Solar Panel questions

System · April 2006

Hey...Am in need of solar panels for my school class project. We are making a go kart which will run off the solar panel. THe solar panel will also charge the batteries ( so we hope! ). The motor and the batteries are both 24V. Hence, need a 24 Solar panel. Have researched various solar panels but for the solar panels, the price does not seem to go below $900.

I explored the option of buying two 12V solar panels but it still turns out to be expensive. buying 1 big solar panel results in a $200 freight cost which increases my total cost

.

Any ideas on how I can reduce my cost?

BB. · April 2006

Re: Solar Panel questions

Depending on where you are at, you may try and contact somebody like Home Deport, BP Solar, a local solar installer, etc. and see if you can get a donation for your school--or offer to purchase any used/demo/old stock panels. Right now, you are probably looking at $4-$5 a watt for solar panels. Please look at your battery capacity to ensure that you can charge in the expected time frame for your project (a 10 watt 24 vdc panel could several weeks to fully charge even two average size storage batteries--assuming that they don't self discharge faster)...

You probably need something like 50-200 watt panels per 1kW of energy storage in your project to make it interesting--There are several threads here about the 5-12% charge rate for wet cell batteries.

Right now, panels are in high demand so they won't have any reason to give you any deal on one panel as a retail buyer.

Offer to contact the local paper and/or slap a sticker on the car... Get a nice looking web page (on school site?) up with an area to thank people/companies for any donations.

Salesmanship!

Good Luck,
-Bill

System2 · April 2006

Re: Solar Panel questions

I hate to down your project but, I feel you should pick something else unless you have a lot of extra cash laying around. You are talking some major bucks if you want it all to be light as possible and to fit on something as small as a go cart. I'll try to stay close to the cheap products, not what the big boys would use. Like thin film panels and aluminum frame and bearings in the wheels not bushings.

I am sure you will have to run it off batteries because a couple of solar panels will not run a very large motor.

So,

What size of a motor is this going to be and how many amps will it pull or watts at 24 volts?
How long do you want the motor to run?
How big of a battery bank are you planing to have on it in amp hours at 24 volts?
How fast does it have to re-charge the batteries?

Two 12 volt batteries 80 pounds each (100 bucks each)
One 180 pound guy setting on it (1 six pack of beer)
One 100+ pound frame (hired out to build 500 buck)
One motor (Couple hundred bucks would be cheap)
One charge control (150 bucks)
Four or more 60 pound solar panels. (about 2000 bucks)
Four tires (200 bucks easy)
All that goes with the steering and breaks gears for motor to wheel drive and clutch (500 bucks plus)
You will have a 1000 pound plus go cart by the time it is done at close to the price of a new small car.

Will not be easy to cut cost.

System2 · April 2006

Re: Solar Panel questions

One thing to think about - - if you don't need it to go very fast, re-wire your 24 volt battery for 12 volts and run the 24 volt motor on only 12 volts. It will run about 1/2 speed and produce less power, and will draw lass current from the battery too - - unless you overload the motor, it will work fine. Slower, but fine for a demo. With a limited budget, don't expect to run the motor directly off the panel. Use a small 12 volt panel to charge a small 12 volt battery over an extended length of time (days if necessary) and then go for a short, slow drive.
However, if on your limited budget you're looking for a fast car that will travel a long distance - - forget it.
Good luck
Wayne

System2 · April 2006

Re: Solar Panel questions

Wayne from NS Cana wrote:

then go for a short, slow drive.

very short, very slow, IF it's downhill

kidding.. :roll:

seriously Jon has some points, if it was cheap enough for a school project to efficiently make a demo that actually makes someone want to do this when seen, well, then we'd all have solar cars.

how about a solar fountain, lighting system, motorized mobile or something.. ?

we have a little toy 8" solar car (well, before i broke it) which runs direct off the panel, cost was about $45 i thinkm, maybe thats an option? of course we're all assuming you have no money as a school, perhaps you have a grant of something? still, it approaches real a REAL car's cost

System2 · April 2006

Re: Solar Panel questions

Before you get too discouraged you might try talking to your city/county/state department of transportation to see if they have any damaged panels that might be salvagable. In my area there are lots school speed zone signs and other types that have solar powered flashing lights on them. They are constantly being knocked down by moron drivers and are replaced with brand new equipment. You may be able to cut good sections out and solder to other good sections. As Bill suggested, try solar installers, suppliers, etc. Expanding on his suggestion, perhaps you could "borrow" a demo panel or two and offer to give them the cart when your project is finished so they will get something from the deal as well, they could use your project cart to help sell their products. You guys have probably already researched and seen lots of cool EV's but here's a site with an electric Ninja and other interesting EV's (scroll down): http://www.electric-bikes.com/motorcys.htm
What kind of motor are you using? One horse power is about 745 watts so you won't get gasoline powered cart performance from a little solar but you should be able to build a useful fun cart. Keep rolling resistance low with narrow high pressure tires as this will be where most of your performance will be lost. Good luck!

Bad Apple

System2 · April 2006

Re: Solar Panel questions

hey.. Thanks a lot for your replies guys..

We have a total $1000 budget from the school. Our group is using a 24V 300W brushless motor ( which we found for $20 because it was refurbished or somethin like that ) . THe way we have it now, the cart is going to be moving at a crawl anyways... Was trying to ask the motor guy in my group if we could just use a 12V motor, hence saving on one battery but he insists that the "12V motor" will probably be brushed, expensive and will not have a speed controller in it ( hence posing an issue ).. Hence, I guess it now depends on finding a solar panel. I was trying to look for 6V panels which I Could connect in a combination of series and parallel and try and get tthe required power ( a minimum of 100W ). but so far, I have no luck.

Tried calling various Solar Panel vendors and they were not willing to cut me a deal even though It wa for aclass project. Two vendors offered a 3% discount but thats not much.

A recharging time of 4 hours would not be bad. Was planning to use two 12V Universal Motorcycle batteries ( each with a 7AH rating ) . Our demo time will probably be 30 min....

Problem is that we already running short of time.. It is due at the end of may and haven't even ordered any of our parts yet. We had certain "teamwork" issues.. and that put us behind a lot in terms of time but oh well, its all a a learning experience I guess...

Will a 24V motor run with only 12V? I thought that this would not provide the motor with enough current to run it... Not sure though...

Also from a tilt angle standpoint, I used the following link ( http://www.wattsun.com/resources/calculators/photovoltaic_tilt.html ) and entered my latitude as 42 degrees .. It gave me a tilt angle of 22 Degrees.. is this accurate?

Thanks

System2 · April 2006

Re: Solar Panel questions

Being you came back you are for real. Others have come in here trying to get us to do their home work for them or just posted and never come back.

Try this website. I think John Kimbal at Sun Electronics will give you about the best deal around. He has panels that are not UL listed but a go cart will not require listed parts. http://sunelec.com/

You are worried about angle and tilt.. I don't see that as a big deal being your cart is mobile and will hardly ever be with the panels facing south unless parked. I would go as flat as you can (faceing strait up). Unless you know which way the track will be facing. But this may not matter if you are only going to drive it for thirty minutes. I think a panel setup as a spoiler would look pretty kewl. :-D I think it would be best to run 24 volts battery bank with a 24 volt motor. You clould burn it up at 12 volts. So the motor will be 300 watts divided by 24 volts is 12.5 amps. So I would think you would want to use four 7ah batteries or 14ah battery bank at 24 volts. So 10% of a 14 ah battery bank is 1.4 amps charge rate. You know you could get away with two 55 watt panels to make 24 volts which would give you 3.2 amps. Would charge the 14 ah in batteries in maybe in a couple of hours. I would get a cheapie charge control from some where or make sure you do not forget to disconect the panels from that battery bank because it will be putting about 20% charge rate in them small batteries. Could blow them up if left on there all day long with no charge control.

14 ah in batteries should run you at top speed for about the half hour your require. The sun will be high in the sky in June so the panels pointed strait up will help the batteries run the car.

Maybe you or someone else can add to this but it seems correct.

Don't forget to come back from time to time and keep us posted on how the project comes out.

System2 · April 2006

Re: Solar Panel questions

Hey.... Thanks for your prompt response. Definately serious about this project

Shall definately call Mr. Kimbal tomorrow.

The solar panel is supposed to power the go kart as well as charge the battery. The battery is only supposed to power the go kart if its a cloudy day. Buying 55Wx2 panels would cost a minimum of $550 and that would be going above the budget. Can't get more batteries due to budgetary constraints.

Need to find some kind of configurations of smaller panensl ( such as 12v or 6V panels ) to get approx 100W. Do you think this would turn out to be cheaper? So far, I've researched some and checked out a few on the sunlec website ( 2.5W, 6V=$15 ) but it turns out to be expensive if I want 24V and 7 Amps.

Will be running the cart in May... Shouldnt there some kind of tilt angle in case we run it in a straight line?

Thanks much

niel · April 2006

Re: Solar Panel questions

i don't think you will be getting this to run by just using pvs. if you were to mount the modules to the kart it would be too large in area and prone to damage from running into things even at 100w. a 24v pv is possible, but higher in costs that you don't have $ for.
be advised that motorcycle batteries will be destroyed quickly used in the way described as they are starting batteries and not deep cycle. draining even deep cycle batteries that steeply on an overall percentage of their capacity could warp the plates in no time and you should not discharge beyond 50% of the battery capacity.
about the only thing right is the motor as it has just about .4 horse power which is enough to move you even if not very fast. when drawing 300w you need to supply 300w constantly and reliably for the time period indicated. that means at least 300w of pvs. back to the drawing board in economics class as it boils down to not the pvs, batteries, or the motor, but what's available for what cost that'll work. that's your real work. good luck.

BB. · April 2006

Re: Solar Panel questions

The panels should be square to the sun to generate the most energy... The link you gave looks to give results in the ball park. But, remember, the time of day is also important--those numbers are best at 12 noon local time (without Daylight Savings time) pointing due south. If you are within 10 or 15 degrees--you probably will not notice the difference.

In general, larger panels are going to cost less per watt than smaller panels. Again, if you have a Home Depot or can find a local installer, perhaps you can even borrow a couple of larger panels for a few weeks (leave some money as a deposit)--ask if they have any used panels too.

Lastly, decide if the "go cart" is going to stay assembled or is going to be taken apart. You can probably sell/raffle the go cart and or pieces of same. The panels themselves, you should be able to get between 50 cents and 1.00 on your purchase price if you are going to piece the project out.

Also, as Niel said, the batteries are not a great fit... Perhaps you can find some parent with trolling motor batteries you can "borrow or purchase ahead" for the project.

-Bill

System2 · April 2006

Re: Solar Panel questions

Greetings,
If I were you I would take the suggestions from this forum, the $1000 bucks from the school and anything else you can get donated and build "something" no matter what. Back in the old days Ford built a pretty good car called a Edsel. It was a flop but Ford didn't stop building cars over it. This solar thing isn't going away. The experience you get from the project will put you smack dab in the middle of this forum. You will have a head start on where everyone on this planet is going. Just as an aside you may want to investigate recovering some momentum for additional battery charging. I know you are running out of time but any thing that has to stop has power to charge batteries. It doesn't all have to turn into brake dust.
Good Luck
Tek

System2 · April 2006

Re: Solar Panel questions

Thanks much for your replies ...

From what my teammate has calculated, the kart should move ( at a crawl and with a push from us to accerate ) with 100W supplied power.

I had a question though.... As an alternative, I was thinking of getting a 110W 12V solar panel and connecting it in series with a 2.5W 12V panel to get 24V. However, it then stuck me that the current supplied will be only .208 amps ( from the smaller pannel ) and not the 7 amps from the 110W panel which will suck. Hence, I did some research and realized that 12V to 24V converters exist. Do you guys think that this would work:

http://www.gearandgadgets.net/store/product_info.php?products_id=13506

Just as an aside you may want to investigate recovering some momentum for additional battery charging. I know you are running out of time but any thing that has to stop has power to charge batteries. It doesn't all have to turn into brake dust.

Can you please elaborate? What do you mean by recovering momentm for additional battery charging?

Thanks much.

System2 · April 2006

Re: Solar Panel questions

Greetings,
The hybrid cars of today that work are multidimensional. They use battery power with gasoline backup and the recovery of momentum to charge batteries while stopping and alternator charging while running on gas. The problem with the project as I see it is that your school is requiring you to throw in solar panels and the system that allows them to work and kick out all other invention making it one dimensional. This will work on the space station where they have unlimited space and weightlessness. Us earth people have to put up with inertia, momentum, air resistance and the resistance in electrical wiring, motor loss, inverter charger losses and a host of other problems that don’t present much of a problem in a weightless vacuum situation.
A number of years back my son built an electrostatic generator and entered it in the county science fair. He and his project made front page news in the paper. No one else was there. Thing about it is the darned thing didn’t even work. The reporter said he didn’t care because the rest of the projects were boring and my kid’s project was the only one that looked exciting.
Regarding your project you might want to check with your local VW dealer or one of the others. They have the solar panels that mount on the windshield with suction cups to keep the battery charged until the car is delivered. They usually throw these in a pile until they have enough to box-up and send back to the factory. Might be they would loan you guys 4 or 6 of them for your project and return them when finished with the project. All you need is lighter adaptors to hook them up. You could mount them on a spoiler as suggested in an earlier post. Heck, I wouldn’t get involved in whether the things have enough power to charge the batteries. Just make sure you have enough batteries to run the car for 30 minutes. You might be able to borrow some deep cycles from a local auto parts store. You might want to put a small team on the momentum recovery part. I would suggest a very small alternator or dc charger that is only turning when braking. In other words it would use the power of breaking to turn the charger. It would disengage when accelerating. Engage when stopping. You could use a micro switch on the accelerator pedal. That would be cool.
Here’s the thing, you need to get the brain power in your class divided into teams working on the different parts of this project. Get on the internet and find out what is possible.
When I was working on my electronics degree I had a guy on my team that had the lowest grade point average in the school. He had barely enough to stay on the roster. Thing about it is that he could write one great report. Couldn’t build a circuit worth a hoot. When we did our final project I did the directing, another member did the building, etc and he wrote the report. It was a pleasure for me to put my name on a report written by the guy with the lowest grade point average in the school. We made a 100% on the project and even got extra credit. What you need to do is find out what strengths of your buddies are and put them to work. Don’t try to do it all yourself. Someone once said that necessity is the mother of invention. Today, great leadership is the mother of invention. Get those people to build a car!!
Tek

System2 · April 2006

Re: Solar Panel questions

Shall definately look into it although am really short of time.

... Need to fill in order forms,etc tomorrow if we really want to be able to build it .....

We had divided ourselves into three groups: Power, drivetrain ( including the motor ) and frame. Am in the first so pretty much my responsibilty to figure out all of this stuff....

I personally have never dealt with any such project before and I'm pretty sure I'll have no idea what to do when it comes to actually physically building the thing and powering it. I'll probably be the guy who "cant build a circuit worth a hoot" but I guess its all a learning experience.... Although, I've learned more about group dynamics than physicws in this project

).

Would you be able to commnet on the DC converter in my previous post? You think that would work?

Also, from a recharging the battery standpoint, how would it physically work? Could I just connect my solar panel to a diode and then to a battery and It will charge or am I missing something here?

BB. · April 2006

Re: Solar Panel questions

That converter you linked to changes 24VDC to 12VDC (24v truck battery to 12v for a little koolmate electrically cooled ice box).

This will not work for you.

A simple solution for you to create, what appears to be a sort of working solar car, would be for your team to build a 24vdc battery/drive-motor car. And for the solar panels, get two approximately 55 watt solar panels and hook them in series through a diode AND FUSE to the 24vdc battery pair.

The pair of 55 watt 12 volt panels will be approximately the same size and cost of one 110 watt 12 vdc panel.

You can also see if you can find a 110 watt 24vdc panel and get the same effect... Assuming you can find any of the above combinations, all the panels should be approximately the same price (usually charged by dollars per watt).

-Bill

System2 · April 2006

Re: Solar Panel questions

Greetings,
BB is on the right track from my point of view. You also need to keep this thing light as a feather. I would make a rail job out of aluminum angle. Perhaps some 1/4 plywood for the spoiler, which should support most any panel. Definitely want to stay away from steel bar and heavy wood. If you need some aerodynamics on the front, you might be able to use some of that thin aluminum insulation that Lowes sells. You can put a furing strip along the base and just staple it in place. Point is not to have much weight and let the lightest person you have drive for the demo. I would if at all possible have some kind of charging system, if for nothing else, to make it look more techy and allow for the fact that the panel isn't going to get it. Come to think of it Lowes sell some angle in light steel and also aluminum. They also have it in flat pices so you can tie the rails together. The light steel flats with the holes all the way down will form easily. You should have no problem getting the body shape you need. Just some thoughts.
I was a certified master truck mechanic and certified master auto mechanic for 30 years before I went into the computer business, so if I can think of anything else I give you a holler.
Tek

System2 · April 2006

Re: Solar Panel questions

And for the solar panels, get two approximately 55 watt solar panels and hook them in series through a diode AND FUSE to the 24vdc battery pair.

The pair of 55 watt 12 volt panels will be approximately the same size and cost of one 110 watt 12 vdc panel.

Prob is that if its a 55W and 12V panel, then I will get only 4.58 Amps.. Also from a price standpoint, checked for a 55W panel on sunelec.com and found one by Matrix for $260 which will total $520 for 2 without shipping.. Tht will be slightly over budget... How does having two 55W ,12V panels solve the problem though? Is it becaues they both have the same current rating and hence will supply 4.58 A?

Greetings,
BB is on the right track from my point of view. You also need to keep this thing light as a feather. I would make a rail job out of aluminum angle. Perhaps some 1/4 plywood for the spoiler, which should support most any panel. Definitely want to stay away from steel bar and heavy wood. If you need some aerodynamics on the front, you might be able to use some of that thin aluminum insulation that Lowes sells. You can put a furing strip along the base and just staple it in place. Point is not to have much weight and let the lightest person you have drive for the demo. I would if at all possible have some kind of charging system, if for nothing else, to make it look more techy and allow for the fact that the panel isn't going to get it. Come to think of it Lowes sell some angle in light steel and also aluminum. They also have it in flat pices so you can tie the rails together. The light steel flats with the holes all the way down will form easily. You should have no problem getting the body shape you need. Just some thoughts.
I was a certified master truck mechanic and certified master auto mechanic for 30 years before I went into the computer business, so if I can think of anything else I give you a holler.
Tek

Thanks.. Shall let my frame teammates know about your recommendations...

System2 · April 2006

Re: Solar Panel questions

I'm near Houston, Tx. and if you're close I may be willing to loan you one of my Sharp 185's. They're high voltage though but I have an MX-60 as well that could help. As you know they're pretty valuable these days so I'd want to keep a close eye on them. If not close, maybe one of the other forum ladies/gents closer to you would step up.

I'd build the cart to fit the loaner (if this works out), test drive on the battery, have a few test sessions with the loaner panel and then the real time demonstration. When/where/how long is the exibition, how many test sessions do you predict you will need prior to exibit? Forget the regenerative braking, too costly and too complex in this short time frame. Forget the titanium muffler bearings, lighter weight makes for more speed but requires cubic $$$/mph gained at the speeds your cart will run. If you have hills to climb then lightweight alloys might benefit you but only if you are at the limits of performance. Steel is plenty strong, light and inexpensive if applied correctly. On something as small as a cart the frame might weigh ten pounds with aluminum or twenty with steel. Make up a project schedule and post here, it will be helpful to all concerned.

Cheers,

Bad Apple

BB. · April 2006

Re: Solar Panel questions

Let me see if this answers your question...

First, a couple of equations for power:

P=Volts x Current
P=Current^2 * R
P=(Volts^2)/R

So, whether you have, for example, one 12 volt solar panel at 10 amps (120 watts) or two 12 volt panels at 5 amps (each 60 watts), when added together (in series) you get 24 volts at 5 amps (or still 120 watts).

The objective, in this case, is to get 24 VDC to charge the batteries (and, by extension, run the 24 vdc motor) by using two 1/2 size 12 volt (and therefore half cost) solar panels connected in series for 24 vdc.

Think of the two solar panels as being like batteries. If you have them connected in parallel, they should have the same voltage rating (you can't put a 6 volt and 12 volt battery in parallel and expect things to work correctly).

If you connect to batteries in series, the voltages simply add up--however, the current capacity (and charge level) much match. For example, you can put a 12 volt car battery in series with a 12 volt truck battery... However, as you use up the energy the smaller car battery will go dead first.

Just like you were concerned earlier when you had two 12 volt panels, one high current and one low current... Putting them in series would leave you with only the current of the smaller panel (or even a ruined panel).

There are other things that could have been done--but given the time available to you, the costs, complexity, and the danger of hurting yourself and/or others with the (relatively) high current of a typical lead acid storage battery--you are much better off keeping things simple. Frankly some of the other solutions (such as using a 12v to 24vdc boost converter) would be best avoided for younger folks.

I would suggest that you get some help from an electrician or electrical engineer (perhaps a local college can give some help) to ensure that your system is safe (if you are doing much more than an ON/OFF switch). Even simple circuits can do thinks that nobody expected if the wrong components are used, or wire insulation is damaged during assembly.

Believe it or not, you may be in more danger from burns with a pair of 12 vdc car batteries than if you were working with 120VAC wall plug voltage. DC current is more likely to sustain an arc (once started) than AC. So fuses, switches, and such must be rated for the DC currents and Voltages you will be using. A car battery shorted (with no fuses) can give 50x, or more current, than a shorted AC wall outlet.

Do not work around the batteries/energized circuits while wearing Rings, necklaces, jewelry, or use any metal tools in the area. With high current DC it is very easy to "spot weld" tool/wire/jewelry together and cook you finger. It is also possible to weld switch contacts together (if you use the wrong rating switch) and get somebody tangled in the motor drive/etc.

Also, charging Lead Acid batteries generate hydrogen gas--a small explosion (from a sparking tool or wire) can give you burns, shrapnel wounds, and very bad Sulfuric Acid Burns.

You really need to have somebody watching your project and giving informed advise for your project. There are just too many variables and unknowns for someone like me to give you advice over a web site and ensure that you guys are safe.

Wazzup, you did not tell us your age or your experience with electricity, motors, and such. Its fine if you are still young--we all had to start somewhere (high school project?)--but you really need an experienced shop teacher or other person to help.

If you are still using the small motorcycle batteries--they do not have the capacity to cause damage as a car battery (or larger) would--so I feel a bit safer there.

You may be aware of all of the above issues when working around batteries and electric motors--but because I don't know you--I want to make sure that I am very clear about the warnings. I am sorry if they may be redundant for you--just rather be safe than sorry.

-Bill

System2 · April 2006

Re: Solar Panel questions

Greetings,
With regard to the above BB statement on precautions. You should also keep a gallon jug of water handy. I have been to the hospital twice in my auto career due to exploding batteries. Both times no harm was done because I flooded my eyes with water. Once in a tire air leak test vat. Now that was nasty but it worked.
Regarding the momentum recovery idea. I thought of this idea. Get a small alternator. Loaner from a rebuild shop would work. They get their stuff from their junk pile anyway. It should be a alternator without the internal regulator. They could take the regulator out and hang you a field wire through the housing. Put a micro switch on the accel pedal with both NO and NC terminals. Put a continuous duty seloniod on the drive motor. Connect dc to the mirco switch via a on/off switch. Wire the NC terminal of the micro switch to the field of the alternator and the NO terminal to the continuous duty seloniod. This is how it works. When you press the accel pedal the NO closes and the motor drives the car while the NC opens to eliminate field current to the alternator. When you release the pedal the NO opens stopping the drive motor and the NC closes once again putting current to the field of the alternator which would energize same and allow it to charge the batteries while stopping. Because the alternator in charging mode would pull pretty good HP is should stop the car pretty fast and give the batteries enough charge to overcome the inertia of getting going again. I would be nice in addition if the alternator had some sort of magnetic clutch which would desengage it completely while driving to eliminate the drag of just the rotor turning with no charging. Every moving device adds to inertia and momentum. I would also have some other braking device no matter how simple. In my opinion you should heed BB's safty precautions. Batteries can be very dangerous. Were safty glasses also.
Hope this helps a bit. Good comments elsewhere in this post also.
Tek

BB. · April 2006

Re: Solar Panel questions

Regarding using an alternator for a dynamic brake--it will work great in larger/fast moving vehicles. In your case, with a light/slow moving cart--you probably will get pretty close to zero useful energy return.

Remember, one of the physics equations for energy:

E=(1/2)mv^2

If mass and/or (especially) velocity are low, the energy available to recover is also low.

-Bill

System2 · April 2006

Re: Solar Panel questions

Thanks,
BB, you are probably right. A lot would depend on the sprocket ratio. IE how fast the alternator is turning. However the object is really to use the stopping return power to charge the battery. I agree he will need a conventional break in addition to the rest.
Tek

System2 · April 2006

Re: Solar Panel questions

Greetings,
Also, one of the fun things about all of this is the fact that you have to be part electrician, part plumber, part scientist, part phycisist, part mechanic, part mathmatician, part internet genius, part electronics technician, .......................
Tek

System2 · April 2006

Re: Solar Panel questions

Greetings,
Just a thought on batteries. You may want to consider using 1 or 2 Exide Marine/Starting batteries for the car. They don't cost much (I bought some for $54 each) and they have a wopping 165VA in the Group 27 model. Two of these would weigh about 50lbs. Info on this site will tell you that a marine classified battery isn't totally suitable for solar projects but it does have the word Deep Cycle on the side of the case. The object is to get enough power to run the car for 30 minutes. Few people other than the brainiacs on this forum would know the difference between a marine deep cycle and a RV deep cycle. The Exide is a lead acid battery which will release more hydrogen gas when charging, so follow the safty precautions above. The hydrogen discharge is what makes batteries so dangerous. If you doubt this, just set a bank of charging vented batteries is welding shop for a while and see what will happen. Point is you will absolutely not get this car to run completely off the panel. If you could I would hide it until it clears the patent office because you guys are the new billionaires. The first hybrid cars were good for about sixty miles then they had to be recharged somewhere. Today with gasoline assist and energy recovery they don't need a recharge. In real terms you better have some batteries that will do the job. The dc motor techs on this site will probably do the math on this post and comment and it is welcome.
Tek

System2 · May 2006

Re: Solar Panel questions

Sorry for the late responses. Was busy with AP exams and such.

We will be building this in the presence of my Professor and in his classroom.. He is an experienced engineering so hopefully, there shouldnt be any probs in terms of satefy although I will exercise caution. However, your concern is much appreciated

With regards to exide batteries, found this: http://www.kingsolar.com/catalog/cat/batteries/exide.html & http://www.academy.com/index.php?page=content&target=products/outdoors/marine/batteries&selectedSKU=0105-00150-0326&start=0

Although I dont know how to intepret the amps i'd get out of the batteries listed on there.. What does "RC minutes @ 25 Amps: 95" mean? Also, whats the difference between cranking amps and cold cranking amps? The second link has a battery which has 730 marine cranking amps.. Does that mean it supplies 730amps during its lifetime? However, how do I put it into perspective for my project?

Greetings,
With regard to the above BB statement on precautions. You should also keep a gallon jug of water handy. I have been to the hospital twice in my auto career due to exploding batteries. ...........................
s elsewhere in this post also.
Tek

However, wouldnt the inertia be relatively low since the cart wont be moving that fast? Plus for a teenager such as myself, what you explained sounds really complicated

).

Our group was discussing starting up and accellerating the cart using a battery ( which would supply approx 6 amps minimum ) , then via a manual switch, power it by a solar panel when the kart is moving at a constant velocity.. This way, we eliminate the need of getting a high powered solar panel and can make do with two 30/40W solar panels in series.? Would this work?

Thanks much you guys. Really appreciate the enthusiasm

BB. · May 2006

Re: Solar Panel questions

In Order, if I understand the questions correctly...

Battery Question... Make sure you understand the difference between Amps, Volts, and Enery/work....

RC Minutes--don't know what “RC” means, but it appears that this battery is rated to supply 12 vdc (probably ends at near 10 vdc) 25 amps for 95 minutes (1 hour, 35 minutes). This is the ability of the battery to supply a known amount of work (usually Watt*Hours) at a specified rate. I.E., if you discharge the battery faster, internal resistance and chemical efficiencies will usually give you less overall work...

Work (watt hours) = 12vdc * 25amps * 95min/(60 min/hr) = 475 watt hours (V*I*Time)

Basically, this battery can supply roughly 1/2 horse power for 95 minutes before the battery is flat (and damaged). If you discharged the battery slower you will find you can get more watt hours out of it. If you discharge it faster, less overall work.

Regarding cold cranking amps and marine cranking amps, you can probably Google the exact answer--but basically, they are many methods for rating how much current (and energy) you can pull from the battery (usually at a particular temperature and voltage drop).

In general, car type batteries are designed to be light and give very high current for a short time (typically only 20% of battery storage capacity is used). If you took a car battery and used it as a "deep cycle" battery and drew current down to 50% or more of capacity often, the car battery will fail very quickly (probably in month(s) or less).

A deep cycle battery is generally assumed to supply lights and other electrical items for a period of time with the engine turned off (such as at anchor on a boat). So, you can cycle the battery down to 50% of capacity (or sometimes take even more energy) many hundreds of times and have the battery last for 7-15 years.

A marine battery is usually a compromise between a starting battery (high current, short time) and a storage/deep cycle battery (low current, deeper discharge).

And, 730 amps is not a "life time" rating... It is a rating of how much current the battery can supply for a short time. Not really very interesting for you. You only want to draw 10-40 amps, so you are interested in the other number which is a constant discharge at a particular current level (in this case 25 amps for 95 minutes).

Remember, the equations for power is P=Voltage * Current. And work is W= Power * Time = I*V*Hours

If this was a project that you intended to use for several years (like driving around the neighborhood, you would want to pick a Deep Cycle type battery and definitely stay away from a Car type battery. A cheap marine battery may work OK for you--but it will probably not last as long as a true Deep Cycle battery).

Asking about low inertia???? Think of it this way. There are two basic equalities for motion that allow you to make calculations. Conservation of Energy and Conservation of momentum.

Conservation of Momentum is when you throw a tomato against a brick hanging on a string. Ma*Va=Mb*Vb -- in other words, the number represented by the motion (a) of the tomato (say 1 lb moving at 10 mph you can figure out the tomato smashing into a brick on a string by equaling the Mass (b) of the Brick + Tomato times the velocity of both... This is a very handy way of calculating bullet velocities and other things where something gets smashed.

Does not apply to two pool balls hitting each other or, hopefully driving your cart. That would be conservation of energy.

Conservation of energy... Assuming no loss due to friction and such, the formula is E=(1/2)MV^2 (E equals Work) ... So if your cart is in motion, if you know the mass of the cart+rider and its speed, you can figure out how much energy it will take to accelerate, for example, 100 kg from 0 to 20 kps. And, if you slow the cart down by using the motor as a generator, you could, in theory, take all of the energy it took to go from zero to 20 and putting it back into the battery by slowing from 20 to zero. However, if your mass and velocity is small, then the energy you would recover may be lost to friction and heating of the electrical circuits/motor/etc. Notice that Velocity is squared in this equation... So if you double velocity, it take 4x as much energy to get to that speed (or to slow down again). If you doubled the mass, it would only take twice the energy to accelerate to the same velocity.

Basically, in your case, because you are small/light and not going very fast, the amount of energy to be recovered is very small. If you were doing a lot of stop and go driving and/or going up/down hills, it could be worth it (that is what hybrid cars and electric cars do), but in your case it just is not worth the effort (you are going for low power because you want to solar power--your not going 60 mph, etc.)...

The thought of using the battery to accelerate to speed and then the panel(s) to maintain velocity is a possibility--but it is closer to a "marketing trick" than a useful demonstration (you can tell I am an engineer and not a marketing guy).

The down side of doing this... One is you can only go in a straight line as the solar array must point (within +/- 15 degrees) at the sun. If not, you will quickly lose power (and speed). If you had something tracking the sun, that would be interesting, but probably beyond what you where planning and, again, not really very interesting in real life. Plus, you still would be draining the cart's battery during starts so it will still need charging some time... Plus, if the solar panels can supply 60 watts for cruising, then looking at the battery above, it will be able to supply the same cruising current for:

Time = (1/2) * 475 Watt*hours / 60 watts = ~ 4 hours (this is to 50% discharge--very reasonable assumption).

So, basically, you can go straight line (no turns, no shade, no clouds) for 4 hours with solar panels (in full sun), or go anywhere for 4 hours on battery only power, then go back to a base station for a recharge.

The real world application is usually what people need to understand... In your case, demonstrating a solar powered charging station (panels, charge controller, couple of storage batteries) that can collect power efficiently all day long--And a battery vehicle that would "dock" at the charging station for a "quick" fill-up... Ideally, this would take a charge controller from the charging station batteries charging the battery on the go-cart (day or night capable charging). Variations of this charging scheme are what virtually all electric vehicles do today.

Hmmmm, I can think of a really neat demonstration to show the teacher (and others) that you really grasp the implications of the project.

Setup two demonstrations. One where you use a battery with cart mounted solar panels and show that you can get the cart moving on the battery, and may (or may not) be able to continue motion with the solar panels on the cart (maybe even drive the cart into a shaded area)--discuss the drawback such as need sun, aiming panels, wind drag of panels, fragile/expensive solution.

Second demonstration--"Push the cart from where it "died" in the "shaded area or during a turn" back to "home"". Move the panels from the cart to a "charging station". Show the panels charging the base station batteries. And show the cart driving around for hours at speed in sun/shade/etc... Then parking the cart for a quick recharge--and take off again. Discuss that the solar panels can charge on sunny days, electricity from an outlet can charge on dark days, and the cart can be charged "at work" or "on a trip" and still be useful...

I would think it can make a very compelling demonstration. Also, you can discuss advanced projects like "regenerative breaking", more efficient motors/batteries/electronics/etc...

There are lots of interesting points that can be discussed: Standard storage batteries vs AGM storage batteries, vs NICAD, vs Metal Hydride, vs fuel Cell, etc. Different types of motors. Issues with electronics. Body design vs aero dynamic drag (slow speed, little drag, high speed lots of drag--IIRC drag goes with the square of the velocity). Combining Solar Panels with Wind Power for better charging (sunny weather--no wind; stormy weather has wind and no sun; etc.)...

You guys can become real solar vehicle experts in a very short time by addressing some of the above suggested issues.

Please work with your math/physics teacher(s) to get some of the basic math down on electricity, motion, work, potential energy, etc… This will be very important for you guys to understand how all of this is related. You can use analogies to understand voltage and current vs water flow—but without understanding the equations/math, you can only go so far.

There are really only a handful of equations that you will need to understand for the basic operation of your cart. And it will be well worth your time to study them.

Sorry for the long post—I hope that it has been somewhat helpful (and I have not made too many mistakes—it has been a few decades since my physics classes).

Good Luck!
-Bill

crewzer · May 2006

Re: Solar Panel questions

What does "RC minutes @ 25 Amps: 95" mean?

RC is an abbreviation for Reserve Capacity. It defines how long (in minutes) a healthy and fully charged battery at 77 degrees F (25 C) can deliver 25 A until the battery voltage drops to 1.75 V per cell, or 10.5 V for a nominal 12 V (6 cell) battery, while under load. RC 95 means that the battery is rated to deliver 25 A for 95 minutes.

Some manufacturers cite 80 F as the BCI spec. See this link.

Also, whats the difference between cranking amps and cold cranking amps? The second link has a battery which has 730 marine cranking amps.

See these links:

http://www.ibsa.com/www_2001/content/faqs/tech_talk/terms/term_ca.htm
http://www.ibsa.com/www_2001/content/faqs/tech_talk/terms/term_cca.htm
http://www.ibsa.com/www_2001/content/faqs/tech_talk/terms/term_marine_amps.htm

However, how do I put it into perspective for my project?

CA, CCA and MCA are important specs for starting batteries, which are designed to provide short bursts of high current and not be deeply discharged, and are therefore built differently that deep cycle batteries, which are designed to provide long periods of relatively low current.

The key specs for deep cycle batteries are the voltage, the so-called “20 hour capacity” and perhaps the RC. The “20 hour” spec defines how many Amp-hours (Ah) can be supplied by a healthy and fully charged battery at 77 F when supplying a current that will require 20 hours for the battery voltage to drop to 1.75 V per cell, or 10.5 V for a nominal 12 V (6 cell) battery, while under load. A 20 hour rating of 105, common for a Group 27 size battery, means that the battery is rated to supply 5.25 A for 20 hours.

Note that the implied capacity of a battery drops as the output current increases (the "Peukert Effect"). A typical Group 27 battery specs are 105 Ah and RC 160. These mean that it can supply 5.25 A for 20 hours (105 Ah), but 25 A for just 160 minutes, which is 2.67 hours and implies a high-discharge current capacity of just 66.75 Ah.

HTH,
Jim / crewzer

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