In-Flight LiPo Charging via Solar Cells

I am attempting to use thin, flexible solar panels to charge a battery on a plane (RC) for a school project. After fully charged, I will begin flying the plane, and the panels will hopefully extend the flight time by charging during flight. The battery is a 2100mAh, 11.1V LiPo. The solar panels will be placed on the wing, which is 7.5" x 31". I may be able to fit a couple of small panels on the tail as well. The problem I have is finding appropriately sized, lightweight panels that can produce at least 600mA of current. A few of my options are:
1.) 6.0V, 100mA, 4.5x5.9"
2.) 7.2V, 100mA, 3.0x10.0"
3.) 7.2V, 200mA, 5.9x10"
4.) 4.80V, 50mA, 3.0x3.7"
5.) 4.20V, 22mA, 1.5x.3"
I'll need to wire two panels in series to get the necessary voltage, but then I'd need multiple sets of these in parallel to get a worthwhile current going. Would it be possible to wire the large 7.2V, 200mA panel in series with a small 4.80V, 50mA panel? Would that result in 12V and 200mA? If not, are there any better configurations with my options? Otherwise, it looks like I could only get 200-300mA with the larger panels.

• You'll likely want to have some regulation at the output of these panels so don't trust the exact voltage numbers to stack up correctly. Therefore, I would go with the most power per square inch provided.
– scld
Commented Feb 24, 2014 at 21:01
• An interesting question would be whether this helps or hurts flight time. You gain energy but you have more mass to carry and may also increase drag. Commented Feb 24, 2014 at 21:17
• I agree with @scld. You're going to have a switching converter anyway, so what matters is power output per weight. However, have you really shown that the weight of the solar cells won't require more power to keep aloft then they provide? My first gut reaction is that this is a losing proposition for such a small plane. Commented Feb 24, 2014 at 21:19
• "After fully charged, I will begin flying the plane, and the panels will hopefully extend the flight time by charging during flight." If the panels don't produce enough power to run the plane's motor, you will not be charging the batteries at all but, rather, reducing their rate of discharge. If, in fact, you could actually charge the batteries in flight, the panels would be producing more than enough power on their own to fly the plane without batteries. Commented Feb 24, 2014 at 22:17

You could wire the two dissimilar panels together. The max current would be 50mA though, but would be 12v (0.6W).

A key to good solar power output is having matched panels, otherwise you are making an already inefficient conversion even more inefficient since the panels won't be operating at their maximum power point. Maximum Power Point Tracking

That being said, your 1.44W (7.2V @ .2A) panel will only output that at its maximum power point, charging a 2.1Ah battery with full power from the panel would take 12.6 hours to charge it. It would be on the ground well before any useful solar power has charged the battery. Not to mention you just added weight, increased drag, and wasted time.

There is an optimal size for adding solar to have any useful effects. Check out Sky Sailor He is the originator. His site has matlab models/scripts to use that calculate at what point you could be powered entirely by solar.

An excellent school project and it would change the face of the RC hobbyist if you could get it to work, but I'm afraid that the answers you have received are correct, the current technology of solar panels are going to be a little too heavy and large to give you the current AND voltage you want. You might be able to keep the plane in the air a little bit longer though.

For example there are panels that are only 3" x 6" that give you 3.5 amps, but only 0.5 volts, so you would have to put too many in series to make it work for you.

Just to summarize:

1) Solar panels in parallel with the same voltage - you add the current; voltage stays the same.

2) Solar panels in series, you add the voltage, BUT the current you receive will be the current of the smallest panel or panel with the least current. So in your example, 50ma.

So in your examples, wire two of the 6.0 volt panels in series to get 12.0 volts, then several of these 12.0 volt panels in parallel to get the current you want - BUT it will likely be TOO big.

I assume that you understand the complexities of charging lithium ion batteries and that you should not just apply the power from the solar panels directly to the battery without a charge controller.

Good luck.