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POWERWALL - DC to AC inverter not included ? !

POWERWALL - DC to AC inverter not included ? !

The Powerwall specification says it is compatible with the utility grid. The same Powerwall specification says DC-AC Inverter not included.

I thought that a DC-AC Inverter was required between a battery and an AC Grid. How can the Powerwall be compatible with the utility grid when a DC-AC Inverter is not included?

Can someone enlighten me?

ian t.wa.us | 1 mei 2015

And I thought i heard Elon say it was " built in" in the presentation last night?

windex | 2 mei 2015

You can't put an inverter in a battery reasonably because you can't just parallel out of phase AC sources into one AC source.

That means all your batteries need to be DC and they need to be inverted once.

Because battery applications are not really one size fits all it really doesn't make sense to include one with the battery when it varies based on how it's used.

Brian H | 2 mei 2015

AC-DC built in, suited for charging the batteries.

science-isbetter | 2 mei 2015

Good question @flyjeffva. Understood that AC-->DC is built in. But what about inverters (DC-->AC)?

windex | 2 mei 2015

According to the presentation, the unit contains a DC-DC converter, which is not the same as an AC-DC rectifier/charger.

An AC-DC rectifier/charger for a 350-450V battery at 3kW has an internet price of about $2,500.

A DC-AC inverter for such a system is going to cost a similar amount (possibly more), but I can't even find one ( see https://www.littleboxchallenge.com/ ), so you'd probably have an extra 1% efficiency loss stepping the 350-450VDC down to something the inverter can handle (12/24/48VDC).

rlwrw | 2 mei 2015

The Powerwall comes bare bones basic so that owners of existing solar/wind/hydro systems can just about literally plug the pack into their system. Very little, if any, interfacing needed.
New installations would have to also acquire supporting interfaces and controllers.

windex | 2 mei 2015

@rlwrw

The battery voltage indicates the battery is designed for a system using a DC-DC converter styled after this:

http://www.ti.com/tool/TIDM-SOLAR-DCDC

That particular DC-DC converter is relatively low cost but limited to 500W and it gets very expensive very fast if you exceed 500W in that type of design (I would say the cost goes up 3x for every 2x increase in load).

That's a test kit but the pricing seems pretty reasonable for the complexity of the circuit as a manufactured product. It's not a complete device, though. Just a board. By the time you install stuff to deal with cooling, fuses, etc, it'd probably go up quite a bit - at least $1,000.

Because the voltage conversion normally happens right by the batteries, there's no particular benefit to working at this voltage because the efficiency rates of that prototype are 94%. That seems like crap, honestly, but I'm used to working with much lower voltage higher current equipment designed to conserve at all costs. It also increases the complexity of the other end (DC-AC inversion).

For pretty much all but custom-built applications this is not very bare bones basic.

bryan.whitton | 3 mei 2015

I would presume that the output voltage is in the 350 to 450 Vdc range to allow it to serve as an input to the standard solar string inverter. These require a DC voltage in the range of 250 to 600 Vdc.

No need to use a DC-DC converter it isn't needed.

windex | 4 mei 2015

@bryan.whitton

Solar inverters that charge batteries include DC-DC converters.

That particular reference converter linked is a reference design that would normally be used as part of a larger project.

The battery operates at 350-450VDC and the solar panels almost universally at 200-300. A DC-DC converter is required somewhere, because going to AC and back to DC at that low of an amperage would be lossy as hell.

windex | 4 mei 2015

Also note that many older/generally available solar panels operate at 6-48VDC because they're not pushing any amps and are designed to bleed right into batteries with a regulator and nothing else. The newer stuff seems to be high voltage. Solar string inverters are only cheap because there's almost no load except during the battery discharge stage.

grega | 4 mei 2015

It certainly seems that half the question has been clearly answered. How the batteries are actually USED depends entirely on the inverters - and any systems for managing your power requirements.

I suspect the expense of inverters would come down significantly with high sales too - which is what Elon is trying to kick start right?

To some degree I guess that SolarCity provides more of that partner product - and perhaps they should have been part of the presentation.

Generator integrated systems would be another partner product, Home peak reduction would be another (common) setup.

It'll be interesting to see how the battery is applied.

windex | 4 mei 2015

Or they could make a battery system that worked -functionally- identically to every other battery on the market while playing up strengths of Li-Ion and everyone would be able to reap the hard won cost and conversion efficiency decades of investment have wrought on the other end of the system right now.

I mean, Tesla didn't decide to go straight to flying cars, right? :)

(I'm biased.)

grega | 4 mei 2015

If you're right, then someone will take the idea and use the patents, and make a better solution.

If they see money in it :)

bryan.whitton | 9 mei 2015

@windex you are right. To charge the batteries you need the input voltage adjusted from the array to the battery charge level. Once the battery is charged the DC from the array will pass through to the array inverter the same as the DC from the array would. As I don't have specific information other than the datasheet I think that the Powerwall is designed to simply sit between the array and the inverter on a string inverter array.

The input voltage for string inverters is typically between 250 and 550 with an upper limit of 600. ABB inverters can be adjusted down to about 120Vdc but the rest operate at the higher voltages.

Off grid inverters typically work in the 150 VDC range and MPPT charge controllers work at that point as well.

Most all 60 cell solar modules are 36VDC. The 72 cell models work at about 48VDC and you wire them up in series to get the voltages you need. Each string works at about 8 or 9 AMPS. This is all wrong for Sunpower and Silevo modules. They work at higher voltages, 48VDC for the 72 cell units and about 72VDC for the 96 cell models. None of these are intended to work off-grid with batteries. We are in a transition phase where the definition of grid tied and off grid are getting blurry. I think that over the next 5 years everything about solar and utility power is going to change dramatically.

windex | 11 mei 2015

The main problem with the Powerwall as I see it is simple.

The reason the battery is staged the way it is, is simply because it's cheap as hell.

It probably uses low amperage internals everywhere and thin gauge wire for all the integral connections within the battery.

Because of this the 350-450VDV at 6A makes sense and it works with enough solar inverters where it's not a big deal for Tesla's marketing purposes.

But for any OTHER user, it's an enormous expense to convert that to usable voltages and amps. $20-30 of beefier gear in the battery (as a DIY project, let alone at scale) would save thousands of dollars at the rest of the system.

That raises the question of why is Tesla selling this, because it's not to make batteries, and its application range at the discharge rate makes many home installations questionable.

ghana.consultants | 11 mei 2015

@windex and @bryan - The powerwall output voltage of 350-450Vdc is designed for string inverters like the ABB powerone and others that have input voltage from the arrays of about 500Vdc. So the way I see it, the output of the powerwall will be connected to an ATS(A side for the arrays and B side for the powerwall), so during the evening the ATS will switch to the battery so that the battery will feed the inverter.

bryan.whitton | 11 mei 2015

Yep, thatis what I said above. It is designed to sit between the array adn the inverter. It iwll operate at its native voltage so there will be no additional DC-DC conversion saving efficiency. The probelm is everyone wants backup and this model will not allow the Powerwall to be used as a method of backup. If the grid goes down the inverter will stop and that means no backup.

AltEnergy | 12 mei 2015

I think we need a hybrid inverter to do the backup functionality.

windex | 12 mei 2015

But see what you guys are saying is "the cheap battery was made to work with the cheap grid tie solar inverters", which is not a resounding endorsement of the product either. :)

My point is that it's effectively not a general purpose battery which means it's a niche product.

ghana.consultants | 12 mei 2015

So the question is; is there any hybrid inverter in the market that can handle the DC range(350 - 450vDC) of the powerwall?

Bubba2000 | 12 mei 2015

For the underdeveloped market, Tesla needs a plug and play product like a UPS. Connect the battery+inverter between the meter and the rest of the house, or a load shedding breaker. If 2 or more batteries need to be used, a synchronizer can be added or it can be built in.

Dr. Polar | 15 mei 2015

I was wondering about using this battery strictly for DC power. My plan is to rewire my house to use LED lighting and have outlets for various other things, like 5vdc, 12vdc, and 19vdc. The idea is most of anything in the house that doesn't have a motor effectively runs on DC power. So, converting the power from DC to AC just to turn it into DC again over a short distance seems like a waist of power. Is that something that is realistic and achievable? Can I pull multiple voltages off this battery?

SCOOBY | 15 mei 2015

"Can I pull multiple voltages off this battery?"

No, but you probably don't need to. If you really want to run your home on DC, maybe just use a DC-DC converter to bring the Powerwall voltage down to say 125/250V DC. Most anything that uses a switched mode power supply and runs from 125/250V AC will "just work" if you feed it DC instead. But I would not advise you to do this because if you accidentlay connect something that requires AC (e.g. a hairdryer) then bad things will happen :)
"

Dr. Polar | 15 mei 2015

You mean, put the 110AAC plug of a laptop charger into a 110VDC source? Maybe with a toaster?

But I really want to put things in the wall like USB sockets (5vdc), maybe 12V car charger outlets, and just other connectors, such as 19V for direct connection to a laptop. I want to do this because using all these "wall" transformers seems like a total waste. My laptop charger is basically a toaster with the heat it generates. There has to be a better way.

So, you would use a DC-DC converter (or multiples) to bring down the voltages. Is this feasible?

I would still keep the power company 110 for things like the refrigerator, washer, dryer, powertools, microwave, some other counter appliances.

SCOOBY | 15 mei 2015

Yes. DC will work on anything with a switched mode power supply (most any electronics) and anything with just a resistive load like a toaster. Things with a motor not so much.

Low voltage power distribution, even just within the home, isn't a very good idea because the losses are huge.

Brian H | 15 mei 2015

losses → heat.

Dr. Polar | 18 mei 2015

Well, inside the home, the code says that you must have at least 12 gauge wire. I cannot see a 5vdc source losing that much power. However, using this calculator: http://www.bulkwire.com/wireresistance.asp

Say you have 40 feet at 3 amps, the loss is acceptable at about 3% at 12 gauge. 24vdc would be better, but can you step down the voltage efficiently nearer the potential outlets? (without using simple resistors).

hpjtv | 13 juni 2015

@Dr. Polar, I have a 24V battery bank attached to some windmills and solar panels. I run the 24V all over the house and use DC-DC converters (you can get them on eBay for like $1-3 a piece depending on amperage you need) at the load to bring the voltages down from 1.5V-20V. I use it to power my laptops, security cameras, routers, modem, LED's, 12V accessory outlet, etc.

rcpaynewv | 15 juni 2015

Does Tesla offer an Inverter that they say is not included in the $3K-3.5K powerwall price but absolutley necessary if Powerwall is to be used as battery backup for a single refreigerator? Does the inverter have the Name and warranty of "Tesla"? What is the price? Is SolarCity absolutely the only installer to allow the Powerwall/Inverter to be warranteed? If not true, Who else?

Captain_Zap | 15 juni 2015

@rcpaynewv

There was some detailed answers to your questions at last weeks stockholder meeting. You can find the video on the investor's page here.

It was mentioned that Solar City made some statements that are not correct.

Brian H | 16 juni 2015

BBhai;
An inverter supplies DC from an AC source. It does not provide "backup for a battery". It is used to charge or recharge one. A battery is not an original source.

chris | 16 juni 2015

could this battery be used on a houseboat? Using Trojan L16H-AC deep cycle 6 volt now , with a Freedom 5W Inverter/charger. Dont know much about system as was on boat when I bought, but time for new batteries and thought this might be an option. Any input would be great.

THANKS

DougJohnson | 16 juni 2015

Brian H:
You've got it backwards. An inverter converts DC to AC so is used to discharge batteries, not charge them:

https://en.wikipedia.org/wiki/Power_inverter

-- Doug

Brian H | 16 juni 2015

Thanks for rectifying my comment.

DougJohnson | 16 juni 2015

Brain H:
Groan.

Red Sage ca us | 16 juni 2015

I love having a vocabulary!

Captain_Zap | 17 juni 2015

@cris

Is your houseboat a permanently moored houseboat? If so, It seems like it would work. Are you on fresh water? I doubt the power wall is designed for a salt water environment where corrosion is a concern.

It would be awesome if they could be used on all boats. Our lead acid house battery bank exploded once. The lead acid batteries gas off terribly when charging, creating H2S that triggers false O2 alarms. Both are deadly.

I know of others that have had battery banks explode on board vessels too. Thank goodness for secondary containment and the fact that it didn't happen when we were underway.

A built in inverter would be nice. Marine inverters that that we have gone through are not that great.

chris | 17 juni 2015

@Captain_Zap

The boat is in a fresh water lake,it is not permanently moored, So no worry of corrosion.Was looking at this as an option ( if works )to replace the Lead batteries for your points exactly, Plus they only last 5-6 years tops. Just looking for better long term options.

THANKS

Red Sage ca us | 17 juni 2015

I'm not sure if PowerWall uses the same system, but I believe it was reported the coolant for Model S battery packs also functions as a fire retardant/deccelerant.

GregZw | 17 augustus 2015

Anyone know if it might be possible to charge a powerwall with the same circuit I use to charge my Model S?

bryan.whitton | 17 augustus 2015

Not if built to code. The charger is a dedicated circuit as I recall. Therefore nothing else can be on the circuit. Why would you want something like this anyway. If you are feeding that much power why not install a sub-panel and have separate circuits?

tesla | 9 december 2015

You actually can 'parallel out' AC sources into a single AC source, that is exactly what grid-tie inverters do. You can connect as many grid-tie converters in parallel as you want. They just use a PLL to align the phase. Undergrad EE.

Frank.B.Smith | 9 december 2015

This reminds me of the DC (central inverter) verses AC (microinverter) solar panel configurations.

When you have a central inverter you have to figure out the number of panels and orientation and angle to determine how to connect them for optimum production before you can determine the size of the central inverter you require. If down the road you want to add more solar you need to discard the inverter you have and upgrade to another one.

With AC solar panels each panel comes with a microinverter that is perfectly sized to that panel. Adding more panels or even reconfiguring the existing panels is simple because each one is independent.

It seems like the powerwall solution is similar to a DC central inverter solar solution. Until you decide on how many powerwalls you plan on buying and what solar feeds and AC loads that it will need to support it is difficult to provide the appropriate inverter.

Enphase (the leader in solar microinverter company) is currently testing an "AC battery" storage solution in Australia. The batteries are not really storing AC but each battery module has its own microinverter built in. So you can add more AC battery modules as your needs grow. The only negative with this solution is that it is grid tied only. It will not provide backup power if there is a power outage. This solution is all about stabilizing the grid when intermittent renewables are providing most of the power to the grid. They are also used to buy power when it is cheap and sell it back at a higher rate during peak loads.

https://enphase.com/en-us/products-and-services/ac-battery