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After this, I think I will take a course on electrical engineering (fuzzy from school days).

I'm trying to add some flexible panels (they were <14g a panel)) to my drone (HS120D), to possibly give my drone a flying time boost by charging the battery while it's being flown in the sun. I'm aware that doing this will void my warranty, not worried about that. The panels will be connected to the battery with diodes in between (to avoid draining bat when panels are in shade/lowlight for some time).

The battery is 3.7V, 1200mAh and weighs idk (like 1/3 pack of playing cards). I am seriously considering upgrading the battery to a 3.7v 5000mAh cell (4x current capacity) too. But the thing is 76g. The drone weights 220g (7.8oz) w/o battery or blade guards (all 4 might weigh an ounce). I will reduce the weight by removing excess plastic frame, at least 30g (probably more). Drone normally gets 18min flight time/charge (no recording, ~ 15m flight+recording). Wires, diodes, removable frame (paperclips) might weigh 40g combined. So I'm increasing the weight by At least 80g.

Finally, the blades will pass over ~ 5.5 more square inches of panel, distance of 1.5cm below (the closeness will reduce lift, I know).

Tl;dr

Which voltage panels should I use for: A 3.7v 1200mAh battery and A 3.7v 5000mAh battery?

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    \$\begingroup\$ Solar panels would allow a drone to rest unpowered whicle recharging and then fly. In flight the panels weight will probably take more energy than they provide. A key factor missing in your figures is panel area. \$\endgroup\$ – Russell McMahon Nov 14 at 5:30
  • \$\begingroup\$ " I'm aware that doing this will void my warranty" - which drone? "So I'm increasing the weight by 50%" - then you will need ~50% more power. How much power does the drone normally use (or, what is the normal flight time), and how many watts can the panel produce? \$\endgroup\$ – Bruce Abbott Nov 14 at 5:50
  • \$\begingroup\$ "Plus The blades will pass over about 5.5 more square inches of stuff, like 1.5cm below..." exactly what 'stuff' is that? \$\endgroup\$ – Bruce Abbott Nov 14 at 5:51
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    \$\begingroup\$ Since you still haven't provided enough information to give an exact answer I will make a prediction - the added weight and drag of the solar panels will negate any extra energy they provide. Their only use would be - as Russell McMahon suggested - to recharge on the ground. To extend flying time the extra weight would be better put into a larger battery. \$\endgroup\$ – Bruce Abbott Nov 14 at 8:24
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    \$\begingroup\$ Possible duplicate of Wiring battery and solar panel to drone to extend flight time - schematic help - we already have one question here with an answer that explains why this is impractical \$\endgroup\$ – Chris Stratton Nov 14 at 17:55
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TL/DR:

Even assuming an unrealistically large amount of collected energy , it is doubtful that solar panels on the drone will do you any good.


Lets rough out some numbers.

The energy stored in your battery is approximately 4.4 watt hours. That's 1.2 ampere hours times 3.7 volts.

To make the solar cells worth while, you'll need to collect a significant fraction of that energy during flight.

Solar power at the Earth's surface is around 1000 watts per square meter. If you had a 100 percent efficient solar panel of one square meter surface area, you could get 1000 watts of power.

Your drone couldn't fly with that large surface hanging on it, and solar panels are far from 100 percent efficient.

I'll assume a (hopelessly) optimistic efficiency of 30 percent for your solar panels.

I'll also assume a solar panel about the size of your drone. The drone is 27cm by 27cm. I'll go with panels of 30cm by 30cm. That's a surface area of 0.09 square meters.

1000 watts times 0.09 is 90 watts of incoming solar power.

At 30 percent efficiency, that's 27 watts of power from your solar panel.

Take a flight time of 15 minutes, and your solar panel will collect 6.75 watt hours of energy.

That sounds good - it is more than your battery, so it seems like you could get some benefit out of it.

Or not.

Your drone now has to carry the weight of the solar panel and stabilize it in flight.

Taking a WAG, I'd be surprised if your drone battery would last for half the normal flight time while carrying the solar panel (and associated electronics.) That halves the time before you are dependent on the energy from the solar panel. You'll have only gotten about 3.5 watts hours in solar energy by that point.

Your solar energy has to be used to charge the battery, so there will be losses. You will lose energy in converting the voltage to be usable, and you will lose energy as you draw it back out of the battery.

You might effectively end up with a couple of watt hours usable from the whole operating period.

If you lose half your flight time by carrying the solar panel, and then can't make it up with energy from the solar panel, then the whole thing is a loss.

Since I mostly ignored losses and assumed a really optimistic amount of energy from the solar panel, I think you will find that a solar panel in flight is a loss. Even if you manage to make it extend your flight time a little, you still have to deal with your drone being ungainly and instable with the solar panels on it.


The better way to power your drone from solar power is to use a (big) solar charger to charge your drone battery on the ground. Keep a spare battery at hand and recharging. Swap as needed. You won't extend the time of individual flights, but you can make more flights per day that way.

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  • \$\begingroup\$ I should aim for the highest mAh panels I can get right? If I could get 1600 mAh coming from panels(obv only in sun), would that increase flight time (total weight will be about 40g heavier than it is now, cause I'm removing some plastic casing)? \$\endgroup\$ – Code_Student09 Nov 14 at 18:27
  • \$\begingroup\$ Milliampere hours (mAh) and milliamperes are not the same thing. Solar panels are rated by power (watts) or volts and amperes, but not by milliampere hours. \$\endgroup\$ – JRE Nov 14 at 18:28
  • \$\begingroup\$ You want panels that can deliver the most power for their surface area. And, you are limited by the solar irradiance (see wikipedia link above) as to how much power you can get from a given area. \$\endgroup\$ – JRE Nov 14 at 18:30
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1200 mAh over 18 minutes = 1200/ (18/60) = 4A. Probably somewhat less as batteries mAh ratings are usually specified at a C/1 (1 hour) discharge rate, so C/0.3 (18 minute) rating will be somewhat lower - although LiIon capacity degrades less under moderately increased loads than many other battery chemistries.

Your 3 x 1.5V panels will give 4.5V at about 360 mA with a clear in midday sun when oriented optimally. ie - usually less. Lets start with 300 mA - which will often not be achievable.

As a percentage of average load of 4A, 300 mA = 300/4000 = 7.5%.
While actual load will probably be slightly less (see above) PV panel output will also often be less.

So, while you may be able to prolong flight time by about 7.5% x 18 minutes = about 1 minute 20 seconds IF the motor current does not increase, it is likely that the panel load will increase the motor current by substantially more than the current boost from the panels.

However, as noted previously - leave this sitting in the sun and it will self recharge in about 4+ sunshine hours. Typical SSH (equivalent full sun hours / day are about 2 in Winter and 5 to 6 in Summer in many areas. Some are of course much worse in Winter and a few somewhat better in Summer. (Kabul in Afghanistan is among the best).
So, if left properly aligned to sun, the panels would give you about 1 x 18 minute flight per day.

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  • \$\begingroup\$ would the 5000mAh battery of same voltage give me 4x the flight time? \$\endgroup\$ – Code_Student09 Nov 14 at 18:06
  • \$\begingroup\$ A larger battery is heavier, and motor efficiency reduces at higher loading, so there is diminishing return from increased capacity. If you have plenty of excess power now and the battery is a small proportion of the total weight then the increase in flight time should be close to 4 times. However bear in mind that lightweight high capacity Lipos are usually designed for low drain applications and have a low maximum discharge current. eg. 1C = 5A (not high enough for your drone). \$\endgroup\$ – Bruce Abbott Nov 14 at 18:37
  • \$\begingroup\$ @Code_Student09 Ignoring that you are now different asking new unrelated (essentially) questions ... . || Weight the old battery. Weigh the nex battery. Add the weight difference to the drone. Fly with fully charged old battery. Call flight time = Tflight_heavy. || New flight time with new battery will be ~~~~<= mAh_new/mAh_old x Tflight_heavy. \$\endgroup\$ – Russell McMahon Nov 15 at 0:15
  • \$\begingroup\$ @bruce thanks man. \$\endgroup\$ – Code_Student09 Nov 16 at 13:36

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