# Direct HV solar charging question



## dcb (Dec 5, 2009)

I'm not entirely sure I follow, are you using a stationary battery? Or just offsetting grid usage with inverters?

fwiw, 3kw charging my leaf @home ([email protected]) has not been much of an issue for me, not enough to mess with anyway. At first I thought it would be because of all the high power hype, but really home is chill, plenty of time for recharging.


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## AntronX (Feb 23, 2009)

Some ideas:

If you want an option to directly connect PV array into Spark's DC plug, you will have to go with big central inverter and a transfer relay in between. Just make sure your relays can extinguish 600V DC arc. The PV array will have to be left isolated from the ground, as the Spark may check for that during DC charger handshake. You will have to reverse engineer DC charger protocol and make some kind of controller to fool the car into closing battery relays to charge as well as monitoring for BMS signalling to stop charging. 

Not sure if it is worth the trouble compared to plain grid-tie net metering. You will save about 12% in conversion losses during charging, but without MPPT control you may lose some PV production efficiency anyway. But you can then proudly declare that your car's charge came direct from solar, and not via fossil powered proxy. 

I would like to adapt typical J1772 AC plug and socket into DC charging with my Nissan Leaf. Either to get rid of CHADEMO socket and install another J1772 in it's place wired for DC or reuse existing AC socket but with relay switching network to keep AC charging option. Would have to handle digital comms between outboard charger and the car somehow. 

One enticing feature of running 390V DC instead of 240V AC is ~1.75 times faster charging at the same current flowing through the cable. At 32A 390V DC, charging speed becomes similar to 57A 240V AC (after including onboard charger losses). At that power I can charge the Leaf from 50% overnight storage SOC up to 90% departure SOC in only 44 minutes. But I suspect 30A AC rated J1772 plug can safely handle more current as long as there is some kind of provision to automatically decrease current in case power pins get too hot due to bad contact. I have 500A low voltage AC current source for connector torture testing. So far I found that NEMA14-50 plug can operate at 80A continuously without significant heating (NEC be damned!). Also I found that 50A rated breaker's trip point is 56A after 30 seconds. Even at 200A it took few seconds for it to trip. But at 52A it dissipates about 10 watts and gets pretty hot!


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## PStechPaul (May 1, 2012)

It seems like it might work OK to charge directly from the solar panels, as long as you add a rectifier to avoid reverse biasing the panels when insolation is low. And you would also need a way to monitor the battery pack SOC and terminate when fully charged. It may not be quite as efficient as an MPPT DC-DC converter, however, but it would certainly be simpler.


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## bicycleguy (Aug 13, 2015)

dcb said:


> I'm not entirely sure I follow, are you using a stationary battery? Or just offsetting grid usage with inverters?


No stationary, only the car. The plan is to not use the inverters at all for the car charging. I could use one string for the car and one for the grid connected inverter or no car, 2 for grid-inv or no gridinv 2 for car, all done with relays and some control.



> fwiw, 3kw charging my leaf @home ([email protected]) has not been much of an issue for me, not enough to mess with anyway. At first I thought it would be because of all the high power hype, but really home is chill, plenty of time for recharging.


I built the awesome Arduino based OpenEVSE and have had no issues with it charging at 3.3kw during offpeak with time of use metering. I doubt I'll save any money.



AntronX said:


> Some ideas:
> 
> If you want an option to directly connect PV array into Spark's DC plug, you will have to go with big central inverter and a transfer relay in between. Just make sure your relays can extinguish 600V DC arc. The PV array will have to be left isolated from the ground, as the Spark may check for that during DC charger handshake. You will have to reverse engineer DC charger protocol and make some kind of controller to fool the car into closing battery relays to charge as well as monitoring for BMS signalling to stop charging.


I'm using Collin Kidders SavyCan to help with the reverse engineering. I can currently read the cell voltages, ect. The DCFC is CAN controlled. I'll have to buy a few specs to see how to do the electrical interface. I plan on only charging to somewhere before the BMS starts balancing.

I have spoofed CAN before:
http://www.mychevysparkev.com/forum/viewtopic.php?p=14848#p14848



> Not sure if it is worth the trouble compared to plain grid-tie net metering. You will save about 12% in conversion losses during charging, but without MPPT control you may lose some PV production efficiency anyway. But you can then proudly declare that your car's charge came direct from solar, and not via fossil powered proxy.


Yah, that is the main point !! I'm tired of people saying “Your just transferring the xx to bla bla boa”

I've always been an energy nut. This is the ultimate, have my cake and drive it too.

Assuming the panels are clean and even, it looks like ~20% below mpp at dead, 25 below charged but passing thru mpp mid charge.

The ultimate would be to use individual panel mppt with adjustable per panel DCV output. This is exactly what SolarEdge does with their 'PowerBoxes' to make a constant voltage for their main inverter. I believe this is why Elon Musk has bought into the company for his wall mounted battery. The PowerBoxes use CAN of some kind for the communication. I think it might work to just have a few mpp controlled panels.


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## Sunking (Aug 10, 2009)

Couple of oversights and incorrect assumptions here. 

1. The biggest issue is solar panels are current sources, not voltage. Assuming there are enough cell sin series to push current into the battery everything seems fine. I can use your array to charge a 6-volt battery just fine. 9-amps into 220 AH battery is absolutely no problem whatsoever assuming the battery needs a charge. 

That 6-volt battery turns your 3780 watt solar panel into a nice 50-60 watt solar panel until the battery is fully charged up. But as a current source of 9 amps with a 500 volt source pushing the current the voltage just keeps going up and up. Why. You have no voltage regulation, just an unregulated current source.

Same thing with your EV battery, when it is fully charged, the voltage just keeps going up and up. 

Of course the other thing you are doing is screwing yourself and loving it. See a Grid Tied System uses a MPPT tracking and utilizes every watt possible to be used either by you or a neighbor. What you do not use goes out and you get a cash credit. 

With any battery system you are completely screwed, stuck paying many times more for power than just buying it. Go back to my 6-volt battery example. It turns your 3780 watt panel into 50 - 60 watts. With a 4 Sun Hour Day assuming your battery could have absorbed all the power that day, it cannot, you would have stored a whooping 0.2 Kwh or 2-cents of electricity that day. If you had used a GTI you would have generated 10 Kwh or $10 worth of electricity. But somehow you are happy with two-cents worth.

Obviously with a higher voltage battery the losses are not that extreme. Without a MPPT Buck Converter you are still are still loosing a lot of power limited to what the panels can generate directly, and only what the batteries can absorb. You are still throwing money away and loving it. 

Let the utility be your battery. Pump every bit of power the panels can possible generate out to where it will be used, not lost or utilized. Excess you generate will be credited to you to be used later. 

Another thing a 4 Kw solar arrays is not equal to a 4 Kwh charger or generator, not even close. A 4 Kw Soar array can only generate at most 16 to 20 Kwh of usable energy in a day on a bright sunny summer day. 1/3 that in winter months. A 4 Kwh charger or generator can pump 96 Kwh 365 days a year or 500 to 600% more energy.


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## bicycleguy (Aug 13, 2015)

Sunking said:


> Couple of oversights and incorrect assumptions here.


I made no assumptions, all numbers quoted were from my LTSpice models of AstroEnergy 315W panels modeled to give within .5% of the manufactures mppts at both STC and NOCT conditions


Sunking said:


> 1. The biggest issue is solar panels are current sources, not voltage. Assuming there are enough cell sin series to push current into the battery everything seems fine. I can use your array to charge a 6-volt battery just fine. 9-amps into 220 AH battery is absolutely no problem whatsoever assuming the battery needs a charge.
> 
> That 6-volt battery turns your 3780 watt solar panel into a nice 50-60 watt solar panel until the battery is fully charged up. But as a current source of 9 amps with a 500 volt source pushing the current the voltage just keeps going up and up. Why. You have no voltage regulation, just an unregulated current source.
> Same thing with your EV battery, when it is fully charged, the voltage just keeps going up and up.


Please go back and review Electronics 101. The 'unregulated current' is the current that particular array puts out at the particular voltage dependant on the VI curve of the array. As stated this particular setup will actually pass through the panels mppt at 35.8V, 8.8A (315W)per panel or 35.8*12panels=429V at 8.8A=3.8kw per string although the average will be lower. Thats the whole idea people are missing about high voltage batteries. They actually can match well with solar panels without requiring huge currents and the associated costs. A $2.50 600V 10A MOSFET can handle what I'm proposing. 


Sunking said:


> Of course the other thing you are doing is screwing yourself and loving it.
> BLA BLA BLA
> Another thing a 4 Kw solar arrays is not equal to a 4 Kwh charger or generator, not even close. A 4 Kw Soar array can only generate at most 16 to 20 Kwh of usable energy in a day on a bright sunny summer day. 1/3 that in winter months. A 4 Kwh charger or generator can pump 96 Kwh 365 days a year or 500 to 600% more energy.


As noted 20 Kwh of usable energy per 1 string of my 2 string 8kw array would charge my 20kwh Spark battery. On bright winter days it would work even better. Please don't comment as an expert when you obviously aren't.


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## Sunking (Aug 10, 2009)

bicycleguy said:


> Please don't comment as an expert when you obviously aren't.


I have been doing it professionally for 38 years, and solar for 20 years. Your logic is flawed. 

Solar panels are current sources and current is directly proportional to the amount amount of sunlight striking the surface measured in W/m2, and MPPT voltage is virtually flat throughout tracking point. Example a LG315N1C is virtually the same panel as your model. Vmp is the same 33 volts at 1000 watt/m2 (310 watt, 9.5 amps), as it is with 100 w/m2 31 watts @ .9 amps. 

I understand your point if the panel voltage and battery voltage are matched does not make much difference, but that is not the case with any battery. With any series charging using solar as a current source, your charge efficiency is far below 95% of MPPT. A 100S battery assuming discharged to 3.0 vpc is 300 volts with say 9 amps is only 2700 watts, and finishes at 365 volts @ 9 amps 3285 watts. That means your efficiency from a 3700 watt array is only 73 to 88% from start to end. True MPPT charging is better than 95%. 

The other point you are overlooking is you are assuming your battery is fully discharged everyday, and every watt hour of the potential power is being utilized which is impossible. You will not be discharged 100% DOD everyday. Something less. As soon as the battery charges, your panels turn off, end of story. Most likely by noon or early afternoon. If you are grid tied with a MPPT Inverter, your panels will generate maximum power possible they can with Sun Light input from sun rise to sunset at some 95% efficiency. Much more than your batteries can possible hold and you get credit for the excess to use another day. You cannot do that with a battery system. As soon as they are full, your are shut down and loosing money. Smart money is to put your money to work. Operate grid tied and use a commercial AC charger. They you stand a chance at an ROI. Your way is lost money.


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## PStechPaul (May 1, 2012)

I think the point is that the MPPT controllers are rather expensive, while solar cells are now fairly cheap. A 350 watt MPPT is about $160, or about $0.50/watt. https://genasun.com/all-products/so...ium/gvb-8a-li-lithium-solar-boost-controller/

A grid-tie inverter for about 4800W costs about $1600.
http://sunelec.com/datasheet-library/download/Conext-MPPT80600-Datasheet-DS20140811_ENG.pdf
http://sunelec.com/charge-controllers

Solar cells are now in that ballpark. So if your solar array is fairly well matched to your battery bank, you can get at least 75% efficiency. You could spend $1600 to obtain 95% efficiency (about 1000 watts more), or you could spend $1600 for an additional 3000 watt solar panel.


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## bigmotherwhale (Apr 15, 2011)

I think its a great idea, should work very well as the losses are so low when conditions are ideal. 

what about charging a capacitor, and discharging it across the battery when it reaches a certain voltage, like a charge pump, I tried something similar and it worked well.


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## Duncan (Dec 8, 2008)

I'm with Sunking on this - it is a terrible idea

Forget the different inefficiencies - the problem is that you are only using your panels when you are charging your car

All of the rest of the time the power is going to waste 

Connect your panels to an inverter and use the grid as a battery


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## Sunking (Aug 10, 2009)

PStechPaul said:


> I think the point is that the MPPT controllers are rather expensive, while solar cells are now fairly cheap. A 350 watt MPPT is about $160, or about $0.50/watt. https://genasun.com/all-products/so...ium/gvb-8a-li-lithium-solar-boost-controller/
> 
> A grid-tie inverter for about 4800W costs about $1600.
> http://sunelec.com/datasheet-library/download/Conext-MPPT80600-Datasheet-DS20140811_ENG.pdf
> ...


Paul you are over looking a whole lot of facts both technically and economically. 

Economics is a huge issue, and your numbers are all wrong leading you. like the OP, is leading you into wrong conclusion. If you are talking about a MPPT Charge Controllers to charge batteries, they cannot be used as a Grid Tied Inverter, does not cost $1600 for 4000 watts of panels. More like $400 for MPPT, and $150 for PWM. If would take 6000 watts of solar panels with PWM controller, to equal 4000 watts with MPPT controller. Even at your exaggerated 50-cents per watt for panel cost is more than the Controller additional cost. Not to mention all the extra cost in materials racking, time and labor. 

But here is the big one you completely over looked. A GT generates cash. As I stated earlier with a battery system is very inefficient, and when the battery is charged, your panels shut off. His system with GTI can generate 20 to 22 Kwh of usable every day. If it takes 10 Kwh to charge his battery means you are selling 10 to 12 Kwh to the POCO That is $2 to $3 everyday in his pocket. Enough to either Net him 0 Kwh usage in a day, or significantly reduce his monthly electric bill. Al that is lost limiting it to charging a battery. That GT Inverter pays for itself in a few years and starts ROI.


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## AntronX (Feb 23, 2009)

Hmm, lots of negativity in this thread. Sunking, you need to chill and let people have fun.


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## Sunking (Aug 10, 2009)

AntronX said:


> Hmm, lots of negativity in this thread. Sunking, you need to chill and let people have fun.


OK if you call being misinformed and loosing money fun, have at it. Both of us will be be tickled pink buying what you want from me. I have no problems taking your money.


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## Moltenmetal (Mar 20, 2014)

Grid tied would in my case not pay the cost of the separate meter installation, electrical and building permits etc., so for me that would mean no solar- and still does at present. Too little unshaded roof area to pay back those fixed costs in any kind of reasonable period. Cutting down mature trees to remove shade from future panels is not an environmentally acceptable solution, even if it were permitted here which thankfully it is not.

Given a choice between no solar and inefficiently used solar, the choice is perhaps a little easier.


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## AntronX (Feb 23, 2009)

Moltenmetal said:


> Cutting down mature trees to remove shade from future panels is not an environmentally acceptable solution...


Environmentally, solar is a waste of money in your area. Ontario gets 86% of their electricity from nuke and hydro power already. Plus solar resource is poor that far north - about half of California's sun, for example.


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## Moltenmetal (Mar 20, 2014)

Only 9% of Ontario's grid power is fossil and of that, 100% is natural gas. So yeah, we have a great grid already. But to correct your misperception, Toronto is further south than just about all of Germany, so yes solar does make environmental sense here. Do the FIT programs offered here for solar make sense? Totally different question to ask!


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## Duncan (Dec 8, 2008)

Even if a country gets most or all of it's power without fossil fuels solar will still help the system

Hydro power is effectively limited by the water storage - it's easy to add more generation but very difficult to add more storage
So solar power will enable the hydro operator to let less water through the generators when the sun is shining leaving more water for other occasions

This is especially important now with climate change effecting rainfall patterns - you may need to keep that water for when a drought hits


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## Duncan (Dec 8, 2008)

I just had a look
Toronto is closer to the equator than we are!!!!!!!
And my panels work quite well


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## Sunking (Aug 10, 2009)

Duncan said:


> Even if a country gets most or all of it's power without fossil fuels solar will still help the system
> 
> Hydro power is effectively limited by the water storage


So? Sun does not shine 24 hours, and there is no real battery solution to store energy that is worth a damn. Besides electricity from hydro or fossil fuels cost consumers in the USA 11-cents per Kwh on national average. I pay 7-cents in TX. Go off grid and battery cost alone is 10 times that, not counting anything else. You might be willing to pay that, but keep your dang hands out of my pockets.


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## TooQik (May 4, 2013)

I'll never understand why some people are so concerned about what other people do with *their* money. The OP asked a question about the technicalities of his proposal, not whether charging his car directly from HV solar would be the most cost effective use of his money. Some people are not looking for a ROI or require their excess to be exported when buying into solar and yet some people automatically assume that they are.

I personally would also like to build a similar system as some stage to charge my own electric vehicle, so I'm interested in seeing where this topic leads. My proposed PV array would be used solely to charge the EV, nothing else. My main obstacle currently is I need to finish building my EV.


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## Duncan (Dec 8, 2008)

Sunking said:


> So? Sun does not shine 24 hours, and there is no real battery solution to store energy that is worth a damn. Besides electricity from hydro or fossil fuels cost consumers in the USA 11-cents per Kwh on national average. I pay 7-cents in TX. Go off grid and battery cost alone is 10 times that, not counting anything else. You might be willing to pay that, but keep your dang hands out of my pockets.


Sunking
I wasn't talking about batteries - here most of our power is big hydro,
That works with a "fixed" amount of water falling into the lakes (depending on the weather)
With that annual rainfall the generators can change their output - it's not one for one but if they don't need to generate the power today they the water will still be there to generate the power tomorrow

So by my using solar (and sending my surplus back to the grid) the hydro guys can keep that water there and let it flow at night - or during the winter

So even with a "green power supply" I can help it to become more robust by using grid tied solar


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## miscrms (Sep 25, 2013)

Lots of interesting thoughts here.

I do think direct charging into high volatge batteries is an intriguing idea. I'm not a solar expert by any means, though I am an EE and did design and install my own home 5.5kW PV system from start to finish including all the design calculations and documentation required to satisfy both the utility and city inspectors.

My main concern would be that things are likely to be a lot more complicated in real life than they are on paper or in the simulator. An IV curve is a good place to start, but really what you need is a temperature dependent LIV curve that adds the additional non-linear effects of temperature and insolation. The IV curve only gives you the relationship between current, voltage, and power at a given temperature and amount of light. Changes in temperature and amount of light, not to mention cell/panel variation, don't just move you up and down the IV curve, they change the shape of the curve.

The difference between STC (1000W/m^2 @25C) and NOCT (800W/m^2 @46C) ratings already gives you a sense of this. In my experience STC is wildly optimistic, and NOCT is more like what you may see under optimal conditions during a relatively short part of mid-day. Just between these two conditions you can see Vmpp and Voc have shifted by ~10%, and Isc and Impp have fallen by over 22%. MPP output power has fallen 30% from 315W to 220W. Another way to get a sense of just how much the string characteristics may vary is using SMA's (Sunny Boy) online array / inverter sizing tool. For a 1x12 string of ASM6612P-315 in Phoenix, that tool predicts a minimum DC voltage of 342V and max of 588V. That's a pretty huge window to try and deal with, let alone maintain decent efficiency over. 

Additionally a tool like PVwatts can help you get an idea of typical irradiance of the panels at a given angle and location on an hourly basis for each day of the year. There may well be a significant fraction of time when temperature / sun angle / weather doesn't allow you to produce enough voltage to push much of any current into the battery at all. On a clear winter day even here in Phoenix you'll be lucky to get ~4-5 hrs a day where irradiance meets or exceeds the 800W/m^2 of NOCT. With MPPT and buck/boost you could probably still pull 6-8 hrs equivalent out, but without it you will likely loose a fair portion of the shoulder energy. Those winter numbers will drop the further north you are. And on a cloudy day when irradiance is more like 100W/m^2 you probably won't get much of anything. You might try to improve this by increasing the number of series panels in the string, but now you are also increasing Voc max which is already flirting with the 600V DC limit of a lot of common components, even for simple things like wiring and disconnects, driving you to costlier alternatives.

After all these things are taken into account, my concern would be that you will end up throwing away a lot more energy than you think. 

There are also some practical concerns about the effects of not being able to throttle down charge current at high SOC. Charging too fast at too high an SOC may negatively impact battery longevity. Stopping at a lower SOC means throwing away more energy. A hard disconnect while current is still flowing also creates stress in the system (arcing and voltage spikes) that are likely to result in failures over time.

Adding MPPT, dc:dc and PWM would solve most of these issues. IMHO if you are going to spend ~$7000 on 7kW worth of panels, why wouldn't you spend the $1-$2k for the electronics required to get the most out of them possible? IMHO adding a grid tie inverter path (as the OP proposed) to allow use of the solar system when not charging is a minimum. It makes no sense to have that much expensive hardware sitting around doing nothing because you went for a drive in the middle of the day, or already had a full battery that day. 

So it seems to me the question is, once you have a GT inverter, is there any advantage to adding a direct DC path? It seems very likely to me that w/o another set of MPPT and PWM on the direct DC path its going to be significantly less efficient than converting to line AC and back. And its highly debatable how much real gain in efficiency one would get even with that additional hardware. The loss in the GT inverter is about 3%. At the other end for 240V/L2 its probably about the same or better. A direct dc battery charger still won't be 100% efficient. Even it was, charging still wouldn't be 100% efficient due to the losses in the batteries themselves.

That said, these type of power electronics seem to be getting cheaper all the time. So maybe its not that far off to think that the cost / complexity could be justified even though the potential gains might be small.

Rob


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## Sunking (Aug 10, 2009)

miscrms said:


> After all these things are taken into account, my concern would be that you will end up throwing away a lot more energy than you think.


Spot on. This is why I jumped in, he is using a model with unrealistic parameters. I also think he assumes if there is sun out the panel is producing some close to Full Power which is as far from the truth as it gets. Just because you may get 14 hours of direct sun does not equal 14 Sun Hours, more like 5 Hours 



miscrms said:


> Adding MPPT, dc:dc and PWM would solve most of these issues. IMHO if you are going to spend ~$7000 on 7kW worth of panels, why wouldn't you spend the $1-$2k for the electronics required to get the most out of them possible?


Agree with you here except a couple of minor differences. A good MPPT controller is not going to cost quite that much. 



miscrms said:


> IMHO adding a grid tie inverter path (as the OP proposed) to allow use of the solar system when not charging is a minimum. It makes no sense to have that much expensive hardware sitting around doing nothing because you went for a drive in the middle of the day, or already had a full battery that day.


Which is my main point, for a few bucks more you can get paid and recover a lot of your investment 



miscrms said:


> So it seems to me the question is, once you have a GT inverter, is there any advantage to adding a direct DC path?


Absolutely not. Not only are you loosing money, you have to park the car all day to charge , and WTF do you do when you have a cloudy day or a week of cloudy weather. 

What I might add is PV Watts is a good tool if you know how to use it. Most are pretty clueless. Example if you use a battery system unless you change the Efficiency, you are dreaming. A GT system is 80% efficient, but not a battery system. If you are talking PWM, then 50%, MPPT 66%. But there is one more huge catch. PV Watts assumes 100% utilization meaning every watt hour possible is utilized immediately. That is impossible to do with a off-grid stand alone system. In real practice even with MPPT you are lucky to utilize 25 to 30% and even less in Summer. Think about that for a minute. If you have a 7 Kw system over an average of a year generates $3 to $4 of power everyday and you are pissing away $2 to $3 every day of the year you could take off your electric bill. You would be better off to start smoking a pack of cigarettes a day. At least you would die sooner and not waste as much money and resources in the end. 

The right way to find your area Solar Insolation with PV watts is enter 1000 watts DC Power Input, 80% efficiency, default Tilt and Orientation. You will get twelve results, one for each month of the year. That will be your Sun Hours. Example in Seattle in December you will see roughly 1.2 Kwh which equal 1.2 Sun Hours. So if you had a 1000 watt panel, at the terminals you will generate 1000 watts x 1.2 Hours = 1.2 Kwh. That is not how much you can store, that is whet your panel will generate into a GT system. A battery system will be less. In July it goes up to 4.5 Kwh or 4.5 Sun Hours. Your area will be different. In the USA Tuscon AZ has the best Insolation with June/July being around 7 Sun Hours. Do not make a laymen mistake thinking if you have 14 hours of direct Sun Light = 14 Sun Hours. 

Bottom Line with any stand alone battery system there are two things that are impossible. Save Money or Reduce emissions. Put another way. no chance at ROI or EROI. So ask yourself this. Why do you drive an EV? If not to save money and/or emissions, why are you doing it? With Off-grid you cannot do either. You just piss away money and resources.


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## miscrms (Sep 25, 2013)

Circling back to the OPs original desire to charge faster... This seems like a very complicated and not very effective way of approaching that goal. IMHO you'd be much better off just running your solar into the GT inverter full time, and looking into ways other folks are already increasing charge rate. You can use a large external charger through the DC input, but the chargers are still pretty pricey and the interface quite complex. My guess is you'd be much better off trying to integrate a second stock charger to double the L2 charge rate, particularly as it seems like the chargers used by chevy are pretty well reverse engineered and understood at this point and should be available pretty cheap used.

Rob


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## dtbaker (Jan 5, 2008)

this is a topic I'm been pondering for a couple years.... but I'm hampered as a mechanical person without a lot of in depth electrical design training.

I have a PV system on my house, a nice 3.8kw system with a string inverter sitting on the wall just outside my garage that puts out two strings of 250vDC at 3-6amps depending on the time of day. The voltage is always high enough to make it a current limited system if I were to hook directly to my 120vDC pack in the Swift, or even the 160vDC pack in the eMiata.

The problem is how to safely monitor the pack voltage, and open/close the relay without major arcs welding things together.

I've seen some cheap little voltage relays that I could set to my desired end voltage, but then the missing piece is the big fat relay needed to turn on/off the PV power.

I like the idea of going direct dc-dc for the charge, but am unclear on the least expensive, but safe, way to build a simple programmable voltage relay switch I can set to a specific end voltage.

The second reason I haven't really pursued this is that under max output, all I would get thru would be 6 amps (limited by panel open circuit max, right)... which gets me a LONGER charge time than with my little on board 1500 watt Elcons that puts thru 10amp to the 120v pack or about 7 to the 160v system.


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## bicycleguy (Aug 13, 2015)

I seem to have missed notifications of all this discussion in the last few days. Thanks everyone for the input.



Sunking said:


> But here is the big one you completely over looked. A GT generates cash.. .....That GT Inverter pays for itself in a few years and starts ROI.


I thought I started this thread with the presumption that I would be buying some kind of grid tie system and adding the ability to switch it on demand to charge my car. I don't plan on spending $7k on solar panels to sit idle. I currently charge only a few hours a night. A possible scenario could be the DIY controller would monitor the car and the solar and only charge when favorable. The rest would be GT. If the car got to low it would be charged the night before. It's all software!

This is the 'DIY' forum ! Aren't we supposed to innovate? jeese!


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## dtbaker (Jan 5, 2008)

bicycleguy said:


> I seem to have missed notifications of all this discussion in the last few days. Thanks everyone for the input.
> 
> 
> 
> ...


Just to point out the obvious.... if you siphon off DC power before it hits the inverter, you miss out on the net metering benefit for those kWhr, which is probably worth more financially than the power you would 'lose' by inverting twice (once in the string inverter and again in your charger). Especially if you live in a time-of-use area where night time charging is cheaper than the day.... You really want to maximize the net meter credit during the day at peak rate.

The technical aspect still interests me and I'd like to do it just to see if I can.

Going the other direction would be even better though!

I want to use my EV batteries as a 'whole house' backup. I'd like to use my existing PV inverter and basically just plug the car battery bank in place of a PV string at night or when the grid is down. BUT the typical grid-tie inverter needs at least 150vDC input to turn on AND you have to supply a 'heartbeat' on the line side for the inverter... So, I might be able to use my eMiata (with a 156vDC nominal pack), but it would be really close as it would sag a little under load. I'd have to :
- isolate the house from the grid (turn off the main breaker), 
- turn off ALL house loads to prevent overloading 'pacemaker'
- backfeed a small 'pacemaker' inverter+battery thru an outlet to provide a heartbeat the PV inverter can sync to,
- switch the input side of my PV inverter from PV to car

I think this would work, but have not tried it.... would void warranty for inverter and probably break multiple electrical codes and void house insurance.


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## WolfTronix (Feb 8, 2016)

I want to make a whole house UPS (On-line) as well...

Mains --> Bridge Rectify --> PFC --> DC Bus --> Inverter --> Transformer --> House

The DC Bus would be connected to a battery pack.

Connecting to the battery pack could be solar panels, or wind power (each has its own MPPT).

The battery pack could also be connected to your EV via a bi-directional buck-boost converter, so you could Level 3 charge your car, or power the house from it.


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## Stevebequik (Aug 29, 2015)

I direct charge a chevy volt pack directly from my PV and it actually works quite well and efficient I believe. 
Firstly I am a electrician, so I am use to working with high voltages, upmost care need to be exercised at all times.

My PV array consists of 2 strings of 10 x 250w panels. These panels have a NOCT maximum power voltage of 28v. Monitoring the MPPT of the grid tie inverter for a couple of years. The inverter would keep DC output of the arrays between a fairly narrow range of 250 to 280v. This is mainly dependent on temperature. I live in the tropics so get a fair bit of thermal de-rating. At dawn and dusk it may be outside this range but the output is so minimal at these times, its hardly worth worrying about.

My battery pack consist of 12kw 72 cells chevy volt modules.
I keep these cells between 3.5 and 4.0 volts. This equates to 252v to 288v.
I use the 2 HV contactors of the chevy volt assembly as a HV and LV disconnect. All controlled by an arduino.

This feeds a Liebert GXT3 10Kva UPS as my off grid inverter. This normally uses a 240v seal lead acid battery pack so it works out nicely. This feeds about half of the house circuits

Here is Australia, the feed in tariff is less than 1/4 of the power we consume from the grid, so at some point it becomes economically viable.

I have been able to compare the Kwh meter of the grid tie inverter to a Kwh meter on the battery pack. One string connected to each, same orientation.
Generally I am loosing about 10 to 15% into the pack. I believe this would be about the same as most AC chargers.

Steve


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## ElectricBicycleGuy (Mar 9, 2017)

Hi,
I love your idea. In fact, I might just purchase a used Chevy Spark EV for just the purpose you described. For all the folks who wonder why this idea is even considered, here is why I want to do it:

I already have a home PV system rated at 3.2kW dc, 2.72kW ac. When the power goes off, which it regularly does in my area, the dc-ac inverter shuts down to avoid electrocuting line workers. I would like battery backup for my home when my 3.2kW PV system goes offline. 

I already set up a separate 100W PV panel that trickle charges 4 golf cart batteries (24V, 220Ah = 5.3kWh). I can then run emergency loads off of a 2kW dc-ac inverter with 24V dc input. The battery power lasts about a day running a fridge, microwave, lights, garage door opener.

Purchasing a Chevy Spark as a 2nd car will allow me to drive about 95% of my local driving miles powered from the utility-connected PV system through the existing Fronius series inverter. It will also provide me with an 18.4kWh LG lithium backup battery! If the power goes off, due to an emergency (earthquake, etc.) I can use the existing 3.2kW dc PV array through the DC charging plug to keep the car charged up. I can then run a 12V dc-ac inverter off of the 12V battery, which will be kept happy through the onboard dc-dc converter. 

I think I can just purchase a contactor that shuts off when the battery reaches a certain preset level and also when the "Sun don't Shine". That will prevent the LG lithium batteries from backfeeding the PV panels at night. My 14 PV panels are around 450-500V dc and the battery is about 370V (112-S * 3.3V). Seems like a good match.

Any thoughts?


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## bicycleguy (Aug 13, 2015)

ElectricBicycleGuy,

Nice to see this thread is still being looked at. Some comments:

On using the 12V battery. Diagnostic tests for the 'accessory DC power control module' the thing that charges the 12V from the HV, specify 160 amps at 12.6-15.5V, or about 2kW. This level, or possibly any level would require that the cars 'ignition' be on so that all its cooling and monitoring systems function. I have used a 400w inverter at full load with the car off through an accessory plug and it automatically disconnects it after a minute or so if the 'ignition' isn't on. iPad level or no loads are allowed to run about 30 minutes before you have to hold a button down for a few seconds and then run about 10 minutes before requiring another button push. That all said, nothing indicates that the car can't sit there all day with the ignition on running the inverters AFAIK.

The Spark is an extremely managed system. Temperatures, voltages and currents are measured everywhere and communicated between ~20 modules using 3+? different CAN buses. Practically every wet dream of DIYers on this site is implemented, i.e. 3 separate water cooling-heating systems for batteries, inverters, smart cell management, ext, ext. (The only blatant item missing is a heat pump instead of resistance heaters.) My point with this is that I don't think you will be able to just tap into the HV with a contractor. The HV cables and systems are physically and electrically protected for mechanic safety, crash safety and reliability. The system will recognize somethings up and react badly. I think the only reasonable way to proceed is to make a box that looks like a HV EVSE to the Spark and interfaces with the solar system. I think this is doable. It will require some interesting hardware and software for the CAN implementation.

The big question is wether the Spark to EVSE communication specifications allow the current to be reversed so the Spark can power your HV inverters.

Love my Spark


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## ElectricBicycleGuy (Mar 9, 2017)

ElectricBicycleGuy,

Regarding: "I think the only reasonable way to proceed is to make a box that looks like a HV EVSE to the Spark and interfaces with the solar system. I think this is doable. It will require some interesting hardware and software for the CAN implementation.

The big question is whether the Spark to EVSE communication specifications allow the current to be reversed so the Spark can power your HV inverters."

Maybe this is a project I'm not ready to get into. I really thought that all I had to do was tap into the ~400 VDC coming down from my PV panels at the input to the solar inverter and feed it into the special plug that connects beneath the orange flap.
Isn't the DC bypassing the AC charger circuit? Of course, something has to regulate the battery, so no cells get overcharged, but I assumed that was done by the BMS, located inside or near the battery module.

Anyway, I've been burned before by being too simplistic in my approach! I guess I'm a KISS kind of guy ("Keep It Simple, Stupid").


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