We're working on a quadcopter that will carry a solar panel on top that will continually charging the lipo battery of the quad. What's the smallest and easiest way to recreate a charger that will allow safe charging for the lipo battery?
Make sure you understand Lithium Ion battery safety before proceeding.
Lithium Ion batteries have limits on charging current and charging voltage. The typical charging technique is to provide the maxiumum charging current until max charging voltage is reached, then provide the max charging voltage until current drops to zero.
If you can find a solar cell array with Isc less than the battery max charging current and Voc at the max charging current you might be able to hook the solar array directly to the battery.
Otherwise, a DC-DC converter between the array and the battery which will convert the array voltage to max charging voltage, with a max current that matches the max charging current of the battery. You'll loose 15% of your energy though.
If you don't have an electrical engineer (or student of EE) on your team, go find one.
The easiest way to accomplish lithium charging is to use a brownout tolerant DC-DC converter. By brownout tolerant I mean the converter should automatically derate output voltage to maintain the max current limit with no need to reset or recover once the condition clears.
The output current limit of the converter should be less than or equal to the current limit for the constant current phase of the charge curve. The output voltage should be less than or equal to the constant voltage phase of the charge curve.
In this scheme, when the battery is low on charge, it will also be low on voltage. A larger voltage differential will draw more current, up to the current limit on the converter, which should be approximately equal to the constant current charge limit.
The battery charges and, as it does, the terminal voltage rises. At some time the voltage differential decreases to the point that the brownout condition ends and the DC-DC converter resumes rated voltage output, which should now be at the constant charge voltage.
A couple of notes, though:
- As @hauptmech points out in their terrific content, assume the copter runs 10min at 100W for flight. This means the battery stores about 16 Watt-hours, or 1000 Watt-minutes of energy. Assuming the solar cell outputs 1W, this means it will take about 1000 minutes, or about 16 hours, to recharge the battery. When you try to connect the solar cell output to the system to charge, there is a very high likelihood that you will brown out the solar cell because of the power draw. This means the DC-DC converter needs to have a wide input voltage range, conceivably down to near 0V input, AND that the solar cell needs to be short-circuit tolerant, as that's likely to be the normal operating condition. You could get a DC-DC converter with a current limit closer to the equivalent power output of the solar cell, which may help.
- Charging lithium batteries is dangerous. They are banned on most flights, banned on most air mail services, for good reason. See also: hover boards.
- You need to be sure to stay under the max ratings. Exceeding the current limit causes heat issues (not that I think you'd exceed a lithium battery's current limit with a solar cell), but exceeding the voltage limit causes dielectric breakdown in the battery, which also causes fires. That is very possible with a solar cell.
- Again, as hauptmech mentioned, you need an EE. This isn't really something to guess at, and if you're here looking for advice you should honestly consider if you know what you're doing and seek guidance at least from a senior EE student.
I don't know of any application where the LiPo is charged and discharged at the same time. It might not be a good idea, due to the delicate charging cycle procedure that LiPos require.
Have you run a power consumption analysis with the given solar panel figures, to see if it is worth the effort? I suspect that the net power profit will be negative, due to the increased system weight and complexity.