# Volts or Amps:



## Joey (Oct 12, 2007)

I can't answer many of your questions, but here are a few things to think about. With a series string of solar panels (like in the battery pack), you are limited by the weakest cell in the string. So if one of the panels is not receiving the same amount of light (shadowing, orientation), you could end up producing less power in the entire string than if the panels were seperate. If you pair up the panels, make sure they are posisitoned to get the same incoming light.

For a given power, higher voltage means lower current (power = I*R) and lower current will mean less line losses. Power loss = I^2*R, where R is the resistance of the wire.


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## PStechPaul (May 1, 2012)

Generally, maximum power transfer occurs when the voltage drops to 50% of no load. So you would have 9.5 or 19 volts to the motor via the controller. This would be at 11A or 5.5A, for 104.5W. For the motor, it depends on whether you need higher torque or higher speed. A good PWM controller can provide the higher torque from the higher input voltage, so you can have 19V at 5.5A into the controller and 9.5V at 11A into the motor, or even 4.75V at 22A. There are efficiency consideration as well, but probably minimal at these power levels. Also, I assume that you have a capacitor acrosss the solar cells to minimize surges and ripple. 

I'd like to see more details about the components of the design and the requirements. If you are racing, it might be an advantage to store some energy at the start and then use it later at higher speeds, but that would be more like a drag race, and you may be doing something more like a long endurance run.


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## McRat (Jul 10, 2012)

Mostly "goofing off". I read the fastest a solar car has went has been 88kmh without battery assist.

At first, I was considering beating that number. When I realized how much money it would take, we put a budget on it. 1HP of solar panel is about $1500, we are running 1000w ($2000) worth. These are lightweight, semiflexible panels.

I'm a total newbie at ALL of this technology, but I'm learning bit by bit.

A solar panel makes virtual no voltage without resistance, ours make 0.2v at 5.5amps on a dead short. Open circuit is 21 volts. The max power is at 18-19 v with the right resistance to optimize the amps. Not sure what the specific resistance is, but the MPPT device is suppose to handle that automagically.

I don't think the voltage will "pull down" after the MPPT. That's what I'm getting ready to test at noon today. It will delivery it's voltage and amps like a power supply from what I can tell and what I read.

Now, when you have partial sunlight, the MPPT device will adjust the resistance, hence the voltage and amps to get the max watts out the panel.

We will only be running at max output. If we hit 57mph, we are the unofficial world's fastest solar vehicle. 

But stage one is building a "proof of concept" vehicle. Starting with a bicycle fork and wheels, "Can a human balance a 280lb, 25' long two-wheeler?" This is in the shop being welded up out of aluminum. Not carbon fiber and honeycomb like a real solar racer. Just the windtunnel time would cost several thousand dollars. Molds would cost several thousand. Etc. This is mostly .125" wall 2x2" sq aluminum tubing. Just over 1lb a foot, and pretty rigid (6061-T6, with rounded corners). 7075 would be better but is harder to weld correctly.

When we get the Proof of Concept vehicle done, I'll post a movie on YouTube and link it here. BUT... You must PROMISE to laugh at it. If you don't have a sense of humor, you are not permitted to watch it.

If the proof vehicle works, we mount the solar panels, and start tuning it. I have 1 MPPT for testing the cells, but need to decide whether to buy 4 more or 9 more. I'm thinking if we put 19v to a 24v motor we can't get over 500w output out of it. But if we run it at 38v, we might get 1 hp out of it? I have two of these motors.


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## Joey (Oct 12, 2007)

McRat said:


> Now, when you have partial sunlight, the MPPT device will adjust the resistance, hence the voltage and amps to get the max watts out the panel.


The MPPT device will find the R to apply to the panel for max power, but with partial shading, the max power will be much lower than without shade. Each solar cell in a series string with the shaded cell will also have reduced max power.


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## otp57 (Feb 7, 2012)

McRat said:


> This is a solar powered vehicle.
> 
> I can run the panels in either 19v x 5.5a x 10, or 38v x 5.5a x 5 feeding into a permanent magnet 24vdc 500w rated DC motor. In other words, 10 panels can be run in either 5s x 2p or 10s x 1p.
> 
> ...


10 panels would use up the power from the 500 watt motor just to push them? plus the motor could be asking for more current than the panal can put out. the solar are rated at max output and that is not the case in the real world. unless you are using this on flat ground only.


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## McRat (Jul 10, 2012)

Joey said:


> The MPPT device will find the R to apply to the panel for max power, but with partial shading, the max power will be much lower than without shade. Each solar cell in a series string with the shaded cell will also have reduced max power.


Perhaps I should find a 12v 1000w motor, and over-volt to 19v, and run individual MPPT's?

Partial shade is not a real killer, but if you completely block one of the 60 cells in the panel, it's output falls to virtually nothing. 100% block of one cell yields 10% of the rated output.

With 1.4HP and a full weight of about 400lb, it won't climb a steep hill. And it's so long with 12" of ground clearance, that level ground is it's home.


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## otp57 (Feb 7, 2012)

McRat said:


> Perhaps I should find a 12v 1000w motor, and over-volt to 19v, and run individual MPPT's?
> 
> Partial shade is not a real killer, but if you completely block one of the 60 cells in the panel, it's output falls to virtually nothing. 100% block of one cell yields 10% of the rated output.
> 
> With 1.4HP and a full weight of about 400lb, it won't climb a steep hill. And it's so long with 12" of ground clearance, that level ground is it's home.


Why not go with a 36v motor and use 3 12v battires ,controller etc and use 3 solar panel to show the solor concept.
You can go to my website www firstontop com can click on the electric motorcycle to see how I did it with soler panels I hope this helps


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## McRat (Jul 10, 2012)

otp57 said:


> Why not go with a 36v motor and use 3 12v battires ,controller etc and use 3 solar panel to show the solor concept.
> You can go to my website www firstontop com can click on the electric motorcycle to see how I did it with soler panels I hope this helps


Thanks for the link!

3 of my panels is only 315w, or less than 1/2 a HP. It won't get going fast.
I have 12 A123 LiFeO4 (12lb) to run as 3p4s which will be used for testing. A voltmeter is all the cell protection I will use. I also have 4 LiPo RC 22v volt packs that I could use. I have lead acid, but not a lot of room for 3 of them safely.

I made plywood sheets that have the same dimensions and weight as the panels. I mount the plywood, hook a speed controller to it with a circuit breaker/fuse, and leash (if I fall off, a relay disconnects the battery) and run the A123's at 1050w output. If I can balance it, then I fine tune the chassis, and see what the top speed is, then optimize.

Then I put the solar panels in place of the plywood, and let 'er rip, WITH the batteries on a relay and switch. If I can run it without the batteries, I remove them and aim at 90kmh with a push start. 1.4hp with no gearbox will make it difficult to have max speed and enough acceleration to get speed to balance it.

Question: How do I restrict the battery output to 1050w? Watt meter and throttle stop?

DOH!! I haven't looked for a 36v motor. I have 12 and 24 motors handy already (El Cheapo brand).


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## otp57 (Feb 7, 2012)

McRat said:


> Thanks for the link!
> 
> 3 of my panels is only 315w, or less than 1/2 a HP. It won't get going fast.
> I have 12 A123 LiFeO4 (12lb) to run as 3p4s which will be used for testing. A voltmeter is all the cell protection I will use. I also have 4 LiPo RC 22v volt packs that I could use. I have lead acid, but not a lot of room for 3 of them safely.
> ...


Most motors can be over volted, take your most powerful motor and overvolt if it is a 24v you can use 36v


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## PStechPaul (May 1, 2012)

I had to look up information on the characteristics of solar cells. Here are some sources:
http://www.nikhef.nl/~h73/kn1c/praktikum/phywe/LEP/Experim/4_1_09.pdf

So the characteristic is more like a battery and power is not determined by a fixed internal resistance. So, you will have an ideal load with an equivalent resistance for maximum power.

Since this load will be a motor controller, it has the ability to adjust the drive to the motor for maximum power, but that is based on the motor characteristics and the actual physical load. And it is based on the limitations of the controller. Also, the controller will have some capacitance on the input so the solar cells will just need to supply a steady DC current. I don't know if there is a limit to the size of this capacitance, but if large enough it might be used to advantage. 

And batteries would be even better, but if the object is to use only solar energy for the duration od the test, it might be "cheating" to precharge them even if charged solely by sunlight. Obviously, you could use huge batteries and charge them up for a week before the trial, and you could use a 10HP motor and you would definitely win, but someone else might use bigger batteries and charge for two weeks and use a 20HP motor.

So I would assume that you would need to start the trial with minimal capacitance or fully discharged batteries. Ideally you want to supply the full power of the solar cells to the motor, but you also want to store some energy in the capacitors or batteries. If the insolation remains constant and the track is smooth and level, and there is no variable wind issue, then maximum power at all times may make sense. 

If the objective is to achieve the fastest sustained speed, then acceleration factors are not applicable, and it simplifies the design. So I'll use that assumption for now. There should be a predictable and quantifiable power requirement for a given speed, based mostly on wind resistance. So let's assume you want to achieve 60MPH and that will take 1/8 HP or 100 W. That will translate to a certain amount of torque and RPM on the drive wheel. Let's say 600 RPM so torque is 
0.125*5252/600=1.09lbft​If you are using a direct drive hub motor, you need to see where this fits on the efficiency curve. Here is some information:
http://www.ebikes.ca/hubmotors.shtml

Here is their list of motors:
http://www.ebikes.ca/store/store_motors.php

And here is their motor simulator:
http://www.ebikes.ca/simulator/

Using that, with a 500W hub motor, I got this graph at a motor power of 101W:










So essentially you need to use something like this to determine the point of maximum efficiency and select or adjust the torque ratios to get the fastest performance. Obviously here the battery power is 124W so you'd have to adjust the parameters to get the 100W you have available.


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## McRat (Jul 10, 2012)

PStechPaul said:


> I had to look up information on the characteristics of solar cells. Here are some sources:
> http://www.nikhef.nl/~h73/kn1c/praktikum/phywe/LEP/Experim/4_1_09.pdf
> 
> So the characteristic is more like a battery and power is not determined by a fixed internal resistance. So, you will have an ideal load with an equivalent resistance for maximum power.
> ...


Thanks! My total available solar power is currently (pun intended) 1050w at 19vdc, or about 1.4 HP. I am running 10 panels in a 4' x 20' grid.


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## otp57 (Feb 7, 2012)

McRat said:


> Thanks! My total available solar power is currently (pun intended) 1050w at 19vdc, or about 1.4 HP. I am running 10 panels in a 4' x 20' grid.


10 panels take up a lot of space?
Your car will be very long?
I think one is on youtube .


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## McRat (Jul 10, 2012)

otp57 said:


> 10 panels take up a lot of space?
> Your car will be very long?
> I think one is on youtube .


Two wheeler, 8 meter wheelbase. LOOONNNGGGG ....

Yup YouTube, then "The World's Dumbest Drivers" most likely.

First tests involve stability.


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## McRat (Jul 10, 2012)

PSTechPaul?

Is there such a thing as a cheap electric motor dyno? When I race cars/trucks, I do 90% of the tuning on the dyno, then 10% at the track. It would be nice to do that with electrics, but 1.4HP isn't going to read on a car dyno, and it would probably be too long.


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## PStechPaul (May 1, 2012)

McRat said:


> PSTechPaul?
> 
> Is there such a thing as a cheap electric motor dyno? When I race cars/trucks, I do 90% of the tuning on the dyno, then 10% at the track. It would be nice to do that with electrics, but 1.4HP isn't going to read on a car dyno, and it would probably be too long.


I have a 2HP treadmill motor (about $50 on eBay) which I planned to use as a dynomometer. You can connect the output to electric heaters and measure the voltage and current for a good idea of the output power of the motor you are testing. Use a tach for the speed. And you can mount the motor on casters or large shaft bearings so it can rotate at least 30 degrees or so, and then use a torque arm and a fishing scale to read the torque.

For higher power you can get a generator head either from a junk generator for cheap, or from Harbor Freight. 
http://www.harborfreight.com/10000-watts-max-7200-watts-rated-belt-driven-generator-head-45416.html


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## McRat (Jul 10, 2012)

Paul? That is TRULY clever!!! I like it!

I was thinking water brake and flowmeter, but that would be much cheaper and easier.


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