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)?
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Have you checked ReadyMadeRC? Their batteries look pretty good, weight/mAh look better than most. Their listed weight includes wires and connectors.
It depends on your motors, flight load, and wiring.
o Gate 1: In 3S at the AUW you desire to fly, does the copter have enough lift at the end of normal battery drain to keep the copter in the air?
If no. Then must go 4S.
o Gate 2: Is your wire thickness going from the ESCs to the motors tuned precisely for higher volts and thus lower amps?
If yes, the there might be a 0.3% advantage to going 4S.
If no (the wires are plenty heavy), then it is likely that there is a 1% advantage to 3S.
As you can see, there is little difference between the two, with the huge exception of the first gate (having adequate thrust). In general, 4S is better because the copter will run at lower amps (less heat) and be able to fight the wind much more effectively. But for that gain, there is a very small hit on efficiency.
I have had a lot of luck with glacier lipo's from buddyrc, anyone else using them?? Probably bought 25-30 packs from them now, zero issues. 55$ for a 3S 35c 5300mAh, 35C, 11.1V, 3 cells, 138 x 40 x 31 mm, 420g, 14.8 oz...
Forrest ---- Xian06 & Mangus pointed out laptop packs and 18650's, here is a thread for you to read: http://diydrones.com/profiles/blogs/my-97minute-06second-record-qua...
There are others experimenting who have been playing with the Panasonic laptop 18650 type cells/packs for a couple of years. Also, Tesla Auto will have a big bundle of them to energize their car.
My 18650 pack (See attached photo.):
I made a small 3S 5000mAh battery pack ~ 2" X 3" x 1/2" (50mm X 70mm X 12 mm) at 129 grams. The cells are cheap Chinese advertised as 5000mAh with protection circuitry. They have NO protection circuitry or protection other than a vent. I know - I tore one apart. The 3S battery pack is likely 1C to 2C and easily lifts my 450mm Flamewheel clone outfitted with HK 1000kv bldc motors, 30A ESCs and 10 x 4.7 props. Approx weight? Heavy - I've got junk on board. When the quad is powered quickly it will cause the low battery LED to flash but, the copter will fly.
I'm a noob and do not fly well so, I've fallen back to using a Turnigy 3S 2200mAh 30C. The 18650s do not supply enough current with my poor flying skills jamming on the throttle which starves the quad for power and constantly triggers the on board battery alarm. The over power draw without ample supply causes the quad to drop or falter in flight which as you would realize causes more issues I just can't learn through. ;-) But, hey --- the battery pack will fly the quad.
3S_5KmAh_pack.jpg
Hi friend,
Flight time will be more in 5000mah 3c ....in 4c the thrust will be more (you can add more payload)....
Regards,
UNMANNEDENGINEERIA
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.
Any thoughts?
Also watch out for the current crop of HK Zippy's using 8ga wire. Egad! That's a lot of wasted copper and weight.
I have a Flamewheel f550, stock motors, I really like the 4S 30C AGA Power Batteries (4200, 5200 and 6200)
I have tried others but these seem to give me a couple more minutes and not as heavy as some of the others.
I have 10" props, I found them at Hobby Town USA
http://www.aga-power.com/list-10-1.html
let's make an assumption that your motor has a fairly flat efficiency curve (thrust per watt) when holding watts constant but varying voltage by +17%. if so, then it's simple math. it's all about watts in the battery at that point.
3 cell (a proportional number to voltage) x 5000 = 15000
4 cell x 3800 = 15200 so a 1% edge. [if you want the numbers in watts, multiply by 3.7]
but let's say that the response isn't flat and that the rotor (motor+prop) responds more efficiently at lower rpm ... say about 2% more efficient (likely). then the 1% gain is lost and the 3S wins by 1%.
then let's say you decide to size your wires going out to the motors. the lower amp system (the one using the higher volts) can use smaller wires, so you save a few grams and extend flight time. now the 4S fights back.
then let's say that the batteries, because of wrapping the battery surface area and wire lengths used to connect the cells, that the 3S is more efficient (likely in your case) ...
this is adding up to at least a 1% difference (lost track though of which way), which is huge because in a 10 minute flight we are talking a whopping 6 seconds ... oh ... so don't worry about it. look at cost instead and just go fly.
if you really want to pursue this because you want every possible drop of blood out of the turnip, Hugues is hosting a thread on batteries as we speak. tune in there. we are about to do some tests because we, like you, are hoping that the blood in the turnip runs deep.
P.S. in reality, if you tested the two system you would see up to a 7% difference, but you won't know until you tested both.
Assuming you are correct that they have EXACTLY the same weight the 3800mah 4 cell has an 1.3% advantage. So almost nothing. Comes down then to what you have and or which has a cost advantage. If they are not the same weight, I would go with the lightest.
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