Tesla: Help Us Charge Two Cars on a Single Circuit

[Robwoodruff]

I have a 100 amp breaker and tried charging my new Model S at the same time as my old model S, both at 40 amp. One of the cars gave a message saying the current was unstable and it would reduce the charging to 25 amp. I didn't like the way that sounded so I set my wife's car to charge at 1:00 am after my car is charged, that way they can both charge at the highest amps but at different times.

 

[andrewket]

I have a 100 amp breaker and tried charging my new Model S at the same time as my old model S, both at 40 amp. One of the cars gave a message saying the current was unstable and it would reduce the charging to 25 amp. I didn't like the way that sounded so I set my wife's car to charge at 1:00 am after my car is charged, that way they can both charge at the highest amps but at different times.

The first car saw a voltage drop when the second one ramped up. Tesla added a safety feature a few years ago that attempts to look for faulty wiring or adapters going bad. It does this by watching for voltage flucstions. When that happens it reduces the amperage. In your case the voltage drop came from the increased demand of your second car.

I've got two HPWCs each on their own 100A circuit. When we first got our X, I had the S scheduled to charge at 1am and the X at 1:05. I occasionally had the same problem. Now they're both set to charge at 1am, and ramp up together. They both maintain 72A without a problem (our old S maintained 80A, alas the refreshed S has the same charger as the X).

Keep in mind I have 200A service to my garage just for the EVs, which I appreciate most will not be able to do without significant cost.

 

[CmdrThor]

I have a 100 amp breaker and tried charging my new Model S at the same time as my old model S, both at 40 amp. One of the cars gave a message saying the current was unstable and it would reduce the charging to 25 amp. I didn't like the way that sounded so I set my wife's car to charge at 1:00 am after my car is charged, that way they can both charge at the highest amps but at different times.

Are you using Wall Connectors or Mobile Connectors to charge your cars? If you have two NEMA 14-50s on a 100 amp breaker that is a code violation, a safety issue, and you are likely to start a fire. The outlet is not rated for 100 amps. You should replace the 100 amp breaker with a 50 amp breaker and only use 1 outlet at a time, or even better get two Wall Connectors. I don't understand why everyone wants to skimp on the charging equipment for their Tesla when doing so could destroy their car, house, and even cause loss of life.

 

[Akikiki]

You have the power to fix this yourself without asking Tesla. When you plug in, you can estimate the length of time its going to take to top off the first car, so set the timer on the second to start 15-30 minutes later. No brain surgery involved.

 

[[email protected]]

This was helpful. Tesla recognizes the issue: HPWC does load balance so that is cool. On the other hand, most of us don't need HPWC. 14-50 is fine at 29 mph even charging two cars that are driven lots. Tesla also knows that the once charging starts, it will end (as Akikiki notes). the only issue is whether it will end based on a mileage input or a time input. The cars presently stop charging based on the mileage input (which as you all know is pretty damn unpredictable: the rate of charge is variable, so it may not be brain surgery, but if you are doing this day in and day out, why have any risk about this at all). The argument I am making is for the charge to stop on a time input. It should cost nothing, and not require buying an HPWC capacity simply to load balance.

 

[AZ Desert Driver]

I think that's the conclusion that Jeff Dahn came to as well in his University of Waterloo lecture a few years ago.

Please drill this comment down a bit more. I was (told, informed, educated) that charging at 30 amps instead of 40 amps would put less strain on - battery cooling system, circuit breakers, cords...Lower charge rate is preferable to High charge rate for electronic life. If you have all night, why rush it?
Now, along comes this message - charge as fast as you can, not for the time waiting for a full tank, but because the battery likes it.
Please tell me what is the "best" way for charging.

from the Dahn lecture replies:
databeestje1 year ago
Explanation why temperature is a problem @4:00 Why fast charging is a good idea @24:30
1


Lance Pickup9 months ago
"Why fast charging is a good idea" I don't think that's what he's saying at all (although subsequently it has been shown that fast charging can have a beneficial effect sometimes). I think what he's saying is that the way battery testing is normally done, there is no pause between charge/discharge, so if you test using fast charging you get a lot of cycles in in a short amount of time, which doesn't leave a lot of time for the parasitic reactions that would diminish battery capacity to occur. If you were to pause after the charge/discharge and just wait the equivalent amount of time that you would've while doing a slow charge, the effect on the battery for each of the two charging schemes would be roughly the same (which he also shows the data for). Furthermore, consider that he is showing data for which charging takes place at a carefully controlled (and possibly elevated) temperature. As he says (and you referenced), temperature has a dramatic negative effect on battery life, and for a typical user, fast charging will elevate the temperature of the battery more than slow charging, so the net effect is that fast charging, in actual practice, would be worse for you, unless you took steps to actively cool the battery and also immediately used your battery followed by immediately charging your battery repeatedly.

 

[vandacca]

Please drill this comment down a bit more. I was (told, informed, educated) that charging at 30 amps instead of 40 amps would put less strain on - battery cooling system, circuit breakers, cords...Lower charge rate is preferable to High charge rate for electronic life. If you have all night, why rush it?
Now, along comes this message - charge as fast as you can, not for the time waiting for a full tank, but because the battery likes it.
Please tell me what is the "best" way for charging.

from the Dahn lecture replies:
databeestje1 year ago
Explanation why temperature is a problem @4:00 Why fast charging is a good idea @24:30
1


Lance Pickup9 months ago
"Why fast charging is a good idea" I don't think that's what he's saying at all (although subsequently it has been shown that fast charging can have a beneficial effect sometimes). I think what he's saying is that the way battery testing is normally done, there is no pause between charge/discharge, so if you test using fast charging you get a lot of cycles in in a short amount of time, which doesn't leave a lot of time for the parasitic reactions that would diminish battery capacity to occur. If you were to pause after the charge/discharge and just wait the equivalent amount of time that you would've while doing a slow charge, the effect on the battery for each of the two charging schemes would be roughly the same (which he also shows the data for). Furthermore, consider that he is showing data for which charging takes place at a carefully controlled (and possibly elevated) temperature. As he says (and you referenced), temperature has a dramatic negative effect on battery life, and for a typical user, fast charging will elevate the temperature of the battery more than slow charging, so the net effect is that fast charging, in actual practice, would be worse for you, unless you took steps to actively cool the battery and also immediately used your battery followed by immediately charging your battery repeatedly.

I feel that this previous opinion from 9 months ago is not complete and too simplistic. "Fast-charging" is only one variable in a complex equation, and that post fails to take into account the battery's state-of-charge. Heat generated during the whole charging cycle is not constant and/or linear. This is why Tesla has an upper limit of 80%-90% for regular charging.

It would seem that Fast Charging is more healthy for the battery because it minimizes the parasitic reactions, as long as you don't elevate the battery temperature. Therefore, a combination of battery cooling and charging to 80%-90% in addition to the fast-charging is the best way to minimize the parasitic reactions. This is why charging is slowed down when topping off the battery.

In fact, of those 3 variables, battery cooling and limiting charging to 80%-90% are much more important than Fast Charging. Therefore, slow charging isn't going to be much worse for the battery, as long as the other two variables are ideal.

I hope that helps. The take-away point is that Fast Charging is not likely as bad for the battery as we originally thought, and it's actually possible that it's a tiny bit better, as long as you restrain the other parameters.

 

[Rocky_H]

Please drill this comment down a bit more. I was (told, informed, educated) that charging at 30 amps instead of 40 amps would put less strain on - battery cooling system, circuit breakers, cords...Lower charge rate is preferable to High charge rate for electronic life. If you have all night, why rush it?
Now, along comes this message - charge as fast as you can, not for the time waiting for a full tank, but because the battery likes it.
Please tell me what is the "best" way for charging.

from the Dahn lecture replies:
databeestje1 year ago
Explanation why temperature is a problem @4:00 Why fast charging is a good idea @24:30
1


Lance Pickup9 months ago
"Why fast charging is a good idea" I don't think that's what he's saying at all (although subsequently it has been shown that fast charging can have a beneficial effect sometimes). I think what he's saying is that the way battery testing is normally done, there is no pause between charge/discharge, so if you test using fast charging you get a lot of cycles in in a short amount of time, which doesn't leave a lot of time for the parasitic reactions that would diminish battery capacity to occur. If you were to pause after the charge/discharge and just wait the equivalent amount of time that you would've while doing a slow charge, the effect on the battery for each of the two charging schemes would be roughly the same (which he also shows the data for). Furthermore, consider that he is showing data for which charging takes place at a carefully controlled (and possibly elevated) temperature. As he says (and you referenced), temperature has a dramatic negative effect on battery life, and for a typical user, fast charging will elevate the temperature of the battery more than slow charging, so the net effect is that fast charging, in actual practice, would be worse for you, unless you took steps to actively cool the battery and also immediately used your battery followed by immediately charging your battery repeatedly.

You are not considering the relative scale of the numerical values though. As far as the scale he is referring to of charging the battery "fast" or "slow", the differences between 30 and 40 amps at 240V is almost nothing. They're basically the same charging speed as far as the battery is concerned. The battery can take anywhere from about 1kW to about 115kW. You're talking about the difference between 7.2kW and 9.6kW. It's just not much different for the battery. However, with the cable and electronics in the UMC and onboard charger, 40A is its maximum rating, and it generates some noticeable heat when running at maximum.

So you can get some noticeable benefit as far as the electronics by turning it down a little, while the battery is barely seeing any difference. Does that make a bit more sense now?

*Edit* Oh, I forgot, with a Model X, the onboard charger max is 48A, so 40 is already down some. But if you're using the UMC, it does still help cool it down some to lower it a little from 40.

 

[AZ Desert Driver]

Yes - this helps. Let me rephrase: The Battery has one set of standards, the Charging Circuit has another. For the long term health of my home charging circuit, set charger at 30 amps. The system can take 40 amps, but why stress it. The Battery can take anything I can throw at it - from 110 volts to Supercharger volts. The cars battery management system will do the cooling. Fully charge the system on occasion, just not every occasion. Discharge the battery (to 20 miles or so) on occasion, but not on all occasions.

Did I get it right?

 

[Rocky_H]

Yes - this helps. Let me rephrase: The Battery has one set of standards, the Charging Circuit has another. For the long term health of my home charging circuit, set charger at 30 amps. The system can take 40 amps, but why stress it. The Battery can take anything I can throw at it - from 110 volts to Supercharger volts. The cars battery management system will do the cooling. Fully charge the system on occasion, just not every occasion. Discharge the battery (to 20 miles or so) on occasion, but not on all occasions.

Did I get it right?

Right, pretty simple to remember and covers all the bases.

 

[Goran]

I would just set them both to 25 amps.

You would need to set them both to 20A instead. You're only allowed 80% of the breaker rating

 

[AZ Desert Driver]

Compliments to all the folks who post here. There are some smart guys here who understand the details of electricity and are willing to give good advice.Thanks.

 

[Rocky_H]

You would need to set them both to 20A instead. You're only allowed 80% of the breaker rating

It's like...deja vu or something. Tesla: Help Us Charge Two Cars on a Single Circuit

Probably go ahead and look through the rest of the thread to see if someone else has already posted a correction.

 

[ThosEM]

For those who with cars that need different amounts of charging, one not having been used as much as the other, you can always set up your UMC on a 120V circuit and charge one car more slowly, while the more discharged car gets the HPWC. We do that frequently and then an issue comes up if one car needs to enter or exit the garage while the second car (farther from the charger) is charging. Here's our solution to driving over the cable, with a cost of about $15 and 15 minutes time:

 

[AZ Desert Driver]

For those who with cars that need different amounts of charging, one not having been used as much as the other, you can always set up your UMC on a 120V circuit and charge one car more slowly, while the more discharged car gets the HPWC. We do that frequently and then an issue comes up if one car needs to enter or exit the garage while the second car (farther from the charger) is charging. Here's our solution to driving over the cable, with a cost of about $15 and 15 minutes time:

Makes sence to just charge one car, then the other - sequential instead of simultaneous. I would interrupt one car - unplug and move the cable out of the way, back out the car, then plug back in. I see no good reason to drive over the cable, even if you have protected it as well as you did.

 

[ThosEM]

Makes sence to just charge one car, then the other - sequential instead of simultaneous. I would interrupt one car - unplug and move the cable out of the way, back out the car, then plug back in. I see no good reason to drive over the cable, even if you have protected it as well as you did.

Saves on trips to the garage, i guess. .
And what can happen does happen, so I wanted to protect the cable in the future.

 

[AZ Desert Driver]

Saves on trips to the garage, i guess. .
And what can happen does happen, so I wanted to protect the cable in the future.

shurr [and of course] - coming home, you would have to park in the drive way, unplug, park and re-plug...
I was just thinking about departure and not return.
As I only have one EV, I always get the left slot in my garage. If I had two---I might lace a permanent wire over the garage door to make getting to car-2 handy, and then plug/unplug into my 14-50.