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I'm a bit of an electronics noob and haven't been able to find a straight answer to this question despite doing quite a bit of research, so I figured I'd ask it here. I am working on an autonomous RC-scale airplane that will be powered by solar cells on the wing during flight. There is also a 3S 4200mAH Li-ion battery on board to serve as a backup in case of cloud cover/higher power demand, like during takeoff and while climbing. There will be 20 solar cells on the airplane which will produce a peak voltage of about 12V in direct sunlight, but this may drop depending on conditions.

Now on to the question: How do I use the solar cells to power the on-board electronics, motors, and servos and also charge the battery when power demand is less than production? I estimate that the plane will use 35-45W of power while cruising and will produce 72W of power in direct sunlight. While climbing, it could use up to 350W of power. I bought a cheap BMS module a little while ago thinking it would be all I needed, but now I'm not so sure since the input voltage will vary and the BMS should be primarily for protection anyway.

Thanks in advance!

Edit: I forgot to mention that extending the lifespan of the battery is a priority. Since the airplane will hopefully have very long flight times and won't need to dip into battery power very much, fast-charging the battery every time it discharged at all and/or charging it to 100% capacity would likely result in it wearing quickly.

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  • \$\begingroup\$ This is very difficult with consumer-grade components. Most consumer photovoltaic cells are too heavy, for the energy they provide even in direct sunlight, to sustain lift, unless the air-frame and propeller is made out of exotic, expensive materials such as carbon nanotubes covered by a fragile membrane. Recalculate the energy supply and requirements -- keeping in mind that quoted photovoltaic power is in direct sunlight, perpendicular to the cell. If you do go with this project, just use the photocells to keep the battery charged, and run the motor off the battery. \$\endgroup\$
    – DrMoishe Pippik
    Commented Jul 10, 2023 at 4:02
  • \$\begingroup\$ The solar cells I'm using are flexible and very thin. They are quoted at 3.6W per cell and since they're about 6" by 6", I'm able to fit 20 on the wing of the airplane comfortably and the total weight comes out to a couple hundred grams for the cells alone. The airplane itself is designed to be efficient (it's pretty underpowered for its size and is basically a powered glider), and I included a pretty large margin for error in my drag calculation. We'll see what happens! Worst case scenario, I have an airplane with significantly extended flight times. \$\endgroup\$
    – jdsp522
    Commented Jul 11, 2023 at 13:33

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You need a battery charger, and you need an MPPT (maximum power point tracking) voltage converter, probably as part of the same module, called a "solar charge controller". Combining them works best because the charger is aware of how much power than MPPT can supply at a given moment and adjust the battery charging rate accordingly. The solar panels connect to the MPPT input, the battery is connected to the charger, the loads are connected to the battery.

Your BMS won't do either of those things — as you say, it's a protection circuit.

Make sure to pay attention to datasheet specs — you want a unit that has a maximum input voltage higher than your cells' Voc. If your cells are able to supply reasonable current at a voltage above your target battery voltage then a buck-only MPPT will do; if not, you may be better with a buck-boost one.

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  • \$\begingroup\$ Thanks! Would I be able to find an MPPT solar charge controller in a relatively small, lightweight form factor? A quick search on Digikey didn't yield many results and googling the same thing gets me a whole bunch of devices with housings, displays, etc. which are things that would just serve to add weight in my application. Ideally, just a PCB would work best, do you know if anything like that is commercially available? \$\endgroup\$
    – jdsp522
    Commented Jul 11, 2023 at 13:39

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