Hey all,
Exciting to see the development and integration of Li-Ion batteries. In the past, this was not possible due to the low C-rating of Li-Ion batteries and the large current draw from multirotor aircraft. However, now that multicopters are as efficient as ever and Li-Ion batteries has matured - the pairing of both technologies should provide for some insane flight time/range.
Although most of your analysis regarding Li-Ion is correct, I wanted to touch up on the C-rating of a battery. A C-rating is an industry standard that is used to define the max amount of amperage that can be drawn from the battery without damaging it. Just like you stated, C-rating * capacity = max amp draw allowed. I think it is important to note that this is only a limit of current draw (amperage) and not energy storage (amp-hour). You mention amp-hour in your post so I just wanted to correct this. Examples as follows:
3C 10Ah Li-Ion Pack --> Max Current Draw: 30 Amps
3C 20Ah Li-Ion Pack --> Max Current Draw: 60 Amps
5C 15Ah Li-Ion Pack --> Max Current Draw: 75 Amps
So, yes, if you believe that the amp draw will be too tight on a smaller pack then adding more capacity is the correct choice. This allows for more energy stored onboard (most likely higher flight times) and less stress on the battery. When designing Li-Ion aircraft I like to have my takeoffs to pull a 3C rating from the battery - with all other flight staying closer to 1.5C to 2.5C. Anything more may result in the temperature constraints you state.
As per charging, Li-Ion batteries are best charged at 0.5C. Thus, the amperage the battery is charged at is calculate the same way as the discharge rate. This means a battery will take just over 2 hours to charge - assuming your charger can support the 0.5C rate. This is different than most Li-Po batteries that can be charged at 30 to 60 minutes. Please take this into account when designing the craft for your mission.
When it comes to vendors, I would stick to the big three companies: Samsung, Sanyo, and Panasonic. All other vendors don’t have the same level of quality control, which can prove disastrous in the future. Also, if possible I would set a battery up on the bench and run a 3C sustained discharge on it to see what the cell temperature looks like. This can be done by taking a singular Li-Ion cell and running a sustained 10A draw from it.
If you need any more help - let me know! Also, I am linking below the two interesting batteries that I mentioned in the other post:
https://www.foxtechfpv.com/foxtech-diamond-6s-22000mah-semi-solid-state-lipo-battery.html
https://www.foxtechfpv.com/diamond-6s-30000mah-semi-solid-state-lipo-battery.html
These batteries use a completely different battery chemistry that I am not familiar with. So, I don’t know how valid their claims are. I know if you contact the company they can provide a full testing and specifications document.