# Trying to figure out an RC Plane Solar Charger

So for a project I am looking at making a solar charge controller for a 3S Li-Ion battery system used in an RC Plane.

The issue I am running into now is that I am unsure how to go about designing the charge controller for specific performances.

So currently our battery pack is going to be 5000 mAh. However based on our calculations the maximum current from our solar array is atleast 6 Amps. The problem is that we want to be able to control the maximum current coming from the solar panel to the battery while it is in use.

What I mean is that while we are flying we might be drawing 5 amps from the battery for the motors but charging 10 amps from the solar array which leaves us at a safe 5amp charge. However, if we go below this in motor draw, say 4 amps, that means we are now charging at 6 amps which is over the recommended charge rate for the li-ion batteries. Does anyone know of a way we could somehow adjust the current going to the system to prevent the battery from overcurrent? The mppt charge controllers we have looked at like the LT8491 do not have a method of controlling the maximum current except for the resistance values you pick when you create the PCB.

I wasn't sure if there was a way to just shunt of the current safely without wasting a lot since we are trying to get the RC plane flying as long as possible.

• Spend some time reading up on the amount of engineering which has gone into the very very few demonstration sort of aircraft which have done this. In effect, you are "in way over your head" and this effort is not going to lead to success without already having a very thorough knowledge of the constituent technologies. The question is far beyond the scope of what can fit on a stack exchange site. Jan 5, 2021 at 3:55
• You need to measure battery current and PV current separately then regulate charge current with a series pass regulator to CC max then spend as little time as necessary during CV mode before going to float where all the current goes to load at say 3.8V per balanced cell. Then keep it there while at the same time optimizing the MPPT by V/I =Z matched to the Zo of the PV panel. These two voltages might not be the same so you must decide what PV efficiency you need. And how to achieve that. Normally Voc= 25% above MPPT voltage Jan 5, 2021 at 3:59
• ... a MPPTvoltage ranges from 72 to 82 % of Voc on PV’s for most Jan 5, 2021 at 4:05
• How long does the plane fly on a fully charged 5Ah battery? Jan 5, 2021 at 16:46
• With the current consumption calculations, we believe it will last roughly 75 minutes. Jan 5, 2021 at 17:45