I can now charge at 80A, should I?

[Steve2498]

I finally got my car and garage set up so that I can use a Tesla wall charger to charge the car at up to 80 amps. I did this to have the flexibility to charge the car quickly if needed but the reality is, more often than not, I have overnight to charge the car and so can do so at a much lower amperage if there is some advantage to doing so.

The question is, is it more efficient to charge at a lower rate? I notice the wires and breaker get noticably warmer at 80A than at 40A or 50A. Is it easier on "the system" (battery, charger, breaker, etc) to use a lower amperage?

Thanks for your thoughts.

 

[wk057]

While I'm sure many others disagree, the chargers are designed to work at their capacity. If you were charging at 40A, then one charger was working at 40A. At 50A, both chargers were at 25A. At 80A both chargers are at 40A. So 80A and 40A makes basically no difference to the car. 20kW charging is ~1/4C for the 85/90 packs as well, not even enough to heat them up much at all through a full charge.

Long story short, charge as quickly as you need for your needs and don't worry about it too much. For your efficiency question, the chargers seem to operate most efficiently at near full power, but there are some resistive losses in the lines and connector of the HPWC that lower overall efficiency by a bit.

I charge both of the Model S in my garage at 80A only, unless I have some reason not to (low solar power or something). They're made to handle the power, and they can. Unless you have demand charges (at a business or some unusual residential electric rate) then it really doesn't matter enough to worry about it.

Edit: Further, the Gen2 chargers in the car (forget when the VIN switch was, but I know everything since late 2014 has them) are actually 48A rated and Tesla only allows them to be used up to 40A in the USA on single phase since there are no provisions in the J1772 spec to make a 96A EVSE.

 

[Cosmacelf]

Well that would be an interesting equation to figure out. There are going to be more line losses when charging at a higher rate, but bear in mind that you will be charging longer, so the lower line losses will happen for longer. Also, there is some car overhead that occurs when charging, so if you charge for longer, you will use up more electricity powering the car computer, pumps, etc.

All in all, I would bet the difference is minor.

 

[MP3Mike]

While I'm sure many others disagree, the chargers are designed to work at their capacity. If you were charging at 40A, then one charger was working at 40A. At 50A, both chargers were at 25A. At 80A both chargers are at 40A. So 80A and 40A makes basically no difference to the car. 20kW charging is ~1/4C for the 85/90 packs as well, not even enough to heat them up much at all through a full charge.

Is that how it really works? I could swear that I read that the first 40A goes to the master charger, and then the secondary charger kicks in for everything over that. (So 50A would be split 40A/10A.) I even remember seeing that someone said that they couldn't charge past 40A even with dual chargers and it turned out that their secondary charger was bad but not through a code. (Of course maybe that was just the fault tolerance kicking in.)

 

[wk057]

Is that how it really works? I could swear that I read that the first 40A goes to the master charger, and then the secondary charger kicks in for everything over that. (So 50A would be split 40A/10A.) I even remember seeing that someone said that they couldn't charge past 40A even with dual chargers and it turned out that their secondary charger was bad but not through a code. (Of course maybe that was just the fault tolerance kicking in.)

0-40A goes to the master charger (slave idle). 41-80A gets split 50% to the master 50% to the slave. Confirmed via CAN analysis.

So 40A is Master 40A slave 0A, 41A is Master 20.5A and slave 20.5A. 60A is Master 30A slave 30A, etc etc.

 

[MP3Mike]

0-40A goes to the master charger (slave idle). 41-80A gets split 50% to the master 50% to the slave. Confirmed via CAN analysis.

So 40A is Master 40A slave 0A, 41A is Master 20.5A and slave 20.5A. 60A is Master 30A slave 30A, etc etc.

Ahh, so that was the details I was missing. Thanks again for all your research and your willingness to share it!

 

[jbcarioca]

Opinions vary.

However, my understanding is that the 12v system charges from the external power while charging the whole system, but otherwise charges from the main battery. I have heard people dispute that but so far rely on the statement of a Tesla Service Manager to me. That argues for charging whenever practical.

Generally li-ion cells like moderate rates of charge and discharge and longevity is best at the equivalent of about 50% SOC. Tesla's Martin Eberhart made some comments about Roadster battery treatment, but none have been forthcoming about the various versions of S batteries. There are Model S around with >100,000 miles on original batteries with >80% capacity remaining. In addition I know one owner who has more than 70,000 miles and routinely uses Superchargers to range charge, then runs down to less than 10% regularly, while keeping >90% original capacity.

Realistically there is lots of data about ideal charging and discharging for various li-ion chemistries and formats. Tesla has such superb battery management that it seems there really is little impact regardless of choice. Honestly, we all can expect that "babying" might help, but there is no objective evidence of that with Tesla.

That said, I tend to charge at home nearly every day, typically at 208v/5a, and maintain about 70% SOC or so. I do use Superchargers often, I charge at 240v/80a when there is no Supercharger available. My own rated range at 20,000 miles is the same as it was whenI took delivery.

In sum, do whatever you feel like and you'll not do much harm. There is no need to worry about it.

 

[Saghost]

The system is designed to take it.

However, if charging at 80 A isn't necessary for your use, you will stress the components less by using a lower charge rate.

I read someone here on the forum arguing for a 64 A setting as the best choice for a dual charger car on an HPWC when you aren't in a hurry, and it seemed to make sense.

You get 80% of the speed, but only 64% of the heat (a function of current squared,) and split the load across both chargers.

 

[AWDtsla]

Edit: Further, the Gen2 chargers in the car (forget when the VIN switch was, but I know everything since late 2014 has them) are actually 48A rated and Tesla only allows them to be used up to 40A in the USA on single phase since there are no provisions in the J1772 spec to make a 96A EVSE.

So the only thing standing between us and charging at 48A on a HPWC is a software update? :frown::frown::frown:

 

[LoL Rick]

I generally follow Cottonwood's advice and set my car at 56 amps. But by all means, use 80 if you need it. I did that today.
HPWC IR Pics (more fun!) - Page 3

 

[David99]

According to Tesla's charge calculator on their own website, it is slightly more efficient to charge at 80 Amp compared to 40.

In terms of battery health, it makes no difference that would justify one or the other. Both rates are pretty low in relation to the battery size.

WK is absolutely right that these chargers are most efficient at their designed power level. Running them at lower rates doesn't increase efficiency nor does it help battery health long term. So do whatever is best for you.

 

[Ed Hart]

Ahh, so that was the details I was missing. Thanks again for all your research and your willingness to share it!

Ah...I have been looking for that info, too. I put in a 100 amp circuit at home for an HPWC (80 amp) and ordered twin chargers in my S. I guessed that at 60 amps, I would charge fast enough, but the workload would be split between the two, meaning less strain on either. I apparently made a good guess!

But...the real reason for getting dual chargers was to make on-the-road charging faster. Well....so far in 30,000 miles of travel I have stopped at over 80 different Superchargers - SC bypasses the in-car chargers - and a few resorts with HPWCs, but only a few stops at resorts with J1772 chargers. So far, none of my situations has required the dual chargers...and I conclude I would not buy dual chargers were I doing it again.

 

[FlasherZ]

I follow Tesla's advice - don't worry about it. Both our Model S and Model X charge at full power, always (80A/70A). I don't worry about it. Then again, I don't worry about charging to 100%, I don't worry about leaving within 35 seconds of the range charge completing, I don't worry about how much I supercharge.

I enjoy life.

 

[Cottonwood]

I generally follow Cottonwood's advice and set my car at 56 amps. But by all means, use 80 if you need it. I did that today.
HPWC IR Pics (more fun!) - Page 3

I generally charge at 56 Amps because of the symmetry and reduced thermal stress. The charging rate is 70% of max, but the resistive losses heating everything are 50% of max. Heat breaks things, and usually I don't need 80 Amp charging at home. That being said, I have no qualms about charging at 80 Amps whenever needed, and in cold weather, when you are trying to heat the battery, 80 Amps is better. All the circuits are designed to work at 80 Amps and the reduced stress is small at best.

With all that, I have limited my charging to 16 Amps for the last 3 weeks. With my normal driving habits, 16 Amp, 12 mph charging is just fine. That gives me 144 rated miles in a 12 hour, overnight charge.

I would never give up my 80/100 Amp charging (usable/breaker-size), but for most users with power limited panels, 24/30 or even 16/20 Amps at 240 Volts would be fine in the vast majority of situations.

 

[bollar]

I attempted to answer this question and basically found no difference at any setting between 20-80 amps:

What is the Most Efficient Charging Amperage?

 

[Quantum`]

Steve, your branch circuit is under-designed! The lines and breaker should never never get warm.

Good electricians will install over-designing by 1/3, so if you intend to charge at 80A the circuit should have at least 100 ampacity, meaning 1 gauge for runs up to 100'. And breakers to match.

If your house panel isn't in the garage, generally you're supposed to put in a branch breaker box or at least a quick cutoff in the garage. You must have two hots of #1 and one ground (which can be a bit smaller) going the run, assuming one of any of the connectors or the wiring insulation is rated 60°C. No need for a neutral to the outlet under Code, but jumper ground to neutral at the plug.

If you just don't worry about it, you'll have a fire. And insurance might not pay. Until you can upgrade that circuit, keep charging at 40A.

 

[AWDtsla]

Steve, your branch circuit is under-designed! The lines and breaker should never never get warm.

Good electricians will install over-designing by 1/3, so if you intend to charge at 80A the circuit should have at least 100 ampacity, meaning 1 gauge for runs up to 100'. And breakers to match.

If your house panel isn't in the garage, generally you're supposed to put in a branch breaker box or at least a quick cutoff in the garage. You must have two hots of #1 and one ground (which can be a bit smaller) going the run, assuming one of any of the connectors or the wiring insulation is rated 60°C. No need for a neutral to the outlet under Code, but jumper ground to neutral at the plug.

If you just don't worry about it, you'll have a fire. And insurance might not pay. Until you can upgrade that circuit, keep charging at 40A.

You understand that 60C is beyond the threshold of pain right? The wires and breakers will get warm.

 

[Quantum`]

lol, you're funny...

 

[Steve2498]

Steve, your branch circuit is under-designed! The lines and breaker should never never get warm.

Yawn - thanks for your concern, but the circuit has been designed appropriately.

- - - Updated - - -

Steve, your branch circuit is under-designed! The lines and breaker should never never get warm.

Yawn - thanks for your concern, but the circuit has been designed appropriately.

 

[FlasherZ]

Steve, your branch circuit is under-designed! The lines and breaker should never never get warm.

Good electricians will install over-designing by 1/3, so if you intend to charge at 80A the circuit should have at least 100 ampacity, meaning 1 gauge for runs up to 100'. And breakers to match.

If your house panel isn't in the garage, generally you're supposed to put in a branch breaker box or at least a quick cutoff in the garage. You must have two hots of #1 and one ground (which can be a bit smaller) going the run, assuming one of any of the connectors or the wiring insulation is rated 60°C. No need for a neutral to the outlet under Code, but jumper ground to neutral at the plug.

If you just don't worry about it, you'll have a fire. And insurance might not pay. Until you can upgrade that circuit, keep charging at 40A.

This is incorrect. At 80A of continuous current, the conductors and breaker will indeed become warm. Touch-warm is fine, touch-hot or "hot electronics smell" is not.

On the conductor size, you're incorrect - there is no "over-designing by 1/3" rule. Electricians follow the NEC (or CEC in Canada), as well as local amendments. NEC dictates that an EV charging loads be considered "continuous loads". Continuous loads require that conductors, over-current protection devices (breakers and fuses), and load calculations be rated for 125% of the charging current. In this case, that means the circuit must be sized for 100A. If you consult article 310 of the NEC, you'll find an ampacity table. #3 wire is good for 100A at the 75 degree termination rating, which is the rating for most modern breakers, panels, and other equipment (older equipment may require 60 degree termination, which would be #2). NM cable ("Romex") is not permitted for 80A HPWC installs because it must be used at the 60 degree column (see section 334.80) and that's only good to 95A (remember, 100 is required).

Your statement on ground and jumping to neutral is not only wrong, but completely unsafe and illegal. The EGC is never permitted to act as the neutral. In all new construction (post-1996), neutral and ground are separated, and joined in only one location at the main service panel.

For more information, see my FAQ located here:
FAQ: Home Tesla charging infrastructure QA