I am building a flamewheel 550 with dji 2212-920kv motors and 30A opto esc,and APM 2.6....I have the choice of 3 or 4 cell lipo....what will give me the longest flight-time of 5000mah 3c and 3800mah 4c (ca same weight)?
I'd really like to get back to the original questions before the redundant 'Zippy vs Premium' thread, and see if we can collect more data samples for 3S vs 4S.
My own interest is in maximum 'hover time' rather than performance or lifting power. To that end, it seems like I could test by simply using the lightest 3S battery possible, and measuring power (current * voltage) in a hover. Then, by adding representative weights to simulate larger batteries, draw the chart.
Then do the same for 4S, measuring power at each weight.
The only complication would be if the added weight increases current draw (it will) and *that* sags the voltage more than an actual bigger battery. Seems like a small risk.
I have a hard time using the 'C' rating to describe the hover power, because it suddenly brings the mAh capacity of the battery into a calculation that has nothing to do with it.
The power needed to hover is simply the current draw at a specific voltage, Watts.
For example, with my Zippy 3S 5000, I have one steady-state data average that shows 11.4v and 28.4A at the APM Power Module. That's 323 Watts for this weight (which I should go measure). On the battery analyzer, this battery holds 58.1Wh under 150W draw to 10v cutoff.
Once you know how many watts your copter draws, you can extrapolate that to any battery capacity and move to 'energy' (Watt-hours).
If I take that 58.1Wh / 323 Watts, that's 10.8 minutes of hover, which is dead-on. I land at 8 minutes of mild flight, or see a 'forced descent' around 10 minutes.
The 'C' rating is only interesting in ensuring you have enough headroom on what the battery can do, or an indication of how taxing it will be over the long term.
Your observations on battery composition are a real interest, especially as HK moves to 8ga wire on their Zippy packs for no good reason.
The trade depends on how far you you are willing to take it. Here are the trades to which I'm aware:
o Battery Efficiency - 4S = 3S since they are all built from the same cells. Packaging would be the only difference and from the extensive list I have, I see no general trend either way.
o Rotor Efficiency at 4S versus 3S - a 3S rotor will be 2% to 5% more efficient depending on the rotor if tested over the same thrust levels. All of my tests under controlled conditions show this.
o Wire Weight - At 4S the amps are 33% less for the same watts and about 28% less for the same thrust. Amps drive wire size (within minimums). If the wire-runs from the ESCs to the motors are significant in length, this can lead to savings in wire weight, which would reduce the amp requirements of the ship in flight. For example, if the wires weigh 80 grams, this might allow you to save 0.12 grams per inch of wire, depending on the amperage requirement. This then gets multiplied on the X2, for example, by 0.192%/gram to see the impact on amps (in this case about 2.5%).
o Powering the APM and Servos - 3S going to 5VDC is done at less of a heat loss than doing so on a 4S. I've never run the math on this.
To summarize: one tie, a 2% to 5% gain for 3S, a 2.5% gain for 4S, a wee gain for 3S. On the right day, either team could win.
The best way to fly duration is to:
o lose weight on the frame and electronics platform. this is huge. really huge.
o fly 3 or 4 S and not higher (go higher and the ESCs take too large of a weight hit). Also conversions to 5VDC takes too much heat loss the higher you go (small + for 3S).
o pick the most efficient combination of batteries within the 4S or 3S family. For example, if you need 3S 15000mAh, then pick the most efficient 3000 (five), 3750 (four), or 5000 (three) 3S battery. The answer isn't always the highest mAh.
o reduce weight to a point where you are flying safely at < 20C so you can take advantage of the lighter batteries (higher C batteries weigh more). The X2 hovers at about 2C. The X2 is fairly optimized for weight but I can still remove about 200 more grams is I'm smart.
o choose the most efficient rotors.
Are we back on track now :-)
The reason why C is important is for two fold:
1) You stated that a concern is duration. If it is, then you have a target duration in mind and know the mAh it will take to reach it. In that calc you also know there is a weight penalty that increases the amps non-linear. The X2 looks like:
2) Higher C batteries weigh more (they need larger wires), thus affecting duration.
So knowing the hover C and max C are important for battery selection because not much else makes a difference in battery selection. I am however testing various 3S batteries to see if their internal wire and covering weights vary the efficiency. While I don't expect to gain much, I am going for a LIPO flight duration record using a standard copter so even a 3% gain is worth pursuing.
Anyway, that's why I make the calculation.
P.S. MaxAmp has a 3S 11000 5C battery that is really light (compared to the typical 20C). The theoretical limit for LIPO battery (no wires or containment) is about 245mWh/gram. The Turnigy nano-tech is about 145. If the MaxAmp specs are right, then it will be about 190 mWh/g.
Notebook battery: 3S 3700mAh 185g. In plastic? case.
4X 3700mAh = 14800mAh , 740g
Where do I get one?
That comes out to 222 mWh/gram!!! 52% better than the Turnigy Nano ... that can't be right ...
If you are looking for cells with high mWh/g I recommend Panasonic NCR-18650B
Zippy Flightmax 5000mAh 3S 30C is what, 136mWh/gram?
The difference is that you can suck the Zippy dry in 10 minutes without it venting or sagging. :)
I didn't see a maximum discharge rate, but if you assume 2C (6A) on those Laptop/Tesla 18650 batteries, and 3200mAh capacity, something like a 3S10P (30 cells) would be 1425gr and 346Wh. That's an interesting concept.. big battery, but 78% more energy density than LiPo packs. Anyone flying something like that?
Here's a detailed article on different 18650 cells and their behavior under relatively high discharge:
Samsung ICR3000 is interesting in that it's 4.35v, but still has a hard time with 5A discharge. The 2A discharge curves look like a reasonable maximum, which means this is going to be one huge battery pack.
For safe discharge, most articles say 1C to 2C is safe. Since many pilots poof their 25C batteries, this would need to be tried with thought.
The X2 Black Momba would use 0.2C at 3S, 0.82C at 3S6P, and 0.85C at 3S10P.
Even when batteries can discharge higher than 1C, the mWh starts to drop rapidly down to LIPO levels. 2C seems OK.
I might try this.
We've got an X8 wing flying with 6S4P NCR18650b battery packs, 13600mAh. Haven't tried yet for ultimate endurance but got 70min/84km on about 75% of the pack at a higher than optimal speed.