Hi all,

I am currently working on a project and it is almost done. I am making a rc airplane that is primarily battery powered, but solar cells are added to increase flight time. I am not going for any records or anything, will mainly be trying to fly during the best parts of the day and see how long I can go, maybe 2 hours (the plane will go about 1 hour as it is now). The plane is built and the cells will be wired up soon, I am just not sure what is the best way to connect the solar array to the battery. Right now it is battery->esc->motor. And so now I am thinking it will go like solar->mppt/charge regulator->battery->esc->motor. Much like Tony's diagram in the Solar Car discussion. I guess my concern is I will be using lithium-polymer batteries instead of the capacitors you guys were using on the cars. Is this a problem? I currently have a 3S4P lipo 8000mah 11.1v battery, the solar array will consist of 28 cells: Voc = .623V, Isc = 8.24A, Vmpp = .516V, Impp = 7.71A. These are factory specs so are of course in ideal conditions (1000W/m2 AM 1.5). I was looking at Genasun MPPT's (GV-9li) and they told me those would work for my application, but I guess I am not 100% convinced. Moreover, a GV-9li comes with a fairly steep price. I was thinking something no more and $100USD. Another plus is that I could get it ASAP since this is the last issue holding up my project.


DB

Hi DB,

This looks like an interesting project. Your right, solar->mppt/charge regulator->battery->esc->motor, I think is the best way for connecting the electronics. Just make sure you dont let your MPPT overcharge your batteries.

Now, also, you said your using 28 cells at .623volts. That will give you an open circuit voltage at 17.444volts. Now, the maximum power point of Grade A solar cells is usually @ 0.7 * Voc, so thats at 12.21 volts. When your Solar panels heat up (and they will, because your using naked cells), Your MPP will drop even more, maybe another 15% volts, to around 10.5 volts. You will need to make sure your MPPT can boost the voltage to get it the power into the batteries, as the battery voltage will be higher than your solar arrays maximum power point. Or you could add another 4 - 6 cells in series with the array to compensate. Just my thoughts.

Looking forward to seeing how you go. How about some pictures??

Cheers.

Tony,

I think you are right about the cells and voltage and probably would be better if we had more cells. Unfortunately 28 is all we have . The manufacture states that Vmpp is .516V which is about 83% of the Voc. I am hoping this the case though it may not be. Also, since the cells will be on the wing with air going over them I am again hoping that this will keep them cooler? I would think at least cooler than if you typically have them stationary. Also, I have not found an MPPT (perhaps yet) within my price range, do you know of something that will work? While a MPPT will certainly be more efficient, I do not need one per say if I cannot find one. I do, however, need something in between the solar array and the batteries, I am just not sure what I need. It doesn't need to be the most efficent device ever made, just something that will get the power from the solar array to the batteries - safely (i.e. without the battery igniting or exploding which lipos are known to do if you do not charge them correctly). I was thinking maybe something like this: http://www.batteryspace.com/index.asp?P ... rodID=4008. However, if my panel voltage is lower than the batteries, I guess I will need some sort of MPPT? Let me know what you think.

I'll try to get some pictures up by mid week.


Thanks,

DB

What sort of price are you prepared to pay for a MPPT??

Well right now it is looking like Genasun is the only place I know that makes a MPPT for li-po's. I have been searching for others to no avail. Their GV-9 (9A max) is $170 USD. I was looking for hopefully pay no more than $100 USD. I have been thinking I might just go over budget and maybe go with their GV-6 for $145. I am really trying to find another option though.


DB

For 99 USD i could sell you a one of my FATMAX MPPTs, i can program it to charge to a maximum of 12.6 volts on the output to protect your lipo's. What do you think?

fatmax-high-power-maximum-power-point-tracking-unit-t142.html

Tony,

Sorry for the somewhat late reply, I thought I replied when I read your latest response a couple days ago but I guess I just thought it and didn't type it out. Anyhow, I would have thought it would have been a great idea, if I would have read your response a few earlier in that day. It just so happens I worked something out with Genasun for about the same price at about the same time of your last post. Also, Genasun is located in the states so shipping and whatnot would be cheaper and less of a hassle. In fact, it should be here tomorrow or friday, so I am pretty excited to just get something and try it out. Thank you for the offer, though, I do appreciate it, I guess it is just a case of bad timing. Hopefully everything goes well, I am a little worried that the panel may not put out of enough voltage on a really hot day. Since I can not get more cells (plus the wing has already been sized to fit the 28) is there anything else I can do to ensure I high enough voltage? For the most part the plane will be flying level at a constant altitude. However, when we turn we might lose the sun for a second? Ii this alright? Does it just mean that for that second we won't be getting any power from the solar, only from the battery?


DB

DB_Flyer,

Can you post some pics of your plane and tell me a little more about it, like dimensions and what kind of cells you are using? Obviously power-to-weight is a huge deal for a plane. That's one of my main concerns with using an MPPT. Do you know how much the Genasun MPPT weighs, and are you planning to cut it down to save weight?

I'm working on this for my thesis, but I'm not nearly as far along as you.

Yeah, we are doing this for our senior design project as ME's (which is a little odd because it is more aero and electrical than anything else but I think we all have learned a lot). Our project is a little different because we didn't get to design our own plane. We were given a kit to build, then once we built that plane we were given a strict budget to work with to increase the autonomy of the plane (the budget is strict enough that not everything on the plane would be able to be changed). I will post some pictures when I get a chance. The MPPT is 6.5oz, which I think is fairly light as far as MPPt's are concerned. We are likely to take off the case to maybe save a couple ounces. The project will be done in a few weeks in which I will have more time to share what I have. Look up Sky Sailor, that should help you out some more.

DB

Hey Tony,

Quick question for you about something you mentioned earlier.

"This looks like an interesting project. Your right, solar->mppt/charge regulator->battery->esc->motor, I think is the best way for connecting the electronics. Just make sure you dont let your MPPT overcharge your batteries."

What exactly do you mean by make sure to not let the MPPT overcharge the battery? Isn't part of the MPPT's job is to make sure the voltage does not go above the float voltage, in my case about 12.6V? Also, as of right now, my battery connects into the ESC and that is the end of it. It connects using about 12 gauge wire. It also has another connector coming out of it - 4 fairly small wires (3 for each cell and 1 ground I presume). Can I use this connection to connect to the MPPT? just simply take off the connector, put the 3 wires to the positive terimal on the MPPT and the black/ground wire to the negative terminal?

Thanks,

DB

Can I use this connection to connect to the MPPT? just simply take off the connector, put the 3 wires to the positive terimal on the MPPT and the black/ground wire to the negative terminal?

No, you will short your battery and potentially cause a fire.
A maximum power point tracker makes you solar panel operate at it optimum operating point, they are not necessarily used to charge batteries, however some have built ion chargers.

Hi everybody!
I just discovered your forum! DB_Flyer proposed to look up Sky-Sailor, and I worked in fact 5 years building this airplane for my PhD Thesis. You could perhaps be interested to read the following report:
http://www.sky-sailor.ethz.ch/
http://www.sky-sailor.ethz.ch/docs/Thesis_Noth_2008.pdf
It contains a design methodology, but also lots of concrete details about the prototype we entirely built. Last year, on the 21st of June 2008, it flew 27hours only with solar energy and autonomously, with an homemade low power autopilot system.
Hope it helps,
Best regards,
ANoth

Hey anoth, im currently downloading your thesis, looks great.
BTW, where did you get your cells from?
Also, can we get a closer look at the electronics?

Well done and cheers,

Tony.

Hi Tony,
The solar cells we use were from RWE Space, now renamed Azur Space. It was the S32 type (16.9%, 0.32mg/cm2, 0.13mm thick) http://www.azurspace.com/index.php?page=11
However, they were extremeley expensive (50Euro for 74x32mm in 2004), but what I tried to present in the Thesis report is that the good design is as important, or even more important that the products themselves. So the sizing of each part (battery, solar cell area,...) is crucial.
Concerning the electronics, I can answer precise questions if you have some.
Best regards,
Anoth