Li-Ion vs LiPo… why not both?

[Flyboy]

TL;DR Summary

Can you parallel a Li-Ion and a LiPo battery?

I’m looking into electric RC aircraft, especially for endurance flights, and I keep running into a debate over what is the best battery design for a high-endurance/long range electric plane.

I have gathered that LiPo batteries are outstanding for high-power applications, such as racers, eVTOL, etc, where you need large amounts of power constantly.

On the other hand, Li-Ion batteries are increasingly used for high endurance FPV drones that are not running wide-open throttle, as the lower current output of the batteries is less of an issue, and the better energy density is advantageous.

I would love to use Li-Ion batteries for a future design where endurance and efficiency are the driving factors, but I am concerned about the limited current output, especially at takeoff. My thought was to parallel the main Li-Ion battery with a small LiPo battery that would provide the extra current needed for takeoff and climb out before switching to the Li-Ion, but I have seen no examples of this having been done before. Thus, I am wondering why it isn’t done. Obviously, there’s the issue of different cell voltages and current outputs, so directly paralleling them is probably not safe, but given the current state of solid state relays, it would seem to be a relatively simple task to make them separate circuits that would switch between them after you reach a safe cruising state.

I am not an electrical expert by any stretch of the imagination, so what am I missing aside from the additional weight and complexity?

 

[Borek]

Endurance flight is about keeping weight as low as possible, so it is probably much wiser to use longer runway (or start from a catapult, even DIY style) than to make model heavier.

 

[Vanadium 50]

I would come at it from another direction. You want a high power/weight ratio. You can do that by either getting the power up, or the weight down. A battery's power depends on its volume, but a battery also has a casing: a non-power producing region that scales as the area. But still adds weight. So one big battery is, all other things being equal, better than two smaller ones.

Exotic batteries, like Lithium-Polymer or Lithium-Metal-Phosphate do a little better than Lithium-Ion, but not hugely better. Enough better to justify the additional dead weight? I don't know, but probably not.

If you have someone who is willing to build a batter for you, I'd look into making the battery casing some other part of the aircraft, so it does double duty. Also, there is a trade-off between recharge capacity and power density. If you only wanted to charge it, say, 30 times, you could do better than one you want to change 500 times. Maybe.

 

[Rive]

...to parallel the main Li-Ion battery with a small LiPo battery that would provide the extra current needed for takeoff and climb out before switching to the Li-Ion

If it's to 'switch to', then it's a bit more complicated parallel then a simple 'parallel battery'. And given the different chemistry/parameters, some kind of switching/matching would be clearly required.

Doable, but not trivial. I too would rather recommend a catapult or such as @Borek already suggested.

Maybe you could take a look at (proper) military drones.