Voltage Regulator

Discussion in 'The Projects Forum' started by DB_Flyer, Apr 3, 2009.

  1. DB_Flyer

    Thread Starter New Member

    Apr 3, 2009
    3
    0
    I’m currently working on a project where I am attaching solar panels to an RC Airplane. The plane has been designed/modified and the battery/solar panels decided on. Unfortunately, I’m a ME at a small school with no EE’s so I am looking for some help.

    My question or concern is with connecting the solar panels to the battery. The battery is a 11.1V 8000mah Li-po and the solar cells are rated at .623V open circuit with a maximum power current of 7.71A (another specification provided was the load current at 0.5V is 7.83 – 7.92A). The cells specs are under standard test conditions of 1000W / m2, AM 1.5. I was planning on having 24 cells wired in series…. But I am not sure if this will supply enough voltage? From my understanding li-po batteries have very specific charging characteristics and I would basically be charging the battery with the solar cells. The charging voltage has to be the same as the voltage of the battery and it can not be charged faster than 1C.

    Now, I choose the 8000mah battery to try to avoid having to use some kind of regulator. 1C would be 8A and the solar cells under bright daylight produce 7.8A so I should be safe here, right? Or is that pushing it? Also, would I need some sort of regulator for the voltage or if I choose the right number of cells will I be ok? I know I will need some sort of diode between the cells and battery to prevent voltage going back into the cells. However, I am just not sure on this whole regulator business. If I do need one, can I purchase one for the requirements listed above? Where? If not, would I have to make one?

    I know that was a tad long, any help or guidance would be appreciated. Thanks.
     
  2. theecoop

    New Member

    Apr 3, 2009
    9
    0
    li-po batteries require a balance charge,maybe you could run solar panel output to a cellpro4 balance charger and then you would have less chance of a fire (which is common on li-po batteries. IMO
     
  3. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    How many amps does the plane's motor use?

    With enough cells, the motor could run from the solar cells, and the receiver/servos from a smaller onboard battery, or vice versa.

    Charging Lithium batteries requires precision. If solar doesn't provide enough power for takeoff/climbout, but does provide enough for maneuvering when at speed, change the speed controller supply from the battery to solar through the remote?
     
  4. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    Read about lithium batteries before you have a fire:
    1) Each battery (you have three in series) must have its own charge controller to balance each battery's charging. Multi-cell charge controllers are available.
    2) A lithium battery cell is nominally 3.7V (11.1V for three) but fully charges to 4.2V (12.6V for three). You need more solar cells.

    Solar panels are big and heavy. Can a model airplane lift them?
     
  5. DB_Flyer

    Thread Starter New Member

    Apr 3, 2009
    3
    0
    thatoneguy,

    Yes and no... i think. It is sort of a dilemma in my mind. To slow fly the plane requires an average of 7 to 8 Amps (more for takeoff, etc). So with the 24 cells we have, in ideal conditions, you would think we could fly as long as there are those ideal conditions. The conditions are never ideal, but I am located in CA where the conditions are better than a lot of other places. Each cell is rated at a little less than 4W, so 24 would be a little less than 96W. So (8A the motor needs) x (11.1V of the battery) = 88.8W. So why not go with 30, or 40, or more cells? Well, you are correct that li-po's require precision charging. They charge in 2 phases and need constant voltage in one phase and constant current in the other I believe. Thus, if a have a 11.1V/8000mah battery, it wouldn't matter how many cells I have because I can not charge the battery faster than 1C and the charging voltage cannot be greater than that of the battery. So even if i got 96W or more from cells, I couldn't use more than 88.8W anyways. Right? I think that is right, but I am not sure. And even if I choose to connect the cells directly to the motor, wouldn't I still need some sort of regulator to keep the voltage constant when it needs to be and ditto for the current? I have been doing some more research and have come across Maximum Power Point Trackers? Is this maybe what I need?
     
  6. DB_Flyer

    Thread Starter New Member

    Apr 3, 2009
    3
    0
    Audioguru,

    I have a charger for the li-po. It hooks up to a standard 12V car battery. However, when the plane is in the air, the cells well basically be charging it as I understand it. So I think i need to simulate what the charger does, but in the air - and that is basically my question - how? If I have 24 cells rated at 0.5V I will have 12V which is what currently charges the battery, correct? Or do I need a voltage higher than that..? Solar panels are big and heavy, however, the cells that make up solar panels only weigh a few grams (the ones I have weight about 12g and are 156mmx156mm). Thats 288g, add EVA sheeting, wiring, and some other stuff, and it's about 0.5kg at the most, the plane will fly.

    DB
     
  7. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    The crux of the problem is that the charger reads the battery while charging, either the voltage in a current cycle, or current in a voltage cycle, or the voltage in a rest cycle (these cycles are fractions of a second, not "stages", as in a car battery).

    If there is a load on the battery, such as the motor drawing 8A, the charger will at best stop due to a failed pack, and at worse give you a replication of the Hindenberg.

    I have an electric glider with an 80" wingspan, and can keep it in the air an hour without spinning up the motor about 2 days a year. The rest of the time, I fly "slimers" and have fun with combat flying, which I've found to be 'second most fun'. First most fun is quarter scale Aerobatics, which is crazy expensive.

    Electric has advanced quite a bit, especially with the brushless outrunner motors. However, they can't give unlimited vertical and 30 minute run time at the same time, like a Nitro powered plane does.
     
Loading...