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

[TSLA Pilot]

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.

If the PW voltage is ~350 volts, this is an easily solved problem: for DC-to-DC Direct Tesla charging, there must be at least two PW's installed. If wired in series, that would give us ~700 volts available for charging, no?

Or perhaps Tesla's PW v3.0 will have voltages that are better aligned with DC charging for Tesla's own vehicles?

Even if the loss is less than a 5% (each way?), the fact that there's any still strikes me as dumb. DC power should stay DC as waste is still waste; given the problems we have with so much GHG dumping, every little bit we can do enhance the new protocols, the better.

Tesla should really work on this, perhaps be partnering with a builder (like ours perhaps?) to create a new DC standard for new/retrofit in the residential markete. Our builder is making solar and home storage standard in his latest developments (but why he picked imported batteries from Sonnen in Germany over Tesla Powerwalls from the nextdoor State of Nevada is hard to fathom):

Mandalay to build 3,000 Arizona homes with solar and Sonnen batteries

October 14, 2016. REUTERS/Axel Schmidt

(Reuters) - German battery maker Sonnen GmbH on Thursday said it would partner with home builder Mandalay Homes to outfit 3,000 new Arizona homes with batteries to store the excess energy generated by their rooftop solar installations.

[A lithium battery unit of the startup "sonnen", formerly known as Sonnenbatterie . . . .]

Mandalay plans to build the homes in Prescott, Arizona with the batteries, which can cost between $10,000 and $20,000. The cost would be part of the home’s sale price and wrapped into the mortgage.

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[MP3Mike]

If the PW voltage is ~350 volts, this is an easily solved problem: for DC-to-DC Direct Tesla charging, there must be at least two PW's installed. If wired in series, that would give us ~700 volts available for charging, no?

Well you can't just put 700 volts into the car, you have to have a DC-DC charger to handle that. Which will introduce lose as well. (Just less than the DC-AC-DC that is currently happening.)

Also, I doubt Tesla is going to series wire Powerwalls as I think safety and wiring gets to be a big problem.

 

[landis]

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.

My 'daily' commute is 92 miles (or about 25kWh.). I currently telecommute 1 day/week (Friday) and could do 2 days (Tues,Thurs).
I.e. commute M,W,F, charge from PV during T,T,S.

The theoretical advantage I've not seen discussed yet is whether DC/DC taped from the PV charging side would be practical in this sort of scenario where primary charging sessions are during PV availability. I presume 1 Powerwall might still be minimum, which I am interested in anyway due to bad utility reliability history in my neighborhood. I am patient and willing to wait for such potential increased efficiency until a Tesla Solar (tile) roof can be installed. ;-)

 

[mongo]

The theoretical advantage I've not seen discussed yet is whether DC/DC taped from the PV charging side would be practical in this sort of scenario where primary charging sessions are during PV availability.

If this was the main goal, there is no reason that one couldn't make solar controllers to convert each PV panel's (or string if shadowing isn't an issue) output to charge a Tesla directly.

 

[Ampster]

I have no need for DC charging. When I consider that transmission and distribution losses are over 30% when using the grid, I am not concerned with the small losses by comparison in converting DC to AC and back again in my garage. Besides, I recover some of the heat produced with my heat pump water heater. I put my inverter near my water heater for that reason. In my own all electric home, I am directing my resources toward making my building envelope less lossy. That is where my low hanging fruit lies. Over 50% of my charging is at Superchargers anyway, so I will let Tesla figure out their priorities.

 

[TSLA Pilot]

I have no need for DC charging. When I consider that transmission and distribution losses are over 30% when using the grid, I am not concerned with the small losses by comparison in converting DC to AC and back again in my garage. Besides, I recover some of the heat produced with my heat pump water heater. I put my inverter near my water heater for that reason. In my own all electric home, I am directing my resources toward making my building envelope less lossy. That is where my low hanging fruit lies. Over 50% of my charging is at Superchargers anyway, so I will let Tesla figure out their priorities.

All of the factors listed are outside of Tesla's control. The unnecessary DC to AC to DC conversion is ALL within Tesla's control, and within Tesla's ability to fix as well . . . .

Lastly, even if it is a small loss (which we can't confirm), small losses multiplied by many millions of Teslas, add up to large losses. I expect future versions of the PW will have an internally and automatically switched DC or AC tap so that Teslas can be directly charged via DC, or switched over to AC when conditions require. After all, the DC Supercharger plug appears to be the same as the AC UMC plug, and I don't think the AC to DC conversion is taking place in the UMC brick, right?

 

[oxymai]

I am new here and please help me understand a few things about PW2.
14kWh PW2 seems not to be enough for an average house consumption per day even though study shows 6000kWh per year for one house.
That is probably why PW3 will go to 26kWh and you might need 2 PW2s for your house. Does Tesla know that they are wasting one/mutiple power conversion on the second PW2 as battery capacity increasing is the only thing we need? Going to 3 ,4, 5 as for self consumption is something pretty interesting!!!
Can anyone here know how many power conversion levels are inside PW2 - AC?
Thanks,

 

[dhanson865]

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

Assuming the connection and logic isn't an issue and we are just talking power and energy:

1 powerwall for every 30a x 240v equivalent charge rate you want (call it 2 powerwalls for a single car to do a nice 40a 240v equivalent pace) x however many sets of those you need to finish the charge without draining the powerwalls to nothing (call it 20 kWh per set of 2)

So if you want to add 20 kWh to a single car 2 "powerwall 2" are needed not counting the ones you need for your normal home needs (refrigerator, AC, and such).

Surely you can get to a Supercharger or Chademo or public L2 of some kind once you have more than 20 kWh in your cars pack.

No reason to be using poweralls to charge a car outside of an emergency.

 

[MP3Mike]

14kWh PW2 seems not to be enough for an average house consumption per day even though study shows 6000kWh per year for one house.
That is probably why PW3 will go to 26kWh and you might need 2 PW2s for your house. Does Tesla know that they are wasting one/mutiple power conversion on the second PW2 as battery capacity increasing is the only thing we need?

Well in my case I would need more than the 5kW that a single Powerwall can put out to run my house, so scaling energy with power makes sense.

 

[GenSao]

I am new here and please help me understand a few things about PW2.
14kWh PW2 seems not to be enough for an average house consumption per day even though study shows 6000kWh per year for one house.
That is probably why PW3 will go to 26kWh and you might need 2 PW2s for your house. Does Tesla know that they are wasting one/mutiple power conversion on the second PW2 as battery capacity increasing is the only thing we need? Going to 3 ,4, 5 as for self consumption is something pretty interesting!!!
Can anyone here know how many power conversion levels are inside PW2 - AC?
Thanks,

First, There is no PW3 yet. Anything you read online is conjecture. Honestly, there is enough demand for the PW2 that Tesla is likely in no hurry for another version.

One Powerwall 2 (13.5 kWh usable energy) is generally limited to 30A max or 5 KW of power (7 KW Peak) enough for partial home backup and some time of use shifting. It is not enough to cover the whole home, but more than enough to cover all the 20A circuits and perhaps a 30 A circuit or two. Loads greater than 30A (such as AC, EV charging, electric ranges, electric cloths driers) are excluded from backup. In the event of a power outage, one can manage their loads to fit within their power production/storage capacity.

Two Powerwall 2 (27.0 kWh usable energy) is generally limited to 60A max or 10 KW of power (14 KW Peak) enough for most household use. Most household circuits can be connected. Just add more Powerwalls if more power is needed +5 KW of power (+7 KW Peak) each. Check out, How long will Powerwall last in an outage? for an idea of power use.

If a home consumes 6,000 kWh in a year, the daily use is 16.4 kWh in a day (6,000 / 365 days/year). batteries with solar can permit a home to effectively be off the grid. The batteries can be used for night time loads while solar can power the home during the day and charge the batteries.

Each Powerwall has its own inverter. There is approximately a 10% in-and-out loss converting from AC to DC and back to AC. So 10 kWh of energy in is 9 kWh of energy out. There is an added loss from the solar panels from DC to AC. Hopefully there is a future PW that is DC tied to minimize the energy loss.

 

[miimura]

I am new here and please help me understand a few things about PW2.
14kWh PW2 seems not to be enough for an average house consumption per day even though study shows 6000kWh per year for one house.
That is probably why PW3 will go to 26kWh and you might need 2 PW2s for your house. Does Tesla know that they are wasting one/mutiple power conversion on the second PW2 as battery capacity increasing is the only thing we need? Going to 3 ,4, 5 as for self consumption is something pretty interesting!!!
Can anyone here know how many power conversion levels are inside PW2 - AC?
Thanks,

Regarding power conversions - the battery is DC and the interface is AC. So, to charge the battery it uses a variable current rectifier, commonly called a battery charger. To discharge the battery, it uses a variable power inverter. The inverter can operate in grid-tied mode and off-grid mode. When you charge an EV, the car's on-board charger is converting the AC power to DC to charge the battery. If you wanted to charge an EV directly from the Powerwall battery, you would have to step up the DC voltage, which is commonly done by inverting to AC, using a transformer to raise the voltage and then rectifying the AC back to DC. So, this is not necessarily more efficient than what is provided by the Powerwall 2 AC.

 

[TSLA Pilot]

Regarding power conversions - the battery is DC and the interface is AC. So, to charge the battery it uses a variable current rectifier, commonly called a battery charger. To discharge the battery, it uses a variable power inverter. The inverter can operate in grid-tied mode and off-grid mode. When you charge an EV, the car's on-board charger is converting the AC power to DC to charge the battery. If you wanted to charge an EV directly from the Powerwall battery, you would have to step up the DC voltage, which is commonly done by inverting to AC, using a transformer to raise the voltage and then rectifying the AC back to DC. So, this is not necessarily more efficient than what is provided by the Powerwall 2 AC.

While I have no idea of what the DC voltage is of the PW2, I'd hope that it is higher than the nominal DC voltage of the Tesla EV's fleet?

If not, perhaps PW3 will consider this in the design phase as it may be convenient for future use, although I do concur that the energy capacity of the PW2 battery makes it sub-optimal for use in charging pretty much any Tesla EV (60, 70, 75, 90 and 100 kWh EV batteries, thus far, vs. ~14 kWh for a PW2?)

 

[MP3Mike]

While I have no idea of what the DC voltage is of the PW2, I'd hope that it is higher than the nominal DC voltage of the Tesla EV's fleet?

Nope, the internal voltage is kept nice and safe at 50 volts.

 

[MP3Mike]

Hopefully there is a future PW that is DC tied to minimize the energy loss.

They had originally planned to make one, I even have the spec sheet for it, but apparently they decided it was better to make a single version that would work for almost any install. (The round-trip efficiency was only 2.8% better.)

 

[miimura]

They had originally planned to make one, I even have the spec sheet for it, but apparently they decided it was better to make a single version that would work for almost any install.

I think they did a pretty good job covering almost all installs. The exception is generator support. Remote start and an additional independent AC input for battery charging would be very helpful for extended grid outages and off-grid installations.

 

[oxymai]

Regarding power conversions - the battery is DC and the interface is AC. So, to charge the battery it uses a variable current rectifier, commonly called a battery charger. To discharge the battery, it uses a variable power inverter. The inverter can operate in grid-tied mode and off-grid mode. When you charge an EV, the car's on-board charger is converting the AC power to DC to charge the battery. If you wanted to charge an EV directly from the Powerwall battery, you would have to step up the DC voltage, which is commonly done by inverting to AC, using a transformer to raise the voltage and then rectifying the AC back to DC. So, this is not necessarily more efficient than what is provided by the Powerwall 2 AC.

This has nothing to do with EV car. PW2 is a complete different product. EV get charged power directly from the grid.

First, There is no PW3 yet. Anything you read online is conjecture. Honestly, there is enough demand for the PW2 that Tesla is likely in no hurry for another version.
Only time can tell and we are talking about the product line, not a very specific case.


One Powerwall 2 (13.5 kWh usable energy) is generally limited to 30A max or 5 KW of power (7 KW Peak) enough for partial home backup and some time of use shifting. It is not enough to cover the whole home, but more than enough to cover all the 20A circuits and perhaps a 30 A circuit or two. Loads greater than 30A (such as AC, EV charging, electric ranges, electric cloths driers) are excluded from backup. In the event of a power outage, one can manage their loads to fit within their power production/storage capacity.

You are completely right about this, the second diagram of PW2 powering the whole house is really misleading.


Two Powerwall 2 (27.0 kWh usable energy) is generally limited to 60A max or 10 KW of power (14 KW Peak) enough for most household use. Most household circuits can be connected. Just add more Powerwalls if more power is needed +5 KW of power (+7 KW Peak) each. Check out, How long will Powerwall last in an outage? for an idea of power use.

Technically, Dryer ~ 5.6kW, Range ~4kW, Microwave~1.6kW and 5kW power conversion is perfect for this product as the big loads mentioned do not run at the same time very often, but having second 5kW when you want to increase battery capacity is a waste and the 5kW power conversion costs around 30% of the PW2 unless Elon gives out for free!

If a home consumes 6,000 kWh in a year, the daily use is 16.4 kWh in a day (6,000 / 365 days/year). batteries with solar can permit a home to effectively be off the grid. The batteries can be used for night time loads while solar can power the home during the day and charge the batteries.

Running off grid will not be able to maximize your investment, you do not want PW2 shutdown when turning on big loads (Dryer ~ 5.6kW, Range ~4kW, Microwave~1.6kW) by accident.

Each Powerwall has its own inverter. There is approximately a 10% in-and-out loss converting from AC to DC and back to AC. So 10 kWh of energy in is 9 kWh of energy out. There is an added loss from the solar panels from DC to AC. Hopefully there is a future PW that is DC tied to minimize the energy loss.

You are right from technical view point, less power conversion, less energy loss.
I guess this ship has sailed as DC Powerwall was a complete mistake and it seems Tesla has learnt this lesson.
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Inside DC powerwall and also Powerpack (16 DC powerwalls) , there is a boost DC/DC converter, anyone here knows it is GALVANIC isolated or non-isolated?
Thanks