Thinking about moving to 18650 based power, opinions?

evranch

Well-known member
Hover test results in! I tie-wrapped my wattmeter under the quad with the pack to keep an eye on usage through the tests. It hovers at 9 amps.

Did an outdoor hover test, quit at 3v/cell under load after 11 minutes. This was 1600mAh burnt, just over half the cell. Not warm but neither was the air.

Recharged and the wind was howling so after putting my daughter to bed I did an indoor hover test to the recommended 2.5v/cell under load. 15 minutes and 2200mAh burnt. Slightly warm. The cells rebounded to 3v after resting a few minutes, I'm charging now to verify the usage.

Much below 3v the throttle levels have to be increased significantly to compensate for the lower voltage, resulting in loss of power, higher currents and exponential depletion of the remaining charge. I'd say below 2.8v it's time to head for a landing without delay.

However, 15 minutes is over half the hover time I got from the bulky 7700 LiPo, from a cheap pack that can be slipped in my back pocket. What's more important is that it handles much lighter and still likely flies long enough to perform most single operations without swapping the pack. I was considering doing 3s2p 6000mAh but it probably works better to have a couple slim, easily carried, easily swapped packs than fat ones.

Hoping to take it for a rip tomorrow, draw some surge currents and see how the flight performance is with a lightweight pack.

If you think hovering an 8" quad in a farmhouse spare bedroom for 15 minutes sounds like an exercise in white knuckles, you'd be right. Not much wiggle room, and it's not trimmed all that great because it's usually flown in acro in open spaces.
 

LitterBug

Troll Spammer
If you think hovering an 8" quad in a farmhouse spare bedroom for 15 minutes sounds like an exercise in white knuckles, you'd be right. Not much wiggle room, and it's not trimmed all that great because it's usually flown in acro in open spaces.

Extreme indoor flying. LOL I don't even like hovering my 2" builds inside the house. Tiny whoops I'll fly all day long.... as long as the fam isn't home. They totally despise the noise of my toys.

You might want to consider building a 2 into 1 connector to double up two packs. That way you can fly a single pack, or create a 2P pack when you need/want more torque. I have one of those for my 650 quad

Cheers!
LitterBug
 

evranch

Well-known member
consider building a 2 into 1 connector to double up two packs.

Great idea as I have room for 2 of these packs on the bottom of the quad side by side but didn't want to build a clunky flat 6 cell wide pack.

I assume all the standard paralleling rules apply, same state of charge, same brand of cells etc.

Do you have any tips on charging these? I have been cutting off at 4.15v/cell as I usually do on LiPos to prolong their life as they don't have much extra capacity near the top. Am I missing out on significant capacity with LiIon?
 

evranch

Well-known member
So I tried to rip it and... it doesn't rip. It flies great for quiet cruising around, but if I tried to punch it the quad would start to roll over to the right. Climb rate also wasn't great considering how light it is. I think 1 or 2 of the ESCs were going limp due to the voltage sag? Max current recorded 25A and only 240W, that's less than 10v. I think without the sag I would have delivered a lot more current as well.

Trying to figure out if this was a tuning issue or a power issue I foolishly continued to do punchouts until one of the ESCs actually quit and put the quad into a full tumble. Luckily I had enough altitude to keep it off the ground and that was the end of that. Back to mild cruising around.

After cruising 11 minutes I decided to bring it in at 2.6v/cell. I had voltage sag throttle compensation on, which I think I will turn off as it covers up the fact your throttle range is diminishing. I swung in to land but suddenly found I didn't have enough power left for the flare, and hit the ground a little hard. Right on the battery pack which did hold up much better than a pouch cell would, not a mark on it!

Disappointing thing was I didn't even burn 2000mAh before I had lost enough voltage that the quad was almost unflyable. Might need to go 2P or 21700 for practical performance.

I'm going to fly a 2200 LiPo on the same quad today, which is the same weight, and compare the performance. Also I've loaded this wide pack into my Spear (with the wide battery bay) and am going to try it for fixed wing. I think with a motor that draws a max of 20A and cruises below 10A I should be able to get a lot more usable capacity out of it.
 

Hondo76251

Legendary member
I was wondering how this project was turning out, Thanks for keeping us updated!

I've never tried to used a liIon anywhere near its capacity and I've certainly noticed the sag in some of my fixed wing setups. Was curious how it would do under hard use. I think, at least for me, the lipos really seem to shine when used in low demand, endurance setups...
 
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evranch

Well-known member
They will never be flying them in DRL, that's for sure. But for a UAV-type quad they are so, so close to being usable. I rarely punch the throttle to 100% when I'm chasing sheep, after all. The main issue is not getting to use all of the charge due to the sag, which could be resolved with parallel cells or with a higher C-rating.

I feel like if I had a 30A continuous cell like the Samsung 40T 21700, which is 50% more, it would be enough to fly this quad just fine. Unfortunately, 18650s are about all I can get in this country.

It might be time to bite the bullet and risk a purchase of 21700 or 26650 cells on Banggood. Almost all cells in this form factor are rated over 30A by Mooch with the exception of a couple really crap ones. If you have bought on Banggood and can point me at a brand/seller that performed well, that would be great.
 

Hondo76251

Legendary member
I have several but I haven't had any real serious testing on them so I'm not much help there but I do plan to have a couple of packs built soon and, if you dont beat me to it, I'll let you know!
 

evranch

Well-known member
I did see some guys building packs on YouTube with LiitoKala cells, as well as decent results from the vape community so I might give them a shot.
 

LitterBug

Troll Spammer
Sounds like the results I would expect. Could be challenging to fly in high or gusty wind. 2P would probably make it more liveable.

I have thought about throwing a lipo in my nano goblin to see what I'm missing performance wise. It is current limited by the 18650 packs as you can tell by the sag in the OSD at full throttle. Bumping from 2S to 3S made a good deal of difference, but still not the raw torque of having excess C in the tank.

Cheers!
LB
 

evranch

Well-known member
Interesting results. The quad still rolls over on punchout with the LiPo, though it's drawing 300W instead of 240W and climbing faster. Must be a tuning fault that was covered up by dragging around the large batteries. I'll have to figure out what's going on here before I make any judgements.

On the 20A fixed wing, the pack is basically indistinguishable from the LiPo for performance, and my thumbs froze before I could run it down low enough to see what happened towards the end of the pack. Telemetry was screaming at me for voltages in the 10s whenever I was at WOT but the plane still went like a bullet. It will be interesting to see how the LVC deals with this lower-voltage pack and if it needs to be adjusted.

I'm thinking the best way to deal with the lower voltages is just to go up from 3s to 4s, which will cut down current draw as well as give me more voltage headroom.