v input. (to cover the range of global voltages from Japan to Europe)

Running them at say 208v vs. 100v saves a couple percentage points of power from what I have been told. Hence slightly more “efficient”. Also- Power supplies can often put out a higher current on the DC side when powered by higher voltages on he AC side. So many network devices for instance require 200+ volts in order to operate.

 

[SSedan]

If we assume that cooling pumps work the same with 120V or 240V charges, then of course, less time you spent on charging, less time you spent on pumps working. From other sede, I do not know if cooling pump needed when you charge slowly with 120V, I assume there is some temperature sensor. Generally it is a big area to research.

There is more electrical overhead than coolant pumps, it is the computers and chargers.

 

[BestHand]

There is more electrical overhead than coolant pumps, it is the computers and chargers.

Well, of course, I was just replying to another post talking about pumps

 

[SSedan]

It seems to me it depends on each individual’s situation.

Being retired I typically drive say 15 miles per day. In my case, the manual and videos tell us it is better to keep the battery plugged in and topped off rather than to deplete the battery to say 50% and charge back to 80-90%.

I have a 120V, 15A outlet in my carport where I currently keep my car. I also have a 240V, 30A dryer outlet in my garage. For daily driving I plan to use the 120V outlet for topping off each night. At my off peak rate of $0.07/kWh and a charging rate of 5 mph, it will take about 3 hours to charge at about $0.30. Taking in to account the 12% efficiency loss in the table above, it will cost me $0.34. No big deal to me.

When I want to fully charge for a trip I will use the 240V dryer outlet.

Being a EE I could easily install a 14-50 outlet or HPWC but it is not worth it to me. YMMV!

Winter might change your mind.
I used a 10-30 last winter here near Green bay with a 15mile round trip commute and put in a wall connector recently. Warming the 1200lbs battery in my MS takes a fair bit of energy. Granted it doesn't get nearly as cold by you but in the high 40s regen can be limited which increases energy use on your normal drive as does cabin heating. Even when doing morning warmup off the wall power at 24amps of 240 my WH/M doubled due to the warmup when leaving work.

The effects of cold weather on BEV ownership are far too often swept under the rug. The instant heat is AWESOME but the greatly increased energy use needs to be understood ahead of time. It really caught me off guard last fall. Longer the drive the warmup effect is diluted, my short commute and bitterly cold climate are a worse scenario than most will see, but it is a trend to be aware of before it is 30f and you decide you need the 240 outlet.

 

[jdcollins5]

Winter might change your mind.
I used a 10-30 last winter here near Green bay with a 15mile round trip commute and put in a wall connector recently. Warming the 1200lbs battery in my MS takes a fair bit of energy. Granted it doesn't get nearly as cold by you but in the high 40s regen can be limited which increases energy use on your normal drive as does cabin heating. Even when doing morning warmup off the wall power at 24amps of 240 my WH/M doubled due to the warmup when leaving work.

The effects of cold weather on BEV ownership are far too often swept under the rug. The instant heat is AWESOME but the greatly increased energy use needs to be understood ahead of time. It really caught me off guard last fall. Longer the drive the warmup effect is diluted, my short commute and bitterly cold climate are a worse scenario than most will see, but it is a trend to be aware of before it is 30f and you decide you need the 240 outlet.

Thanks for the feedback in cold weather. I do have a 240V dryer outlet that I can use during cold weather.

 

[fiatlux]

Thanks everyone for the replies + interesting details about efficiency etc. Echoing jdcollins5's comment, people have different situations and should decide according to their needs & priorities.

Some additional details in my case that led me to be happy with this decision:
* Free charging at work from 100% renewable energy sources. Could meet all of my charging needs (but occasionally inconvenient/full)
* Our city utility is 100% renewable for electricity, so it's purely an economic choice.
* Our winters are very mild in coastal CA. Winter only has lows around freezing a few weeks per year and the rest of the time it's 40s & 50s as the lows.

Cheers / enjoy your M3s all!
The Tesla grin is real!

 

[AWDtsla]

208/10a is better then 110/20a even though on paper it’s the same.

Not going to make a difference charging a Tesla. Auxiliary draw (pumps, computers, etc) is the problem, not anything else.

 

[ratsbew]

Why do people continue to measure charging in miles per hour? UGH!!! And of course charging at a higher total wattage is more efficient at many levels. Pumps, contactor, etc. etc.

Because that is the practical metric that matters. I drive about 40 miles per day and charge at 15mph....It takes about 3 hours to charge.