Why can't we charge our Teslas directly from PW, DC to DC?!?

[TSLA Pilot]

This is currently posted on the PW FAQ:

Can Powerwall charge my Tesla vehicle or an EV?
Yes, Powerwall can provide stored solar energy to your EV through your home electrical panel. However, there is no direct connection between Powerwall and the EV wall connector.

Powerwall FAQs

Color me very confused.

We're coordinating for two (or possibly more) PW's for a PV install on a new home in Phoenix, all with Tesla Energy/SolarCity.

SolarCity failed to submit the application on time (GRRRR!), so now we've lost out on being grandfathered into the barely decent APS "old" net metering, and now we'll be forced to use the new, "garbage net metering" with absurdly low buy back rates. Hence, our new need to capture our own power.

With both a 90D and a P100D, we would like to keep as much of our "home power" usable as DC because my understanding is that one loses about 10% in the conversion from DC to AC (and another 10% on the way back), and since our new home should be pretty efficient, our greatest power demand will likely be the Teslas.

So here's what Tesla/SolarCity is having every Tesla owner do with their PW?

1. Generate solar energy: DC.

2. Store solar energy in PW: Always DC.

3. Convert PW energy into AC for generic home use: DC to AC, lose ~10%?

4. Charge Tesla car batteries (batteries are always DC): Convert AC back to DC, lose another ~10%?

Especially considering that many PW's are installed in the garage, why is there no "Bypass AC Inverter/DC Out" option on a Powerwall?!?

Why is so much power being wasting and for no good reason?

Obviously, Tesla is filled with smart people doing smart things; what am I missing here?

 

[macpacheco]

In theory its doable, the PW would need to have a compatible Tesla charger cable with the proper signaling to talk to the vehicle.
The key issue is a single PW v2 is a wee bit too small to charge any Tesla.
And the overwhelming majority of home charging take place at night, when solar isn't producing, so the DC-DC pathway couldn't be used.
Finally the car would have to be fairly close to the PW, due to the signaling issues, or a dedicated DC-DC box would have to be developed instead.
And how would it work if you had 2 or 3 PWs ?
People tend not to have high voltage DC cables running inside their houses. Direct DC PW output would be >300 Volts.
It seems the idea is a bit too unpractical so far.
Hopefully in 5-10 years Tesla will have a bigger PW or smaller PP that would actually be practical for this need.

Plus does a PW v2 even has a DC output ? Or is it AC/DC input and AC output only ?

Notice most of those losses when charging PW->Tesla car is at the battery rather than inverter.
Charging and discharging a battery produces lots of heat.
It might be more efficient to charge at night directly from off off peak electricity and use PW juice for day-to-day needs.

 

[mongo]

1. Generate solar energy: DC.

2. Store solar energy in PW: Always DC.

3. Convert PW energy into AC for generic home use: DC to AC, lose ~10%?

4. Charge Tesla car batteries (batteries are always DC): Convert AC back to DC, lose another ~10%?

Especially considering that many PW's are installed in the garage, why is there no "Bypass AC Inverter/DC Out" option on a Powerwall?!?

Why is so much power being wasting and for no good reason?

Obviously, Tesla is filled with smart people doing smart things; what am I missing here?

Unless you have the old version Powerwall, the Solar to Powerwall connection is also AC.
So solar(DC) -> AC -> Powerwall (DC internal) -> AC -> Tesla (DC internal)
Even at 95% efficiency per step, that's 0.95^4 = 81%, 19% loss
If the Tesla is home during the day, loss is 10%.

The root issue is that none of the DC items use the same voltage whereas AC is standard. If you could reduce that intermediate step you'd have.
Solar (@some variable voltage) -> PW (ideally higher than the solar max solar voltage) -> Tesla (ideally lower than PW voltage)
Two stages of conversion again, 10% loss at 95% a stage.
So it would have half the loss and 10% more overall efficiency, but require separate converter circuits.
Tesla could make a PW to Tesla converter which then gets you:
Solar -> AC -> PW -> Tesla
3 stages of conversion 14% loss, 5% total loss different from current, 25% reduction in loss from current.
Plus as @macpacheco noted, combining PWs would be interesting..

Most efficient route is solar to Tesla which could be done also.

Long story short: AC is a standard, DC is not, but it can be done how you envision.

 

[TSLA Pilot]

Long story short: AC is a standard, DC is not, but it can be done how you envision.

Concur, but here's the rub: Tesla Energy is Tesla. Why doesn't Tesla do this and effectively set the DC standard? Surely the total energy needed issue will be addressed by our multiple PW's, and by the passage of time: PW v3.0, v4.0, and so on will have even more power.

I can easily see a future where Tesla is designing off-grid communities with massive PV Arrays and Powerpaks, and a lot of home internal DC, but I'd hope we can just start by keeping DC as DC--it's what we end up with from the Sun, and it's what BOTH the PW's and our Teslas need.

Seems like it's not rocket science.

 

[TSLA Pilot]

p.s. As for the voltage and voltage, heck, the status quo can have us running at the full 40 amps, 250 volts; that's 10,000 Watts, or 10 kW. I can't imagine DC being much more dangerous than what's streaming through the UMC and the NEMA 14-50 on the wall?

 

[mongo]

p.s. As for the voltage and voltage, heck, the status quo can have us running at the full 40 amps, 250 volts; that's 10,000 Watts, or 10 kW. I can't imagine DC being much more dangerous than what's streaming through the UMC and the NEMA 14-50 on the wall?

DC is a lot worse to deal with in terms of switching and isolation. Switching because whereas AC goes to 0 volts 120 times a second, DC is constant, so if a switch arcs, it will keep arcing until the contacts melt far enough apart to break the circuit. That is why relays have lower rating for DC vs AC. Ways around this are gap distance and magnetic quenching.
AC is easily isolated and adjusted with a transformer, the transformer can also be used to limit output power. DC requires a switching converter and UL puts extra requirements on circuit reliability if the feed is not isolated.
DC is also a problem in that your muscles clench, but don't release if you are electrocuted.

 

[TSLA Pilot]

Yes, thank you for the info.

Just to be clear, however, every Supercharger in the World is DC powered and up to 135 kW (for now; going higher soon).

Surely if it's something that John Q. Public can handle without gloves in a thunderstorm and while standing in a puddle of water, Tesla can find a way to make DC safe at home as well?

All those percentages of wasted energy add up, and we can't afford to be wasting that much power; the planet is counting on us.

 

[DriveMe]

How many Powerwalls would it take to charge a Tesla car?

 

[mongo]

Yes, thank you for the info.

Just to be clear, however, every Supercharger in the World is DC powered and up to 135 kW (for now; going higher soon).

Surely if it's something that John Q. Public can handle without gloves in a thunderstorm and while standing in a puddle of water, Tesla can find a way to make DC safe at home as well?

All those percentages of wasted energy add up, and we can't afford to be wasting that much power; the planet is counting on us.

Ev charge cables have ground fault interrupt circuits that can detect current leakage in the tens of milliamps. I expect Superchargers have something similar. They also have interlocks such that the cables aren't live until fully seated in the port. So similar circuits could be used to increase safety for a DC centric system (AC is easier to detect a fault on though, imbalance in current causes imbalance in magnetic field which us sensed by a transformer. DC requires high precision current sensors or other means)

 

[electracity]

Why? Because Tesla put a number on the value of charging from DC at home. The value is pennies.

Tesla will probably do it eventually, because a lot of people would like to power everything from solar.

I would pay for the capability, but just because I like the idea,

 

[animorph]

The Supercharger, or the charger in the car, has to adjust the DC voltage in order to control the current going into the battery. In order to use a Powerwall to charge a Tesla car battery, a DC to DC converter would have to be used. That would allow the DC output voltage to the car to be controlled. Probably more efficient than converting to 60 HZ and back to DC, but roughly the same process.

 

[mongo]

The Supercharger, or the charger in the car, has to adjust the DC voltage in order to control the current going into the battery. In order to use a Powerwall to charge a Tesla car battery, a DC to DC converter would have to be used. That would allow the DC output voltage to the car to be controlled. Probably more efficient than converting to 60 HZ and back to DC, but roughly the same process.

Yep, and according to the literature, each of the 16 modules in a powerpack has it's own DC-DC converter to control charge/discharge.

 

[mongo]

The Supercharger, or the charger in the car, has to adjust the DC voltage in order to control the current going into the battery. In order to use a Powerwall to charge a Tesla car battery, a DC to DC converter would have to be used. That would allow the DC output voltage to the car to be controlled. Probably more efficient than converting to 60 HZ and back to DC, but roughly the same process.

Correction to this, the supercharger DC is likely not ground referenced, so there is no ground fault protection involved., any leakage current would need to flow down the opposing wire, and that cannot be directly measured. It could detect a conductive path to earth/ chassis ground (wiring fault).

 

[TSLA Pilot]

How many Powerwalls would it take to charge a Tesla car?

You must not have a Tesla?

Actually, it would take less than one Powerwall to charge a Tesla.

The average daily mileage in the US is around 20 or 30 miles, IIRC? Thus, that's a mere fraction of the battery capacity on a Tesla, say 10% of a 100D.

Thus, only 10 kWh, which is less than the capacity of a single Powerwall.

 

[TSLA Pilot]

Why? Because Tesla put a number on the value of charging from DC at home. The value is pennies.

Tesla will probably do it eventually, because a lot of people would like to power everything from solar.

I would pay for the capability, but just because I like the idea,

Ditto--I too would pay for an optional "DC Out" port to allow direct DC to DC charging, Tesla PW to Tesla cars.

The waste of the energy in this needless conversion process is bothersome, and worse, it's probably all turned into heat? In Arizona, any added heat is undesirable given the temperatures frequently exceed 110F.

Thank you for all the informative replies here; let's hope that Tesla is reading this and takes the appropriate action.

 

[DriveMe]

The average daily mileage in the US is around 20 or 30 miles, IIRC?

It’s actually more than 45 miles per day for an average male driver (source: Average Annual Miles per Driver by Age Group)
Likely even more than that for an average Tesla driver!
So a single Powerwall is certainly not going to be enough, even on average.

And if you can’t fully charge the car from the Powerwall (can’t even recoup your daily commute) then this at-home DC charging wouldn’t make much sense, IMHO. Are you going to be switching from DC charging to AC charging in the middle of the night?

 

[mongo]

It’s actually more than 45 miles per day for an average male driver (source: Average Annual Miles per Driver by Age Group)
Likely even more than that for an average Tesla driver!
So a single Powerwall is certainly not going to be enough, even on average.

And if you can’t fully charge the car from the Powerwall (can’t even recoup your daily commute) then this at-home DC charging wouldn’t make much sense, IMHO. Are you going to be switching from DC charging to AC charging in the middle of the night?

From OP's OP

We're coordinating for two (or possibly more) PW's for a PV install on a new home in Phoenix, all with Tesla Energy/SolarCity.

So it doesn't look like switching from DC to AC would be a problem.

But to answer your question, it takes 8 Powerwalls to fully charge a 100kWh pack (assuming minimal losses).

 

[DriveMe]

it takes 8 Powerwalls to fully charge a 100kWh pack (assuming minimal losses).

Assuming minimal losses and assuming fully charged Powerwalls...

Although I do see some scenarios where this might work, I question how common those scenarios would be and whether it would be feasible for Tesla to offer a charging solution that only works under certain conditions.

If they could come up with some home charger that would automatically switch from DC to AC charging, it would make more sense of course. But without that, I can’t see them implementing it.

 

[MP3Mike]

Ditto--I too would pay for an optional "DC Out" port to allow direct DC to DC charging, Tesla PW to Tesla cars.

How much would you be willing to pay? That "DC out" port would need to have a DC-DC convertor/charger as part of it, which would probably cost about $2,000. (I think they cost about the same as a AC charger.)

And then there is the issue of the additional wiring, and electronics, between Powerwalls that would be needed to support more than a single Powerwall being able to supply energy to that "DC out" port.

Originally Elon announced a new charger would be released along with the Powerwall 2 but it never happened. I suspect it ended up being too expensive to integrate it into a system and actually make it work well.

 

[nwdiver]

So here's what Tesla/SolarCity is having every Tesla owner do with their PW?

1. Generate solar energy: DC.

2. Store solar energy in PW: Always DC.

3. Convert PW energy into AC for generic home use: DC to AC, lose ~10%?

4. Charge Tesla car batteries (batteries are always DC): Convert AC back to DC, lose another ~10%?

Especially considering that many PW's are installed in the garage, why is there no "Bypass AC Inverter/DC Out" option on a Powerwall?!?

Why is so much power being wasting and for no good reason?

Obviously, Tesla is filled with smart people doing smart things; what am I missing here?

DC to AC conversion is a bit more efficient than that... losing <5%. Most of the loses in the round-trip efficiency of the PW are chemical not the actual DC-AC conversion. The voltage of the PW IIRC is ~350v so if you have a PW that's <50% SOC charging a Tesla to >50% SOC you're gonna need to boost the voltage anyway. It's easier just to convert it to good 'ol 240vac and let the onboard chargers you already paid for do the work.