Announcement

Collapse
No announcement yet.

Question about solar panel voltage near the same voltage as the battery

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Question about solar panel voltage near the same voltage as the battery

    Hi guys. I'm in the design process for adding solar panels to my rc plane. My current plan is to use 30 Sunpower cells in series which will put out ~100 watts (17v 5.8A). Here's the tricky part. I can't afford the weight penalty of a big heavy charge controller. So my plan is to use a lifepo4 battery (highly resistance to overcharge) which at fully charged will be 14.6v and match it with my solar panels which will be ~17v. Under load the battery can drop to as low as 12.8v which should give a nice voltage difference. During cruising it will probably be less though.

    So here's my question. Without me needing to buy the panels and test this, will this work the way I think it does? My plan is that even when the battery is fully charged while flying it won't overcharge itself to badly. In theory the battery could charge all the way up to the voltage of the solar panels and not damage the battery in any way other than longevity so I'm assuming that's worst case.

  • #2
    Using your scheme if the battery is fully charged, is connected to the solar panels and the plane electrics are turned off the battery voltage could get to 20.6V (VOC of the panels 0.687*30) or even higher if the temperature is < 25C. If you are using a 4 cell 12 volt LiFePO4 (LFP) battery this equates to ~5.2V/cell which is much higher that the maximum operating voltage of 4.0V/cell!

    You could get around this problem if your battery has a BMS installed that will disconnect the battery if the charging voltage goes too high. It would also protect your battery from being damaged by over discharge and keep it balanced.

    Using 25 cells would give you roughly the same amount of power as from 30 cells unless you use an MPPT controller.

    As weight is critical, I am surprised that you aren't using a LIPO battery or similar as they have much higher energy and power to weight ratios than an LFP battery.

    How much power does your plane use when flying?

    What size battery were you going to use?

    Simon

    Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
    BMS - Homemade Battery logger github.com/simat/BatteryMonitor
    Latronics 4kW Inverter, homemade MPPT controller
    Last edited by karrak; 02-07-2017, 11:55 AM.
    Off-Grid LFP(LiFePO4) system since April 2013

    Comment


    • #3
      >Using 25 cells would give you roughly the same amount of power as from 30 cells unless you use an MPPT controller.

      I could just stick 5 of the cells in parallel if need be. The .687 is only if there is no load which will be a very short period of time which I'm not worried about. They should be around .57v a piece when flying maybe lower. Looking at it a bit closer I would do 2 rows of 16 cells (16 on each wing, 85"ish wingspan) so 32 total would make the most sense.

      >As weight is critical, I am surprised that you aren't using a LIPO battery or similar as they have much higher energy and power to weight ratios than an LFP battery.

      The battery is mostly just a way to smooth out the voltage of the solar panels. It will be much smaller than what I normally run. When flying normally I use a 4s4p of NCR18650B's which gives ~1hr flight time. I'm estimating it uses around 80 watts when cruising.

      >What size battery were you going to use?
      Something as small as possible. a 4s1p of NCR18650GA's probably which would be 3500mah.

      I'm open to a controller of some kind as long as it doesn't weigh a lot, but I couldn't find anything that was less than 100 grams. If it's going to work properly, I'd like to just match the solar panel voltage up to the battery voltage and call it a day. My next best alternative would be an electronic switch where I have to manually monitor the battery voltage and flip a switch on my transmitter accordingly.

      Comment


      • #4
        Originally posted by scotty562 View Post
        >Using 25 cells would give you roughly the same amount of power as from 30 cells unless you use an MPPT controller.

        I could just stick 5 of the cells in parallel if need be. The .687 is only if there is no load which will be a very short period of time which I'm not worried about. They should be around .57v a piece when flying maybe lower. Looking at it a bit closer I would do 2 rows of 16 cells (16 on each wing, 85"ish wingspan) so 32 total would make the most sense.

        >As weight is critical, I am surprised that you aren't using a LIPO battery or similar as they have much higher energy and power to weight ratios than an LFP battery.

        The battery is mostly just a way to smooth out the voltage of the solar panels. It will be much smaller than what I normally run. When flying normally I use a 4s4p of NCR18650B's which gives ~1hr flight time. I'm estimating it uses around 80 watts when cruising.

        >What size battery were you going to use?
        Something as small as possible. a 4s1p of NCR18650GA's probably which would be 3500mah.

        I'm open to a controller of some kind as long as it doesn't weigh a lot, but I couldn't find anything that was less than 100 grams. If it's going to work properly, I'd like to just match the solar panel voltage up to the battery voltage and call it a day. My next best alternative would be an electronic switch where I have to manually monitor the battery voltage and flip a switch on my transmitter accordingly.
        Have you checked out the RCGroups forum. I remember seeing some threads concerning people that have used solar cells to extend the flight time of their gliders.

        I do not remember what type of battery they used or how many solar cells they mounted but I do remember the plane had a large wing span.

        Comment


        • #5
          I did post over there, but I'm not finding what I need to know over there. They are more interested in debating whether or not it's possible to fly on solar power.

          Comment


          • #6
            Originally posted by scotty562 View Post
            I did post over there, but I'm not finding what I need to know over there. They are more interested in debating whether or not it's possible to fly on solar power.
            Haha. They are still arguing that action.

            Well I can see their point because you are running into the same problem which is how do you charge a Lithium chemistry battery without a BMS system.

            Based on what I remember it was a dice throw because you might end up over charging the battery or not charging at all.

            I think someone used one LiPo battery to power the prop to get height. Then a second battery (can't remember the voltage or type but was not Lithium chemistry) which was connected to the solar cells for charging, was used just to control the trim surface and rudder servos. Since it was a glider the idea was to use the solar cells to keep the control battery happy but not the main power motor.

            Comment


            • #7
              Originally posted by scotty562 View Post
              Hi guys. I'm in the design process for adding solar panels to my rc plane. My current plan is to use 30 Sunpower cells in series which will put out ~100 watts (17v 5.8A). Here's the tricky part. I can't afford the weight penalty of a big heavy charge controller. So my plan is to use a lifepo4 battery (highly resistance to overcharge) which at fully charged will be 14.6v and match it with my solar panels which will be ~17v.
              First off, use LiCo and not LiFePO4. They are much lighter per watt-hour.

              Options:
              Genasun GV-5 MPPT controller - 80 grams and will give you 16.8 volts to charge a 4S lithium ion battery - and do MPPT
              Roll your own controller (hard but can be super light)
              Do a linear regulator (less efficient but easy)

              Notes:
              You might want to check out Alta Devices cells. 30%+ efficient, very light and flexible. Very expensive of course.

              Comment


              • #8
                Originally posted by jflorey2 View Post
                First off, use LiCo and not LiFePO4. They are much lighter per watt-hour.
                Wrong answer, he does not want to use either. You are out of your area of expertise.

                Both are inferior to to RC LiPo. Similar to LiCo but on Steroids with higher density and unstable. RC LiPo has the highest energy density of all the Lithium batteries and the only one that can actually be discharged at 20 to 100C. LiFeP04 density is less than 100 wh/Kg with a C-Rate of 3 to 5 C, LiCo is 150ish wh/Kg and very low C-Rates of less than 5. LiPo approaches 250 wh/Kg and C-Rates up to 100C. No contest LiPo wins and why they are used in RC Planes. It is the only battery that can do the job. Only other type is NiCd which has poor energy density so very short fly times. I have 7 planes, 3 of which are 3D and around 200 batteries. My Addiction XL uses two batteries. One LiPo for the motor, and one LiFeP04 for the RX/Servos

                To the OP you do not need to worry about a charge controller in this application. LiPo Batteries are cheap so if you should happen to over charge one no big deal, and it is almost impossible to do with Solar. You would have to leave the plane outside turned on for a day or more in the sun with the battery in it. No one in their right mind does that as it is a huge Safety No-No.

                If you use 3S your target Vmp voltage is 12.5 to 13 volts, 4S is 16.6 to 17.3 volts. Any higher and you are just throwing power away and adding dead weight. But you have another issue. 100 watts of panels into a 3S battery is 8 amps. LiPo's can be charged fast up to 3C, with 1C being normal. If you go trying to dump 8 amps into say a 2200 mah battery, you are going to puff the battery, destroy it in short time, or catch fire. At 3S with 100 watts you need to be talking a 4000 to 8000 mah battery. Otherwise you are in trouble with physics, and physics does not give a damn what you want.

                You are running into the physics road block and why electric planes are not commercially doable. Flight flight times are limited to minutes and not hours. 1 hour charge gets you 5-10 minutes fly time. You charge at 1C, and discharge at 20 to 50 C. The math does not allow for improvement for RC planes. The weight required for 100 watt of Poly cells takes 500 watts to fly not including the plane and motor weight.

                FWIW ignore Karak, he is a fraud and a liar and has been banned here multiple times. I have no idea why they do not make it permanent. He is dangerous. .
                Last edited by Sunking; 02-07-2017, 04:17 PM.
                MSEE, PE

                Comment


                • #9
                  Originally posted by Sunking View Post
                  Wrong answer, he does not want to use either. You are out of your area of expertise. Both are inferior to to RC LiPo. Similar to LiCo but on Steroids with higher density and unstable.
                  Lithium cobalt refers to the CATHODE. Specifically, lithium cobalt oxide. LiCo cells are the highest capacity cells out there. They are also not the most stable. When you want the highest capacity per pound, you use LiCo.

                  Lithium polymer refers to the ELECTROLYTE. The organic electrolyte is replaced with a solid polymer electrolyte, usually something like poly/ethylene oxide. When you want odd form factors or pouch-enclosed cells (which are a bit lighter than metal-case cells) you go with LiPo.

                  Most lithium polymer cells are LiCo cathode for energy density. If you are buying high energy density polymer cells for RC, you are almost guaranteed to be getting a LiCo battery.

                  You should know this.

                  Comment


                  • #10
                    Originally posted by jflorey2 View Post
                    Lithium cobalt refers to the CATHODE. Specifically, lithium cobalt oxide. LiCo cells are the highest capacity cells out there. They are also not the most stable. When you want the highest capacity per pound, you use LiCo.

                    Lithium polymer refers to the ELECTROLYTE. The organic electrolyte is replaced with a solid polymer electrolyte, usually something like poly/ethylene oxide. When you want odd form factors or pouch-enclosed cells (which are a bit lighter than metal-case cells) you go with LiPo.

                    Most lithium polymer cells are LiCo cathode for energy density. If you are buying high energy density polymer cells for RC, you are almost guaranteed to be getting a LiCo battery.

                    You should know this.
                    You are preaching to the choir.

                    Lithium polymer refers to the ELECTROLYTE. The organic electrolyte is replaced with a solid polymer electrolyte, usually something like poly/ethylene oxide.
                    Dead wrong. In a LiPo it is a gel with a porous polymer separator, not a solid polymer. Do your homework. The combination of a gel electrolyte, LiCo cathode, and a coffee bag pouch makes them lighter and hotter raising the voltage to 3.7 to 3.9 volts. Traditional LiPo you speak of have no commercial application as the Ri is way to high to be of any use.

                    Shoot one with a 22 or nail gun. You will see the purple gel come shooting out with fire and smoke.
                    Last edited by Sunking; 02-07-2017, 04:49 PM.
                    MSEE, PE

                    Comment


                    • #11
                      Ok I missed a bit so I'm going to do some quotes here.

                      >First off, use LiCo and not LiFePO4. They are much lighter per watt-hour.

                      The energy density is irrelevant for me. For normal flying yes, I use lithium ion batteries. I'd fly a lead acid battery if I could overcharge it without any problems with my solar scenario.

                      >Genasun GV-5 MPPT controller - 80 grams and will give you 16.8 volts to charge a 4S lithium ion battery - and do MPPT

                      It can only do 5A which isn't enough. They have a 10A version, but it's 185g. I could probably take the plastic off and save a bunch of weight, but I'd rather not butcher a $159 piece of tech. I'd love to find something like this that works for me though.

                      >Roll your own controller (hard but can be super light)
                      Like an arduino with an electronic switch? Should be less than 50 grams.

                      >Do a linear regulator (less efficient but easy)

                      I looked into this, but I don't know enough about it. All of the regulators I could find were for 1.5A or less. How would I go about this? I'm sure this is super basic stuff, but I just don't know .

                      >You might want to check out Alta Devices cells. 30%+ efficient, very light and flexible. Very expensive of course.

                      Solar cells you mean? Hard to beat Sunpower's C60 for $2.39 per cell.




                      >Both are inferior to to RC LiPo. Similar to LiCo but on Steroids with higher density and unstable. RC LiPo has the highest energy density of all the Lithium batteries and the only one that can actually be discharged at 20 to 100C. LiFeP04 density is less than 100 wh/Kg with a C-Rate of 3 to 5 C, LiCo is 150ish wh/Kg and very low C-Rates of less than 5. LiPo approaches 250 wh/Kg and C-Rates up to 100C. No contest LiPo wins and why they are used in RC Planes. It is the only battery that can do the job. Only other type is NiCd which has poor energy density so very short fly times. I have 7 planes, 3 of which are 3D and around 200 batteries. My Addiction XL uses two batteries. One LiPo for the motor, and one LiFeP04 for the RX/Servos

                      Yes Lipo has the highest discharge rates of all the lithium batteries. For low C applications, li-ion is superior due to higher energy density. Won't be doing any 3D on them of course.

                      >To the OP you do not need to worry about a charge controller in this application. LiPo Batteries are cheap so if you should happen to over charge one no big deal, and it is almost impossible to do with Solar. You would have to leave the plane outside turned on for a day or more in the sun with the battery in it. No one in their right mind does that as it is a huge Safety No-No.

                      So if I aim for 16v of solar panels under load running lipos I should be good? No chance of over charging them, but when the voltage goes down the solar panels should charge the battery.

                      >If you go trying to dump 8 amps into say a 2200 mah battery, you are going to puff the battery, destroy it in short time, or catch fire. At 3S with 100 watts you need to be talking a 4000 to 8000 mah battery. Otherwise you are in trouble with physics, and physics does not give a damn what you want.

                      That's a good point. My battery will have to be chosen carefully. Just a quick look gives this guy that should be able to handle it (8C charge rating). https://hobbyking.com/en_us/turnigy-...lipo-pack.html.

                      >You are running into the physics road block and why electric planes are not commercially doable. Flight flight times are limited to minutes and not hours. 1 hour charge gets you 5-10 minutes fly time. You charge at 1C, and discharge at 20 to 50 C. The math does not allow for improvement for RC planes. The weight required for 100 watt of Poly cells takes 500 watts to fly not including the plane and motor weight.

                      It all depends how you like to fly. Just cruising around with no aerobatics and no payload whatsoever you can fly as long as the sun is shining. These guys are my inspiration for this project. https://www.youtube.com/watch?v=rMkPjBf6dNQ These kinds of plans only use 60-100w of power to cruise.





                      Comment


                      • #12
                        Originally posted by jflorey2 View Post
                        First off, use LiCo and not LiFePO4. They are much lighter per watt-hour.
                        The energy density is irrelevant for me. For normal flying yes, I use lithium ion batteries. I'd fly a lead acid battery if I could overcharge it without any problems with my solar scenario.

                        Originally posted by jflorey2 View Post
                        Options:
                        Genasun GV-5 MPPT controller - 80 grams and will give you 16.8 volts to charge a 4S lithium ion battery - and do MPPT
                        Roll your own controller (hard but can be super light)
                        Do a linear regulator (less efficient but easy)
                        Genasun GV-5 MPPT can only do 5A which isn't enough. They have a 10A version, but it's 185g. I could probably take the plastic off and save a bunch of weight, but I'd rather not butcher a $159 piece of tech. I'd love to find something like this that works for me though.

                        I looked into linear regulators, but I don't know enough about them. All of the regulators I could find were for 1.5A or less. How would I go about this? I'm sure this is super basic stuff, but I just don't know .


                        Originally posted by jflorey2 View Post
                        Roll your own controller
                        Like an arduino with an electronic switch? Should be less than 50 grams.


                        Originally posted by jflorey2 View Post
                        Notes:
                        You might want to check out Alta Devices cells. 30%+ efficient, very light and flexible. Very expensive of course.

                        Solar cells you mean? Hard to beat Sunpower's C60 for $2.39 per cell.
                        Last edited by scotty562; 02-07-2017, 05:18 PM.

                        Comment


                        • #13
                          Originally posted by Sunking View Post
                          Wrong answer, he does not want to use either. You are out of your area of expertise.

                          Both are inferior to to RC LiPo. Similar to LiCo but on Steroids with higher density and unstable. RC LiPo has the highest energy density of all the Lithium batteries and the only one that can actually be discharged at 20 to 100C. LiFeP04 density is less than 100 wh/Kg with a C-Rate of 3 to 5 C, LiCo is 150ish wh/Kg and very low C-Rates of less than 5. LiPo approaches 250 wh/Kg and C-Rates up to 100C. No contest LiPo wins and why they are used in RC Planes. It is the only battery that can do the job. Only other type is NiCd which has poor energy density so very short fly times. I have 7 planes, 3 of which are 3D and around 200 batteries. My Addiction XL uses two batteries. One LiPo for the motor, and one LiFeP04 for the RX/Servos
                          Yes Lipo has the highest discharge rates of all the lithium batteries. For low C applications, li-ion is superior due to higher energy density. Won't be doing any 3D on them of course.

                          Originally posted by Sunking View Post
                          To the OP you do not need to worry about a charge controller in this application. LiPo Batteries are cheap so if you should happen to over charge one no big deal, and it is almost impossible to do with Solar. You would have to leave the plane outside turned on for a day or more in the sun with the battery in it. No one in their right mind does that as it is a huge Safety No-No.
                          So if I aim for 16v of solar panels under load running lipos I should be good? No chance of over charging them, but when the voltage goes down the solar panels should charge the battery.

                          Originally posted by Sunking View Post
                          If you use 3S your target Vmp voltage is 12.5 to 13 volts, 4S is 16.6 to 17.3 volts. Any higher and you are just throwing power away and adding dead weight. But you have another issue. 100 watts of panels into a 3S battery is 8 amps. LiPo's can be charged fast up to 3C, with 1C being normal. If you go trying to dump 8 amps into say a 2200 mah battery, you are going to puff the battery, destroy it in short time, or catch fire. At 3S with 100 watts you need to be talking a 4000 to 8000 mah battery. Otherwise you are in trouble with physics, and physics does not give a damn what you want.
                          That's a good point. My battery will have to be chosen carefully. Just a quick look gives this guy that should be able to handle it (8C charge rating). https://hobbyking.com/en_us/turnigy-...lipo-pack.html.

                          Originally posted by Sunking View Post
                          You are running into the physics road block and why electric planes are not commercially doable. Flight flight times are limited to minutes and not hours. 1 hour charge gets you 5-10 minutes fly time. You charge at 1C, and discharge at 20 to 50 C. The math does not allow for improvement for RC planes. The weight required for 100 watt of Poly cells takes 500 watts to fly not including the plane and motor weight.
                          It all depends how you like to fly. Just cruising around with no aerobatics and no payload whatsoever you can fly as long as the sun is shining. These guys are my inspiration for this project. https://www.youtube.com/watch?v=rMkPjBf6dNQ These kinds of plans only use 60-100w of power to cruise.

                          Comment


                          • #14
                            Originally posted by Sunking View Post
                            Wrong answer, he does not want to use either. You are out of your area of expertise.

                            Both are inferior to to RC LiPo. Similar to LiCo but on Steroids with higher density and unstable.
                            Yes Lipo has the highest discharge rates of all the lithium batteries. For low C applications, li-ion is superior due to higher energy density. Won't be doing any 3D on them of course.

                            Originally posted by Sunking View Post
                            To the OP you do not need to worry about a charge controller in this application.
                            So if I aim for 16v of solar panels under load running lipos I should be good? No chance of over charging them, but when the voltage goes down the solar panels should charge the battery.

                            Originally posted by Sunking View Post
                            If you go trying to dump 8 amps into say a 2200 mah battery, you are going to puff the battery, destroy it in short time, or catch fire.
                            That's a good point. My battery will have to be chosen carefully. Just a quick look gives this guy that should be able to handle it (8C charge rating). https://hobbyking.com/en_us/turnigy-...lipo-pack.html.

                            Originally posted by Sunking View Post
                            You are running into the physics road block and why electric planes are not commercially doable.
                            It all depends how you like to fly. Just cruising around with no aerobatics and no payload whatsoever you can fly as long as the sun is shining. These guys are my inspiration for this project. https://www.youtube.com/watch?v=rMkPjBf6dNQ These kinds of plans only use 60-100w of power to cruise.

                            Comment


                            • #15
                              This is how the guys in my post above did their setup. Notice they have the solar panels dumping right into the esc. I thought having two separate power sources like that would cause problems. Why isn't it? If this is the case then the battery may see very little current as the motor will be sucking up most of it as the electricity is captured from the solar cells.
                              Attached Files

                              Comment

                              Working...
                              X