# Solar panel direct charging guide



## Voltswagen (Nov 13, 2008)

Meander
Put the solar panels on the roof of your home. 
You can't load enough panels on a car or truck to make any measurable difference.
A light vehicle will draw 50 - 60 amps crusing at 40mph. A truck...maybe 80 amps....maybe more.

Watts divided by volts = amps
http://www.atlantechsolar.com/calculate_volts_amps_watts_conversion.html

Here are some cheap 200w panels.....note the size. http://www.dmsolar.com/solar-module-2.html

How many of these panels can you load on truck......3? Lets say they put out 26.9v per their spec sheet. 81v total (3) wired in series. How many amps does that convert to? 
Here is another good site I found today for determining Solar Panel output:
http://www.ehow.com/how_4720927_volts-amps-watts-solar-panel.html

It's not about the weight of the panels...it's about the output.


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## meanderingthemaze (Jan 25, 2010)

Thanks Voltswagen for the links and input. 

I'm curious to hear what you mean by measurable difference, though. I've heard that you could increase your range by maybe 5 miles. Plus, I park my car in the sun all day at work. BTW, I'm about 6 degrees closer to the equator than where you are and get a lot more sun. Los Angeles is classified as desert. Another problem I have is that I rent, so I can't really do a solar panel installation at home, when I own I will definitely put them on my roof. 

Anyway i specifically mentioned that I am aware of the controversy and didn't want to create another thread debated the advantage and disadvantages of solar panels on a car. That discussion has been done a million times already. I am making an guess that I will not only have a significant amount of solar paneling (about 20 sq ft), but that it'll be parked in the sun all day, trickle charging the batteries and prolonging their health. I have considered creating a hinge so that I could slant it in the direction of the sun while parked. And, no I am not under the illusion that this is a new idea.

So, I guess the specific questions I have are...
1) Should the paneling have the same voltage as the pack or slightly higher?
2) Or should I consider a transformer to get to the voltage i need?
3) Can i literally hook up to + & - ends of the pack without a charging system? Or should I at least have a shutoff to prevent over charging (for example if I charge it over night and then leave it parked in the sun all day without driving, could I over charge?) Or if the voltage is the same as pack voltage, then it shouldn't over charge, but then would I damage the solar cells if there is nowhere for current to flow?
4) Any other wiring considerations?

I realize that this is not a common practice so I will probably not get very much guidance. Just thought I'd throw it out there though. Thanks


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## Sunking (Aug 10, 2009)

It is just a matter of physics. You cannot add enough panels to make any noticeable contribution. The very best and most expensive panels are only on the order of 17 to 19% efficient. At 100% efficiency (those panels do not exist or ever will) you can only generate 1000 watts/m2. So in So Cal about 6000 wh/m/2. So with 19% efficiency you are looking at 1/5th or about 1.5 Kwh/m/2. How many square meters do you have available on a vehicle? Not enough to make any meaningful difference. Not to mention the extra added weight and drag to carry around.


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## Kelmark (Oct 26, 2009)

I am not a solar expert but have been looking at using solar on my conversion as well. I received the same reality check but have not been deterred. My original goal was to be able to get 4 mile at 35mph extra range. Running some numbers I realized even that would take covering my entire car with high grade cells. $$$

Instead I have now decided not to pursue running solar to my main pack. Instead I will be using them to keep my Aux battery charged. In turn I it will reduce the need from the main pack through the DC to DC converter. This also saves power losses in the converter amplifying the benefit of solar.



meanderingthemaze said:


> So, I guess the specific questions I have are...
> 1) Should the paneling have the same voltage as the pack or slightly higher?


Normally the Voltage is slightly higher I beleive.



meanderingthemaze said:


> 2) Or should I consider a transformer to get to the voltage i need?


If you feed to your aux batt the voltage will be low enough this won't be needed.



meanderingthemaze said:


> 3) Can i literally hook up to + & - ends of the pack without a charging system? Or should I at least have a shutoff to prevent over charging (for example if I charge it over night and then leave it parked in the sun all day without driving, could I over charge?) Or if the voltage is the same as pack voltage, then it shouldn't over charge, but then would I damage the solar cells if there is nowhere for current to flow?


I will be using lithium for my aux batt so I will use some sort of cutoff so I do not damage the cell.



meanderingthemaze said:


> 4) Any other wiring considerations?


They sell small solar car battery chargers, I am going to get one for my petrol car and look close at its componets and wiring. 



meanderingthemaze said:


> I realize that this is not a common practice so I will probably not get very much guidance. Just thought I'd throw it out there though. Thanks


I am still in the early stage of my conversion so I don't have allot of answers yet. I will be using thin flexable solar cells, for your bed cover you can use solid solar panels with higher output. Just start researching weight to power output versus price. 

The panels should help air drag on your truck if used as a bed cover just don't exceed the average weight of other bed covers and I don't see how there will be any disadvantage. The advantage will likely be around 1-2 mile extra range at 35mph so you'll have to determine how much to pay for that. Sorry I am not more help.


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## Voltswagen (Nov 13, 2008)

Meander
Sunking pretty much summed it up for you.

Yes, you do get more daily sun where you are vs my location.
So you might collect an extra 5 miles worth of amps in a day but then subtract for the increased drag co-efficient. I don't know what it would be on a truck but it would depend on the number of panels and the added weight.

There are a number of EV builders here on DIY who seem to have very deep pockets with vehicles surpassing $30K in cost but none to my knowledge who have added solar panels.

But at the end of the day........it's up to you........


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## rmay635703 (Oct 23, 2008)

Sunking said:


> It is just a matter of physics. You cannot add enough panels to make any noticeable contribution. Not enough to make any meaningful difference. Not to mention the extra added weight and drag to carry around.


Dead wrong if you are using lead acid batteries. Even a very very small charge rate 1/250 C reduces the rate of hard sulphation by about 50% in discharged FLA batteries. Meaning it is VERY worthwhile to have a small solar trickle charger mounted to the car, assuming of coarse you drive during the day and charge at night. Even when you are coasting or stopped during a trip solar starts to reduce hard sulphation.

But not worthwhile for range, only worthwhile to extend the life of your $600-$3000 investment. A small long trickle charge can occasionally have the side effect of equalizing batteries as well.

So yes Billy having a SMALL inexpensive solar panel on the car can be worthwhile. Just not for range.

Too bad I can't locate any inexpensive sources of new loose cells anymore 4 years ago they were 0.51 cents a watt, now $1.25 WTF? I would love to utilize the 4x8' roofrack area on my fathers 48v minivan to build a 400-800 watt array. To me that is fairly sizeable? And the 2-4hrs it sits between trips would likely gain back a sizable amount of range.

It would only have a payback to extend battery life but during mid day it would extend range about 5 miles at 25mph if that means anything. The cost of such a panel would not be recovered though in the form of electricity only potentially battery survivability.

So whether solar is for you is far more complex than those here claim, it has benefits beyond range and Depending on the shape and speed of your drive; your vehicle may gain a small additional range at midday. 

Extending the lifespan of the most expensive part of the EV to me is the most important aspect of having solar, but whether it is worth thousands of dollars to add 5 miles range at low speed during midday is up to you to determine. 

I would argue $50 for some little 5 watters to trickle your pack is a good investment though (assuming you have room or mounting to place them when parked). Afterall if your $1600 battery pack lasts even 10% more cycles (conservative estimate, much much higher if you exceed 50% dod occasionally) its probably worthwhile since the most money you will ever spend on an EV that is actually used will always be for batteries and solar panels generally hold value and last decades longer than your pack of batteries.

Food for thought
Ryan


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## green caveman (Oct 2, 2009)

Sunking said:


> It is just a matter of physics. You cannot add enough panels to make any noticeable contribution. The very best and most expensive panels are only on the order of 17 to 19% efficient. At 100% efficiency (those panels do not exist or ever will) you can only generate 1000 watts/m2. So in So Cal about 6000 wh/m/2. So with 19% efficiency you are looking at 1/5th or about 1.5 Kwh/m/2. How many square meters do you have available on a vehicle? Not enough to make any meaningful difference. Not to mention the extra added weight and drag to carry around.


Yes, but since the vehicle is parked all day, you could come up with a creative fold-out system that would then benefit the users of surrounding parking spaces by providing desirable shade. So, if you fold it out to the size of the parking space, you'd have about 12m^2, three parking spaces would give you 36m^2 - 54 kWh (if your 1.5 Kwh/m/2 is correct).

Not judging whether this is good, cost effective, workable, whatever, just trying to not to stifle creativity.


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## rmay635703 (Oct 23, 2008)

green caveman said:


> Yes, but since the vehicle is parked all day, you could come up with a creative fold-out system that would then benefit the users of surrounding parking spaces by providing desirable shade. So, if you fold it out to the size of the parking space, you'd have about 12m^2, three parking spaces would give you 36m^2 - 54 kWh (if your 1.5 Kwh/m/2 is correct).
> 
> Not judging whether this is good, cost effective, workable, whatever, just trying to not to stifle creativity.


A very expensive fold out indeed, would have to make sure it couldn't be easily broken or stolen.

I still will say preventing battery sulphation is a far better use and would save the most money in the long run increasing the battery pack lifespan.


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## meanderingthemaze (Jan 25, 2010)

I have removed the bed as you can see and I am planning to leave it off to reduce weight. But in California, by law you must have fenders or a bed that covers the back wheels as well as mud flaps, so I was planning on installing a flat bed to cover the batteries as well as the rear wheels.



> How many square meters do you have available on a vehicle? Not enough to make any meaningful difference. Not to mention the extra added weight and drag to carry around.


I have about 20 sq ft. If you can explain why a flatbed made from solar panels would increase drag over let's say fiber glass or plywood, I would love to hear. It'll be the same shape and relative weight.



> Extending the lifespan of the most expensive part of the EV to me is the most important aspect of having solar, but whether it is worth thousands of dollars to add 5 miles range at low speed during midday is up to you to determine.


Not to mention the 1500 or so miles a year that would not be powered by the dirty grid.



> Yes, but since the vehicle is parked all day, you could come up with a creative fold-out system that would then benefit the users of surrounding parking spaces by providing desirable shade. So, if you fold it out to the size of the parking space, you'd have about 12m^2, three parking spaces would give you 36m^2 - 54 kWh (if your 1.5 Kwh/m/2 is correct).
> 
> Not judging whether this is good, cost effective, workable, whatever, just trying to not to stifle creativity.


I love it green caveman, rather than stifle, you have inspired me!


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## rmay635703 (Oct 23, 2008)

I didn't mean to discourage you on the contrary I love to see test of concept, usually it costs a tad much for most of us to swallow.

Might I recommend (even though it increases weight) to build a lightweight aerocap type bed out of lightweight materials similar to the one made by bondo on ecomodder. You could easily cover that slant cover with cells and have it on pivits to lift flat when parked, you could then make a very lightweight bed for a belly pan and make fender skirts to cover the rear wheels thus dramatically improving your aerodynamics and range. In other words have a wedge flatbed/cover combo on the back of your truck that pivots up to provide access to the bed and for charging purposes during the day. There have been folks who have made carbon fibre bodies by hand, I would estimate a cheap composite should be easy to make into a form fit flatbed and cover.

Aero makes more difference than weight at speeds above 40mph

Good Luck, I will watch this with interest  Oh and if you find solar cells really cheap in bulk let me know where. I would love to laminate a form fit setup for my roof.

http://www.google.com/url?sa=t&sour...t-H3BQ&usg=AFQjCNHASsDYeSNFLow44ZMNwOhsikKwSg
http://solarglobalgreen.com/pg/pages/view/1875/
Not efficient or light but HV for ev charging



meanderingthemaze said:


> I have removed the bed as you can see and I am planning to leave it off to reduce weight. But in California, by law you must have fenders or a bed that covers the back wheels as well as mud flaps, so I was planning on installing a flat bed to cover the batteries as well as the rear wheels.
> 
> 
> 
> ...


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## green caveman (Oct 2, 2009)

rmay635703 said:


> I didn't mean to discourage you on the contrary I love to see test of concept, usually it costs a tad much for most of us to swallow.


People buy Ferraris and Hummers, if it was just about cost then those would be non-starters. People in So Cal buy Ferraris to drive at 10mph on the freeways in rush hour.

So, if you're going for style, I think that the Space Station style fold out solar panels would be hard to beat. I'd put them high, have the sides go up and out over the parking spaces on either side, so you have the area without blocking the space.

Hey, it's out of my price range and ingenuity, but I'd love to see it. I can think of lots of problems with it (cost and the others you mentioned), but these are just opportunities for creativity!

You could call it a "Plug in Hybrid" partly solar, partly grid (no one's taken that name yet have they ).


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## Sunking (Aug 10, 2009)

green caveman said:


> Yes, but since the vehicle is parked all day, you could come up with a creative fold-out system that would then benefit the users of surrounding parking spaces by providing desirable shade. So, if you fold it out to the size of the parking space, you'd have about 12m^2, three parking spaces would give you 36m^2 - 54 kWh (if your 1.5 Kwh/m/2 is correct).


Point taken but there is a catch. if you were to do something like that, you would want something very light in weight, and flexible right? Well that would mean using thin film panels. The catch is the efficiency drops to below 10%. I would also think in LA or southern CA the panels would have a great big bullseye on them for thieves and vandals. I just do not see it being practical or feasible. That is just me.


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## green caveman (Oct 2, 2009)

Sunking said:


> Point taken but there is a catch. if you were to do something like that, you would want something very light in weight, and flexible right? Well that would mean using thin film panels. The catch is the efficiency drops to below 10%. I would also think in LA or southern CA the panels would have a great big bullseye on them for thieves and vandals.


You know, it's not my project and I haven't really given it any thought (so why am I responding?). It think it's a non-trivial project, and I know I don't have the wherewithal to even start it, but I've done lots of things in my life that other people consider crazy** and, the worst thing, I want to keep on doing them. It always help to know that I have some company.

If you had the only solar car around, and everyone knew it, and it was parked in the same (company) parking lot every day, it would be hard to see that thieves and vandals would be able to get to it because it would be a landmark.

Panel weight is just a technical hurdle.

** I think that building an EV meets this in some people's minds.



Sunking said:


> I just do not see it being practical or feasible. That is just me.


Hey! That's what people say about EVs!! (That doesn't mean it's not correct in the case of the Solar Panels).


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## johnnyfoos (Sep 1, 2008)

"Anyone know of any examples"
OK,
I know of one
"http://visforvoltage.org/forum/solar-vehicles/1589"
But this still requires normal battery charging


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## TheSGC (Nov 15, 2007)

Getting any decent charge from panels is a challenge, costly, not lots of power, etc. Anyways, here is a another way of thinking about it:

Take a look at you Wh/Mi, for instance my test route for my Civic is 270 Wh/Mi @ 35 MPH. Now if I added, say a decent 100 Watts of solar panels, that would add 100/270, or 0.37 miles of range to my entire drive, at 35 MPH. That could be bumped up to 0.5 miles if I was on flatter terrain.

Now, I do know that one of the Solectria's at my college is going to get a solar panel treatment as an experiment, and my quick math shows it might give it 1/2 to 1 more mile of range, and probably do a decent charge of around town charging, like while in parking lots away from the wall socket. They are using about 100 watts of flexible panels, and they cost more than it would for me to fully upgrade my battery pack from 12 KWh to a 16.3 KWh pack and give me another 6+ miles of range.


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## bipole (Sep 8, 2009)

Meander, I'm putting in a few solar cells on my build. I only have a limited space so they're going to be supplementing the 12v, which is the most cosly power in the car since the power goes through several conversions to get there.

Like you, I have space that needs covered anyway, mine on the rear hatch. I can only fit 7.5 watts into two holes that are supposed to be ventilation for the exhaust. I know 7.5 watts is nothin', but it is enough to offset enough power for all my led guages, including tach and digital speedo during the daytime, plus some trickle charging when the car sits. I'll try to get a picture up here soon.

Most people spend thou$ands on options on a car that have zero payback, at least solar has some. Any power that doesn't come from dirty sources earns you big brownie points in my book. Enjoy (and use) the sun.


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## Tinbendr (Mar 6, 2008)

Here is a source for panels.

I've been told they have the best prices.


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## icec0o1 (Sep 3, 2009)

You guys are making this too complicated. Your truck will use about 250-350 watts per mile depending on how hard you like to accelerate. You could probably put 3 or 4 200 watt panels on your truck so you'll get about 2 to 3 miles per hour of direct sunlight. Average of 5 hours = 15 miles per day. 

Is it worth it if every panel costs $340? Yes if you can wait 5 years to recoup the cost up front. But if you're willing to do so, why haven't you installed panels on your house (I'm assuming)? 

You could go the second way and buy the cheap solar cells from ebay and make your own panels. You could make one giant panel and cover it with plexiglass so it can't break easy and i'll cost you a lot less. But it'll be a lot more effort.


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## Jan (Oct 5, 2009)

But how should they connected? All solar cells have a optimum power point. Like in for example this:










The optimal power (=V*A) is for this panel a little under 2 amps. If you draw more, the power delivery will quickly drop to zero.

How to avoid that? Yes, there are special devices: Power point trackers. But they are expensive, not high enough rated for my taste. And they are commited to a certain voltage ranche. Also not my taste. So, how?


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## green caveman (Oct 2, 2009)

icec0o1 said:


> Is it worth it if every panel costs $340? Yes if you can wait 5 years to recoup the cost up front.


It's not always about cost. People buy $30K pickup trucks to drive the grocery stores. SUV's that never go off-road, etc. etc.

So, I have a '94 Suzuki Sidekick, that will have cost me about (at least?) $3000 to convert by the time I've finished. Figure that it probably won't get driven more than about 1000 miles per year - just around town.

I could have bought a nice, working ICE '94 Sidekick for about $1200. It would cost a little more per mile to drive, but not enough more (unless gas really skyrockets) that the break-even point is anywhere in sight.

I haven't actually calculated it because I don't care. I just don't want to drive a gas driven car.



icec0o1;165634But if you're willing to do so said:


> OP is a renter - that was covered earlier.


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## rmay635703 (Oct 23, 2008)

Built Solar panels can be had for a little upwards of a dollar a watt, did anyone see my links?

Those are the cheapest but not most energy dense panels and they are heavy but weight doesn't make that much difference on a truck, aero does however.

Again whether worth it or not is up to the person building it, we all know the expense but I would argue that done right the solar panels will outlive the vehicle and have value after the vehicle is gone.

Cheers
Ryan


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## peggus (Feb 18, 2008)

Go for it! Ignoring cost, there is really no reason not to. Aerodynamics and weight is not much of an issue with a truck.

You should be able to get some 300W so that's about 1mile per hour with decent sun, or about 6 miles per day in LA in the summer. 
And as previously mentioned, lead acid batteries really like some trickle charging. 

Ideally you'd want some kind of tilting arrangement so you can aim it at the sun but that may be more trouble than it's worth. 

I've charged off of a solar panel at work a few times by just hooking it up to my battery pack. It was about 400V open circuit, solar panels are mostly constant current sources so it will put out a constant current across a wide range of voltages. I managed to get around 2-5 amps going into my 120V pack.
I certainly wasn't using the full power output of the panel but with my 10 mile commute it was enough.

You should design you panel array so that its VMP matches your battery voltage, about 14V per battery. You can also look around for some peak power tracking charger, those tend to be designed for 48V battery packs though. 

Put a diode between pack and panel, you can probably leave it connected permanently with lead acid batteries, the panel will provide some EQ charging if your pack is full.




meanderingthemaze said:


> Anyone know of any examples or resources on charging via solar panels?
> 
> I live in So Cal and it'd be a shame not to harnass that solar.
> 
> ...


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## meanderingthemaze (Jan 25, 2010)

Thanks to all who posted info on solar. And thanks to green_caveman & Ryan for keeping an open mind and encouraging this project. I appreciate it. I will definitely post lots of documentation on all this stuff.

Again, I didn't intend for this to be a war around the idea of using solar, to each his own, right, but alas, it's such a passionate topic we had to air out some of our views. 

So to get back to the original intent, can anyone else offer guidance, based on other people's conversions, or ground up ev builds, or just general solar knowledge, what the best way to approach this would be?

I know absolutely nothing about solar-specific wiring, but then again this whole project has been an educational experience. 

I think the best way to maximize the surface area of the 'flatbed' is to buy high-density cells and make the panel custom fit to the truck's shape. Prebuilt panels probably won't be the ideal solution. While money is a consideration, I would like to spend money smartly, and so don't want to cut corners in this way. 

But if that number of cells doesn't add up to the right voltage, then will I need a voltage converter (transformer, etc.), and what considerations are important for converting a solar signal? Or can we say that DC current is DC current and it doesn't matter that it's coming from solar cells?

Also, what type of cutoff would you recommend to ensure no over charging happens? I don't want to damage cells or batteries.

In terms of security from theft, I'm thnking maybe I could use hex bolts, or something less common so that it would be more difficult for a casual thief. Also, perhaps an actual lock, you know those ones that are hard to get a metal cutter around. It's impossible to stop someone from grinding off the metal, but it would at least deter some thieves. Also, I have considered getting an alarm system too. I do live in a town where car theft is more common and so it may be smart to have that too. These are all deterents, there's no way to stop a determined thief willing to go to extreme measures to get what they want, but if the prize isn't worth the energy spent then you will probably succeed in deterring them?

Finally, what type of coating would be strong yet not limiting light so that I could use the flatbed to cart things around once in awhile? Or would you recommend having a removal plexiglass or other laminate that could be put on and taken off when needing to load the truck?

Thanks


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## peggus (Feb 18, 2008)

I presume you missed my post since you posted a few minutes after me, it gives some answers to your questions.

I looked around for a peak power tracking charge controller, but they are very hard to find for a 120-144V system.

You could split your pack up into lover voltage parallel packs and use some of the off the shelf charge controllers for off-grid houses and RVs.

I think your best bet is to pick your panels so the peak power voltage is close to your pack voltage.


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## meanderingthemaze (Jan 25, 2010)

peggus,
yeah, i missed your post somehow. thanks for the info. FWIW, my pack voltage will probably be in the range of 96V-108V.


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## EV-propulsion.com (Jun 1, 2009)

johnnyfoos said:


> "Anyone know of any examples"
> OK,
> I know of one
> "http://visforvoltage.org/forum/solar-vehicles/1589"
> But this still requires normal battery charging


 Here's a customer's hybrid solar pickup- it is supposed to have 4 - 190watt panels for a total of 760W. 
Maybe a little more inspiration for you?
Mike
www.EV-propulsion.com


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## Jan (Oct 5, 2009)

peggus said:


> I've charged off of a solar panel at work a few times by just hooking it up to my battery pack. It was about 400V open circuit, solar panels are mostly constant current sources so it will put out a constant current across a wide range of voltages. I managed to get around 2-5 amps going into my 120V pack.


Hi Peggus,

Does this mean I should read this graph the other way around:










The current will drop to zero if your voltage increase beyond the -in this example- 60V? So if you keep the optimal Voltage close to your nominal batt voltage it would be work fine? 

I thought the voltage would drop to zero if you draw to much current.



> You should design you panel array so that its VMP matches your battery voltage, about 14V per battery.


VMP?


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## Jan (Oct 5, 2009)

> Here's a customer's hybrid solar pickup...


Fantastic.

And yes, if money is your main or only motivation, buy a small, secondhand economic and not-american ICE car. And use your bike with distances less than 15 miles orso. I don't know of any forum where you can discuss this for months though.


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## peggus (Feb 18, 2008)

You can read it either way. As your pack voltage approaches the open circuit voltage of the panel the current drops to zero. 

If you hang a constant current load on the panel that draws more than 2.2A the voltage will drop to zero.

For charging a battery the first interpretations is more appropriate: the voltage of the battery dictates the current.

VMP is Voltage Max Power, this is the voltage where the panel produces the most power. For the graph you included that would be around 45V. This peak power point changes with temperature, solar angle and insolation. 

You can get a special DCDC converter called a Peak Power Tracker that hunts around for the point of max power automatically in order to maximize the output of the panels. However, the benefit is minor and in following the KISS principle it is a lot easier to just hook the panel up to the battery directly (don't forget the diode). 





Jan said:


> Hi Peggus,
> 
> The current will drop to zero if your voltage increase beyond the -in this example- 60V? So if you keep the optimal Voltage close to your nominal batt voltage it would be work fine?
> 
> ...


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## meanderingthemaze (Jan 25, 2010)

EV-propulsion.com said:


> Here's a customer's hybrid solar pickup- it is supposed to have 4 - 190watt panels for a total of 760W.
> Maybe a little more inspiration for you?
> Mike
> www.EV-propulsion.com


Cool, although he must get a lot of drag in that setup. I'll probably be able to fit half of that, just to cover the backend, but hopefully I'll be able to find higher-density cells so I can get close to what he is getting, but over less area...
Thanks for sharing


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## Jan (Oct 5, 2009)

peggus said:


> You can read it either way. As your pack voltage approaches the open circuit voltage of the panel the current drops to zero.
> 
> If you hang a constant current load on the panel that draws more than 2.2A the voltage will drop to zero.
> 
> ...


Thanks Peggus.

I will not forget the diode! Never.


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## Sunking (Aug 10, 2009)

peggus said:


> You should design you panel array so that its VMP matches your battery voltage, about 14V per battery.


This is not accurate. If you are going to connect the panel directly to a 12 volt battery back it should have a Vmp of 17 to 18 volts which is not a problem because every solar panel made for 12 volt systems has a Vmp of 17 to 18 volts. Problem is I do not know of any EV's that work on 12, 24, 36, or 48 volt battery packs. Golf carts yes, EV's?

The other option is to use panels made for grid tied systems because their Vmp is much higher than those made for battery systems. However it would be a challenge to find the right ones to work with a specified battery voltage unless you use a MPPT type controller. But trying that brings you back to 12, 24, 36, and 48 volt systems because commercial MPPT charge controllers are made for 12, 24, 36, and 48 volt battery systems. It would have to be a custom design.


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## Jan (Oct 5, 2009)

Sunking said:


> If you are going to connect the panel directly to a 12 volt battery back it should have a Vmp of 17 to 18 volts


Why is that, Sunking?


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## icec0o1 (Sep 3, 2009)

Jan said:


> Why is that, Sunking?


Because it's 17-18v at max sunlight and the voltage would have to be above 14v at partial sunlight for it to charge the batteries.


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## Sunking (Aug 10, 2009)

Jan said:


> Why is that, Sunking?


Ice0o1 answered it close enough. But it is a moot point anyway because if you buy a panel made for 12 volt battery systems it is going to be 17 to 18 Vmp. 

FWIW Vmp does not change much with irradiance say from 1000 w/m/2 to say 500 w/m/2. It is the Imp that changes with irradiance. Temperature affects the Vmp, the colder it gets, the higher the voltage.


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## Jan (Oct 5, 2009)

Well, I’ve been calculating a bit. Probably wrong. But that’s why I would like input.

I wanted to know what the most effective layout is given a limited area. And by layout I mean, the amount in parallel and serial. And I’m not talking about panels, but cells. 

Given 20 cells with the A x V characteristics shown in graph1.

Where the percentage the power of the sun is. Copied this more or less from this site: 
http://chuck-wright.com/SolarSprintPV/SolarSprintPV.html

Next I took one Sky Energy 100 Ah Li-Ion battery. And copied the charge graph from jrickard’s blog giving attached graph2.

Then I made a graph for every solar power percentage. 60, 80 and 100%. Where I calculated the total charge time for 5 to 20 cells in series. This I did by looking up the amperage the solar cell will give at the battery voltage. Based upon Peggus’s “For charging a battery the first interpretations is more appropriate: the voltage of the battery dictates the current.”

I did this for 5% steps in the SOC. And for those 5% pieces (=5Ah) I calculated the total hours charge time. And added them all up to a total charge time.

Giving me graph3, with the purple line for 5 cells, up to 20 cells in series.

With less than 7 cells it takes more than forever to charge the battery. But with more than 7 cells in series the charge time doesn’t get much shorter. 
So, the yellow line gives the charge time if you have 20 cells and can place them in parallel too. I know you can’t put 20 cells parallel in series of 7. It‘s just theory. You probably have more than a few hundred cells. 

The yellow line shows that with 7 cells you have the shortest charge time. And in this example 7 cells are rated 3,5V. Which means to stay as close as possible to the max SOC voltage. If you want to use a limited surface to the max.

I can’t attach a spreadsheet, so if you really want to check it, mail me.


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## peggus (Feb 18, 2008)

I stand corrected, 17V it is. 

As for total voltage, panels are made from a number of cells in parallel and series, you can get any voltage you want by connecting lower voltage panels in series. Make sure any bypass diodes can handle the full voltage.




Sunking said:


> This is not accurate. If you are going to connect the panel directly to a 12 volt battery back it should have a Vmp of 17 to 18 volts which is not a problem because every solar panel made for 12 volt systems has a Vmp of 17 to 18 volts. Problem is I do not know of any EV's that work on 12, 24, 36, or 48 volt battery packs. Golf carts yes, EV's?
> 
> The other option is to use panels made for grid tied systems because their Vmp is much higher than those made for battery systems. However it would be a challenge to find the right ones to work with a specified battery voltage unless you use a MPPT type controller. But trying that brings you back to 12, 24, 36, and 48 volt systems because commercial MPPT charge controllers are made for 12, 24, 36, and 48 volt battery systems. It would have to be a custom design.


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## meanderingthemaze (Jan 25, 2010)

This is where I need help. I don't know what the available or necessary electronics would be to execute a custom panel for my pack.

If I'm doing 108V (18 x 6V), then the appropriate voltage should be about... 
18 * 9V = 162V [I'm getting 9V from the 12V example given earlier ie: 6V:9V as 12V:18V, right or wrong?]

If so, and solar cells are rated at .5V each, then that would require... 
162V/.5V = 324 cells

Otherwise, I'd have to step up the voltage, which would require other electronics. Does anyone know if the parts required to do the job would be specialty or off-the-shelf radio shack stuff?


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## Jan (Oct 5, 2009)

meanderingthemaze said:


> Does anyone know if the parts required to do the job would be specialty or off-the-shelf radio shack stuff?


Add a 12V battery to your system, if you not already have. Charge this one with solar. And if you want to charge the main pack: add an 12V-220V inverter, and connect it to your onboard charger. With some simple electronics you could start the inverter if the 12V voltage raises eg 13V. And stop it when it reaches 10 or so. Sometimes they allready have such a battery protection system built in. So with a z-diode and a relais you could start it. It's like a bucket you fill up under a dripping tap, and pour into a big tub.


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## Sunking (Aug 10, 2009)

Jan said:


> Add a 12V battery to your system, if you not already have. Charge this one with solar. And if you want to charge the main pack: add an 12V-220V inverter, and connect it to your onboard charger. With some simple electronics you could start the inverter if the 12V voltage raises eg 13V. And stop it when it reaches 10 or so. Sometimes they allready have such a battery protection system built in. So with a z-diode and a relais you could start it. It's like a bucket you fill up under a dripping tap, and pour into a big tub.


Jan I think I follow you but not to sure that would work very good.

First problem I see is the built in charger. I assume it is some type of constant current or constant power supply assuming the batteries need a charge. So for example it needs a solid 3 Kw source. Wel that means you need about a 4 to 6 Kw inverter which would be very expensive if you could even find one. But just how long do you think a 12 volt accessory battery could supply a constant 3 Kw load. Most likely outcome is the inverter tripping off-line immediately from under voltage supply. Not to mention all the losses along the way. I just do not see that as being practical or workable.


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## green caveman (Oct 2, 2009)

meanderingthemaze said:


> Cool, although he must get a lot of drag in that setup. I'll probably be able to fit half of that, just to cover the backend, but hopefully I'll be able to find higher-density cells so I can get close to what he is getting, but over less area...
> Thanks for sharing


If you're only going to use it while parked, you could improve efficiencies significantly by angling the panels. Gary Reysa's BuildItSolar has some great resources. Information on angles is here (and in some of the links from there too). Of course, that adds a level of complexity, since you'd either always have to find parking spaces oriented N->S or have some way to tilt the panels based on the parking space orientation.


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## Jan (Oct 5, 2009)

Sunking said:


> Jan I think I follow you but not to sure that would work very good.
> 
> First problem I see is the built in charger. I assume it is some type of constant current or constant power supply assuming the batteries need a charge. So for example it needs a solid 3 Kw source. Wel that means you need about a 4 to 6 Kw inverter which would be very expensive if you could even find one. But just how long do you think a 12 volt accessory battery could supply a constant 3 Kw load. Most likely outcome is the inverter tripping off-line immediately from under voltage supply. Not to mention all the losses along the way. I just do not see that as being practical or workable.


Yes, I wouldn't take a inverter of more than 1 kW. Something between 600 and 800 watts would be enough. Not all chargers need 3kW. A manzanita (?) can be adjusted to a lot lower amperage. And a Brusa can be controlled with a pot meter.


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## green caveman (Oct 2, 2009)

meanderingthemaze said:


> This is where I need help. I don't know what the available or necessary electronics would be to execute a custom panel for my pack.
> 
> If I'm doing 108V (18 x 6V), then the appropriate voltage should be about...
> 18 * 9V = 162V [I'm getting 9V from the 12V example given earlier ie: 6V:9V as 12V:18V, right or wrong?]
> ...


What's the downside to splitting the pack? It's usually much easier to find electronics rated for, say 40V than for 160V. Or even charge each battery individually?. Someone suggested that this might not be a good idea, but assuming that the panels are match and have equal solar intensity on them is there really a problem.

I'm starting to like this idea in general, although I don't think I'm willing to take this on as a project just yet - at least until the car is finished! It occurs to me that most of the time the car will do, say, 6 miles per day. With some (easy) scheme to pull out the panels whenever it's parked it might not be too difficult to cover most of this from panels.

Unlike adding them to a house, you don't need the permits/grid tie/inverters, etc. which significantly lowers the cost of installation.


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## meanderingthemaze (Jan 25, 2010)

Jan,

Thanks for the ideas. I'm not sure I completely understand, mostly because I've never done a conversion, so I don't have a lot of hands on with these components. But I'll read it a few more times. 


green caveman,

I'm trying to think of a practical way to tilt but also have it be secured while stationary (to carry load), and also have some semblance of an anti-theft feature. I'm thinking of a telescoping ball joint...I don't know. If I want the right angle, I'll need control in all axes. Thanks for the links.


I'm going to call around to some solar companies and see what they say.


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## Sunking (Aug 10, 2009)

Jan said:


> Yes, I wouldn't take a inverter of more than 1 kW. Something between 600 and 800 watts would be enough. Not all chargers need 3kW. A manzanita (?) can be adjusted to a lot lower amperage. And a Brusa can be controlled with a pot meter.


Ok 3 Kw was more of a guess on my part because there would be no reason to have 240 VAC charger for such a light load of 800 watts. I mean my little 48 volt golf cart charger uses more than that on 120 vac. If you have say a 16 Kwh battery pack and want it charged from 100% DOD in 8 hours you are going to need 2000 to 3000 watt input, and that can only come from a 240 VAC 30-amp circuit.


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## Jan (Oct 5, 2009)

Sunking said:


> If you have say a 16 Kwh battery pack and want it charged from 100% DOD in 8 hours you are going to need 2000 to 3000 watt input, and that can only come from a 240 VAC 30-amp circuit.


I though we allready agreed that charging completely from onboard solar is impossible. So, to make some use of onboard solar with just radio shack stuff, this could be an option. You don't need a DCDC converter for your 12V circuit anymore. What helps a little. And in excess of solar energy this mechanisme charges the main pack too. Not full in 8 hours. But a few extra miles a day. And maybe full in a sunny week for a short weekend trip.


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## rmay635703 (Oct 23, 2008)

http://ecomodder.com/forum/attachment.php?attachmentid=278&d=1202049916
http://ecomodder.com/forum/showthre...roduced-aerodynamic-pickup-bed-cap-583-4.html

I had a nice point by point long post worked up but then I got a blasted spyware attack and crashed, everythings reinstalled now, bleh.

Anyway I would strongly consider making a lightweight aero flatbed/belly pan with wheel covers, Then repeat what bondo did above making a tiltable angled cover, I would keep the line of the cover just below the rear window. Since this would be a true boattail you could easily gain 20% more range just from the aero improvement, likely closer to 50% depending on your speed.

I would also attach the panels securely to that cover so although possible to remove eventually it would be big boxy, bulky and downright heavy making it almost impossible to steal.

Having a lightweight bed and cover on the back would allow the batteries to have a nice place to be put out of the weather and accessible. Also they would give you a bit more traction.

As for having separate MPPT systems to charge that is PERFECTLY fine so long as they are equally sized. You will have to do more maintenance on the batteries to keep equalized but you should do that anyway. Also remember panels run separately usually put out more power in total then when run in series but they also put it out unevenly.

Next don't bother building panels from solar cells, If I could still find the individual cells for $0.50 a watt I personally would build a custom panel but remember homebuilt have some major issues
1. Cost of ownership, solar cells die rapidly exposed to any form of moisture and the RTV like compounds to seal correctly are very expensive.
2. Complexity, there are literally thousands of solder joints to make in your project, meaning thousands of chances to get a cold one lighting fire
3. Lifespan as I mentioned most homemade panels, even those done correctly are indoor only and die within about 5 years outside.

So just find panels that will both fit within the area you have and also meet the voltage requirements needed when added together.

Solar panels have two voltages, one is the lower rated voltage that provides maximum amperage output the higher voltage listed is the open circuit voltage.
Generally the minimum voltage panel you need must have ideal amperage output at around your floating voltage, however if you don't want the panels to overcharge you can build an unregulated array that only goes up to the max voltage you want the batteries to see but then they are much less efficient.

I had a much better writeup earlier, ah well.

Cheers
Ryan


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## meanderingthemaze (Jan 25, 2010)

Ryan,

What a drag about the computer. I just had to a wipe a computer the other day. Might I suggest you take a disk image of your newly configured system, just in case it happens again. You can revert back to a clean install in minutes. Don't mean to distract from the forum, but any time you're spending fixing computers is time you're not spending on EVs. 

I was definitely planning some aero-mods...
http://ecomodder.com/forum/showthread.php/belly-pan-fan-concept-12206.html

However, I think the custom bed shell would violate principal #1 of building an EV, namely, that you must like the vehicle you will be driving, otherwise you won't want to drive it. 

But the belly pan, and possibly some ducting to route the cooling air through the motor compartment is definitely a must.

I wonder if you would have the same opinion about custom solar panels if a solar company built it for me? I would totally agree that it is a manufacturing process that could lead to more headaches than help, and for a beginner probably will. But, I would prefer to have it fit the bed properly. I'm staying open to whatever option makes the most sense.

Too bad we missed your 'better' first post, but thanks anyway for reposting.


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## rmay635703 (Oct 23, 2008)

Well to build your own best to use tempered glass and slyguard, thats how they did them in the 70's and many of those panels are still operating.

Avoid any wood products. Plexi and plastics can work but don't pass light as well and also have issues with aging in the sun, unless of coarse special measures are taken.

and check out
http://solarpaneltalk.com/

They have several threads outlining what people have done, what they have used and how long they have lasted. Generally you need to get the cells encapsulated on both sides fully with slyguard after everything is tabbed, soldered and load tested. I believe you can also buy laminated cells which are more resistant to weather out of the box but they come at a premium costing almost as much sometimes more than the various panels on the market.

Also If you go around asking about DIY solar panels be prepared for backlash, many of the solar sites are hostile to that and I haven't even been able to coax any lower cost bulk sources of plain cells out of anyone. I find it very strange that cells cost more now than 5 years ago when I priced them out. Must be a lot of vested interests holding out on me.

Next step is to start bothering manufacturers. Needless to say with panels costing between $1.50-$3 a watt from many sources cells have to be in the $0.50-$1.00 a watt area for the panel to be priced that low.

Interesting reading here
http://www.builditsolar.com/Projects/PV/pv.htm (though not entirely on topic)


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## NZero (Jan 30, 2010)

Just thought I'd throw my 2 cents in here.

My conversion is a 88 Prelude, big bonnet area plus small boot lid and behind the sunroof.

I can fit 2 x 120w panels on my car so your truck should be good for a bit more.

My batt voltage is 120v so even 1 panel would charge them at 1amp.
with 100ah batteries and I work 9 hours a day plus lunch so thats 10hours parked in some sun that my car will get.

10hours x 2 amps = 20 amp hours back to my batteries or 20%.

So much for its not worth it!
And all for another $600!


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## ZipZap (Oct 24, 2009)

After the DC/DC converter failed to operate properly I removed it from the system and have been using a solar panel for charging the accessory battery for about a year now. This way I've gone to using a U1 bat from the grp 27 I had in before. I installed with the panel an inexpensive solar controler. This system has worked fine for me even here in the northwest, Seattle area. It is only connected into the accessory system. I am looking for a better panel, as the one I'm currently using is not the best. As for aiming it, I just mounted it on the lid of my drive battery box. Sitting there flat it gets enough sun, (when we have it!) to maintain the battery, even on cloudy, dark, rainy days.

Jon


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## Sunking (Aug 10, 2009)

NZero said:


> Just thought I'd throw my 2 cents in here.
> 
> My conversion is a 88 Prelude, big bonnet area plus small boot lid and behind the sunroof.
> 
> ...


Well you have made a mistake most folks make about solar. You assume if the sun is shinning for 12 hours per day you get a full 12 hour power production out of the panels. That is far from real world application. Solar insolation is measured in kwh/day/m/2 which can be directly converted to Sun Hours. 

For example I live in TX which has very good insolation. This time of year about 3.9 Kwh/day/m/2 or 3.9 hours even though the sun shines 13 hours per day. So for a 100 watt panel gives me 100 watts x 3.9 hours = 390 wh at the terminals. Once I account for inefficiency only about 250 watt hours of usable energy. At a EV efficiency of 400 wh/mile less than a mile of extended range.


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## meanderingthemaze (Jan 25, 2010)

Ryan: thanks for the tips, I'm looking into those sites/recommendations.

NZero: Good for you. I think Sunking is right about not getting the full draw all day, but it all helps.

Jon: Nice! that's great to hear.

As far as my application goes, I'm looking at some 17% efficient Sanyo panels, but they're ridiculously expensive, at about $5/watt. The price just skyrockets after about 14% efficiency. It'll give me 400W peak total.

I think I've abandoned the idea of building the panels myself. If something does go wrong, it would be nice to be able to either have a manufacturer replace or repair the panel, and if it was my fault for the problems, I could go back and just buy another panel off the shelf. Plus, I think I just won't be able to beat the manufacturing methods that are being utilized for mainstream production (easily).

I think tracker systems are too expensive, but I read an interesting thread on electric linear actuators here on the forum that might be a good way to go for positioning the panels. 

I emailed US Battery and they told me:
"You will need to control the Voltage either by a power resistor in series, or better still with a Voltage regulator that maintains the Voltage at about 2.4 Volts when it reaches there during sunshine."
(Not exactly sure where the 2.4V number came from.)


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## NZero (Jan 30, 2010)

Sunking said:


> Well you have made a mistake most folks make about solar. You assume if the sun is shinning for 12 hours per day you get a full 12 hour power production out of the panels. That is far from real world application. Solar insolation is measured in kwh/day/m/2 which can be directly converted to Sun Hours.
> 
> For example I live in TX which has very good insolation. This time of year about 3.9 Kwh/day/m/2 or 3.9 hours even though the sun shines 13 hours per day. So for a 100 watt panel gives me 100 watts x 3.9 hours = 390 wh at the terminals. Once I account for inefficiency only about 250 watt hours of usable energy. At a EV efficiency of 400 wh/mile less than a mile of extended range.


Yes except that 10 hours is 90% useable sun as per my 3 10w panels I currently charge my ICE vehicles battery with when I removed the alternator many moons ago.


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## green caveman (Oct 2, 2009)

Since the cost/weight/etc. etc. is in the panels. Could you use smaller panels, and a large, folding parabolic mirror?

Imagine some large, inverted, shiny, parabolic, beach umbrellas with a (relatively) small panel at the center. Cooling/mounting, etc. might be a problem, but they'd be readily portable and should be light to deploy. 

Could you use thermo-electric cooling and gain some more power, that is, if you pull power out of a thermo-electric junction would you cool the hot side enough to cool the panels?


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## dtbaker (Jan 5, 2008)

this thread seems to bounce a lot between the practicality, and the original question which is simply what you have to DO to feed lower voltage PV power into a battery pack....

I am not interested in putting panels ON my car but AM interested in building a PV covered carport at work to recharge a little during the day. So the question is, depending o whether I can build a string of enough panels to get over my pack charge voltage, or not... HOW do I regulate the voltage feeding into the pack with enough accuracy to feel good about the batteries not getting fried?

Two different problems really, but go ahead and try to address both!

Let's say I can only find a couple cheap old panels to start with, and the max voltage is LESS than what I need. How do I bump the voltage up to some specific target and let it trickle in at low amps?

Let's say I get a boatload of cheap panels somewhere and can put together a string in excess of my pack voltage. How do I clamp the voltage down to a specific voltage, and prevent things from getting fried if the battery pack actually gets 'full' ?


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## Jimdear2 (Oct 12, 2008)

dtbaker said:


> Let's say I get a boatload of cheap panels somewhere and can put together a string in excess of my pack voltage. How do I clamp the voltage down to a specific voltage, and prevent things from getting fried if the battery pack actually gets 'full' ?


As a fun thought exercise. 
Rediculously expensive.
Just the right thing for a DIY's to play/have fun with.

If it seem worth while/FUN let's start a thread on this and see just how far we can all take it. Baised on the needs of EV's of all types.

This might be a place for those with the specific math and knowledge to teach us something as well.

Anyone have any portion of this already done?



Here Are My Ideas


How about a large, grid tied, low maintenance, UPS battery/solar system, something like an Edison battery inserted into the plan. This way all % of any solar available is used to either charge the UPS battery or kicked back into the grid. 

A bit of developement on a logic to control this, based on average use and needs over time.

This gives you a system at home/work/both that is always productive as long as there is light, even a few watts will run the power company's meter backwards.

From here it is going to be easy to dump charge the EV battery from the big battery at a rate that is best for EV battery life.

In case of emergencies the EV becomes a source of power for the UPS system


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## Woodsmith (Jun 5, 2008)

That sounds much like the 'Car2Grid' idea that has been about for a little while, Jim.

I like it, especially the dump charge bit, enabling a possible quick recharge, like having a gas pump at home.

I guess for those with pick ups it might be an easy case of swapable packs straight off the pick up bed, maybe a demountable rack?


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## dtbaker (Jan 5, 2008)

Jimdear2 said:


> As a fun thought exercise.
> Rediculously expensive....
> How about a large, grid tied, low maintenance, UPS battery/solar system,


I already HAVE a household grid-tied system.  What I am after is a way to build a carport at my parking spot at work.... I happen to have a cousin in the recycle biz, who just bought several hundred 30-40 year old PV panels for scrap (aluminum frames). They still work, but only put out about 6.5 volts per panel.

To avoid the cost and losses of running thru an invertor, and then BACK thru my on-board charger, is there a way to regulate the DC volts coming off the panels to 'exactly' what I want to feed my main traction pack directly?


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## major (Apr 4, 2008)

dtbaker said:


> Two different problems really, but go ahead and try to address both!


Hi dt,

Excuse me if this has been put out here already. I didn't read the entire thread.



> Let's say I can only find a couple cheap old panels to start with, and the max voltage is LESS than what I need. How do I bump the voltage up to some specific target and let it trickle in at low amps?


You need a boost converter.



> Let's say I get a boatload of cheap panels somewhere and can put together a string in excess of my pack voltage. How do I clamp the voltage down to a specific voltage, and prevent things from getting fried if the battery pack actually gets 'full' ?


You need a buck converter. Like a dc/dc converter.

Now the basic boost and buck converters are not isolated, input to output. This may or may not be an issue for you. Often times, dc/dc converters are isolated.

If you end up having excessive voltage from the panels, you could simply figure out how to use dropping resistors to reduce the voltage. This is obviously energy wasteful, but if it is free energy in the first place and more than you need, what the H.

My take on it 

major


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## dtbaker (Jan 5, 2008)

thanks Major, that's a good start! 

Do you have any favorite sources for these types of DC-DC convertors? I am wondering what kind of cost ballpark.... If there is a significant difference between them, it would indicate I would be best to put together a string either slightly over, or slightly under what I want to charge at.

And... can you clarify if it would be ok to have a 'constant voltage' determined by the convertor and just let the panels simmer along all day without fear of overcharging pack if and when it got 'full'? As the pack gets close to full at a fixed voltage, the amps fall off to nil, so I wouldn't have to worry.... right?


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## peggus (Feb 18, 2008)

dtbaker said:


> ....
> Let's say I get a boatload of cheap panels somewhere and can put together a string in excess of my pack voltage. How do I clamp the voltage down to a specific voltage, and prevent things from getting fried if the battery pack actually gets 'full' ?



In this case you really just need a circuit that opens up the connection between panel and battery when the pack is full. It could be as simple as a comparator and latch that opens up a contactor when pack voltage exceeds some level. 

I've charged my 120V pack from a 400V panel at work, works beautifully.


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## major (Apr 4, 2008)

dtbaker said:


> thanks Major, that's a good start!


You're welcome. But I may not be of much more help 



> Do you have any favorite sources for these types of DC-DC convertors? I am wondering what kind of cost ballpark.... If there is a significant difference between them, it would indicate I would be best to put together a string either slightly over, or slightly under what I want to charge at.


Yeah. I really like Vicor. But they are really expensive. For electronic converters, for years, I've used the ballpark of a dollar per watt. If it is a special converter, it may be $10/W. If it is a mass produced converter, maybe $0.10/W. 

Trouble is that what you want is most certainly special. Probably non-existent. But easily built if you're an EE specializing in power and control electronics. Nothing new to invent. Just application of well known converter technology.

Both the buck and boost converter use the same basic components, just different topologies. From that stand point, doesn't matter. But with a solar PV array, you have a variable source depending on the time of day, time of season and cloud cover. So you'd have to look at these variables to figure out the best solution.



> And... can you clarify if it would be ok to have a 'constant voltage' determined by the convertor and just let the panels simmer along all day without fear of overcharging pack if and when it got 'full'? As the pack gets close to full at a fixed voltage, the amps fall off to nil, so I wouldn't have to worry.... right?


Well, again you have this variable input to the converter to contend with. And not only that, but the PV output will vary with load. Theoretically, it is a complex problem of which I have no easy answer. That does not mean that a simple solution is not possible. In fact, there probably is. It just takes someone willing to spend a lot of time fooling around with it. Like I said, if it is free energy to start with, what do you have to lose? 

Regards,

major


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## green caveman (Oct 2, 2009)

peggus said:


> In this case you really just need a circuit that opens up the connection between panel and battery when the pack is full. It could be as simple as a comparator and latch that opens up a contactor when pack voltage exceeds some level.


Does Lee Hart's Shunt Type Battery Balancer, a simple Zener system, add anything useful in this discussion?

I think that there's been a discussion of stopping charging based on a light input (using the sensor from a night light?) using the bulbs in the system as the trigger.


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## dtbaker (Jan 5, 2008)

peggus said:


> In this case you really just need a circuit that opens up the connection between panel and battery when the pack is full. It could be as simple as a comparator and latch that opens up a contactor when pack voltage exceeds some level.
> 
> I've charged my 120V pack from a 400V panel at work, works beautifully.


Sounds like shutting off an over voltage source is the easiest and cheapest? rather than the boost or limiting dc-dc converters?

ok, how about some specifics on parts. Lets say I put together a string that yields voltage a little over what I need.... what do I need to have an accurate (maybe even settable) latch?


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## rmay635703 (Oct 23, 2008)

peggus said:


> In this case you really just need a circuit that opens up the connection between panel and battery when the pack is full. It could be as simple as a comparator and latch that opens up a contactor when pack voltage exceeds some level.
> 
> I've charged my 120V pack from a 400V panel at work, works beautifully.


I agree a simple standalone DIY battery overvoltage cutoff would be a GREAT addition to everyones arsenal since it could be used to prevent overcharging from any DIY charging source.

In this case however might I recommend a 2 stage setup in other words 2 comparators one with a resistor and one without.

Then the solar panel could fully charge the battery (assuming its not super low amps and doesn't need this)

And ideally he wants his panel voltage to be roughly 50% higher than his nominal pack voltage to get maximum charge current.

Cheers
Ryan


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## dtbaker (Jan 5, 2008)

hhhmmm, this is interesting.... my new PV panels are 40v nominal, and the ancient old ones (non-UL listed even) are 6v ! so using the old ones I am unlikely to build a string of 150+v because of the number required. So, realistically for the 'small array' I would have to use a 'buck converter'.

Does anyone have good sources for one perhaps bucking up from 60v panel output to the 140v that a 120v nominal battery pack would get charged to?

With a new large array, one could possibly go the direct DC route to avoid inversion and charger... AND get a quick charge! My house system puts out a max of 22amps at 320v .


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