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Lipo Battery Questions

#1
Hey folks!

I am having mega trouble understanding what battery type will work the best...ie what C rating I will need to use.....I am thinking of buying this motor and esc....


https://www.amazon.com/gp/product/B00XBZWIHE/ref=ox_sc_act_title_3?ie=UTF8&psc=1&smid=AF5VK40LEGWAH


Hobbymate HB2806 brushless motor, 1450 Kv Hobbywing 30A speed controller,

Motor specification:
Kv: 1450rpm/V
Shaft Dia: ø3.00mm
Weight (about): 34g
Unload current: 0.9A (7.4)
Max current: 14.5A (7.4)
Lipo Battery: 2S-3S
Recommend Propeller: 7*6
Thrust: 530g
Hobbywing Pentium 30A ESC specification:

1.1 Output: Continuous 30A, Burst 40A up to 10 Secs


please help!
 

Montiey

Master Tinkerer
#2
You'd obviously need a 2 or 3-cell battery. But, there are hundreds of batteries that would be compatible in terms of current draw. What plane are you hoping to use this on? That will determine the size of the battery- then we can find one with an adequate power delivery capability.

:)
 
#4
Then you would need to buy four of those packages? Those are pretty big motors to build a basic quad. They are more suitable for a 200-300 gram airplane than a small quad. I guess that's what you mean by "smash drone?"

Mike
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#5
Mike,

Smash Drone: http://www.flitetest.com/articles/now-you-can-crash-and-keep-flying


wspaldingstowevt,

From the Spec, that motor will at most draw 14.5A before the heat build-up it creates begins to damage it. The 7x6 prop spec'ed for it on 3S should be safely under that, as probably would an 8x4 prop. If you can fit the 8x4 prop without striking the boom, I'd do it -- longer prop with lower pitch gives you the good low speed thrust (thrust, good) with a lower top speed (speed, not so much). if you can find a pusher or reverse prop in that size, go with that. it's not required, but a pusher prop on a pushing motor will tighten the prop-adaptor where a normal prop will eventually loosen it.

So you're looking for a 3S pack that can source at least 15A continuously. The build specs an 1800mAh pack, but probably anything between a 1300 and 2200 will do -- might have to nudge it around for balance. I mention those sizes because they're a bit more popular, with the 3S 2200mAh pack being the most common pack you'll find in the larger FT planes.

So . . . for a 20C pack, you can expect 26A out of a 1300mAh (1.3*20 = 26), 36A out of a 1800mAh, and 44A out of a 22oomAh pack . . . all with a good margin above your needed 15A.

One last thing to note . . . Battery OEMs are NOTORIOUS for over-rating the C value. There's some good and bad reasons for this, but when picking packs keep this in mind. In this case, I wouldn't go under 20C (hard to find lower than that anyways) and form the numbers I don't see much need to go above 35C (it's just added weight and cost for no benefit to you).

I'd also note . . . a 30A ESC is overkill for that motor. If it's sold in a pack, I wouldn't worry about it, but when you spec an ESC separate from the motor, I advise add 10% to the motor's max current and call it good. so for that motor, a 17.5A ESC would be fine . . . typical next-size-up would be an 18A ESC. getting less chances burning out the ESC, and more is extra weight and cost without much benefit.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#6
Hey folks!

I am having mega trouble understanding what battery type will work the best...ie what C rating I will need to use.....I am thinking of buying this motor and esc....


https://www.amazon.com/gp/product/B00XBZWIHE/ref=ox_sc_act_title_3?ie=UTF8&psc=1&smid=AF5VK40LEGWAH


Hobbymate HB2806 brushless motor, 1450 Kv Hobbywing 30A speed controller,

Motor specification:
Kv: 1450rpm/V
Shaft Dia: ø3.00mm
Weight (about): 34g
Unload current: 0.9A (7.4)
Max current: 14.5A (7.4)
Lipo Battery: 2S-3S
Recommend Propeller: 7*6
Thrust: 530g
Hobbywing Pentium 30A ESC specification:

1.1 Output: Continuous 30A, Burst 40A up to 10 Secs


please help!
From the specs supplied your motor can draw 14.5A @7.4V (2S). Therefore at 3S it will obviously draw somewhat more. My guess is that a 25A esc would do but the 30A is just a little more safety margin.

As for the battery, you need to find what the battery size recommended is and try to use a battery around that size and weight. As for the "C" rating it is simply a matter of multiplying the battery capacity by the "C' rating. As an example a 2200mA 20C battery can supply 2200 x 20 mA which is 44000mA. Divide milliamps by 1000 to get Amps and so 44000/1000 = 44Amp.

This would work but as the battery capacity is reduced then the "C" rating must increase. a 1300mA 3S battery would need to have a "C" rating of at least 30C for a max current of 39Amp. A higher rating than required is not a bad thing but a lower rating will give a very short life for the battery indeed.

Just a note ensure the battery and ESC get some cooling air as well.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#7
From the specs supplied your motor can draw 14.5A @7.4V (2S). Therefore at 3S it will obviously draw somewhat more. My guess is that a 25A esc would do but the 30A is just a little more safety margin.
Hai-Lee . . . that's not quite how it works.

If they spec 14.5A as the maximum, it's the maximum current regardless of the voltage. Going over current risks the motor, regardless of the voltage driving it.

It's the current that generates heat in the wires, not the voltage. Voltage merely drives the current against the load -- while it allows a higher RPM (bigger magnetic field, but bigger back-EMF), it's the current through the wires that generates the limiting heat buildup. To set that maximum current for a given motor and voltage, you need the appropriate load (right sized prop). For a higher voltage, the motor will need a smaller prop to keep the current constant, but that smaller prop will spin proportionally faster to make up for it . . . generating more thrust overall than the longer, slower spinning prop.

From experience, 3S spinning a 7x6 prop on a 1450kv motor will draw around 15A, so their spec for the prop is about right for 3S . . . Quick check and eCalc agrees, estimating 13.6A on a gemfan 7x6. Drop down to 2S and that same prop will demand a mere 8A of current -- A waste of motor weight for how little power that will provide. To make the motor pull it's weight at 2S you'd need closer to a 10x4.7 (lower voltage, bigger prop), which is estimated to draw of 14.2A on 2S.


As for the C-rating math . . . checks out, but that motor will smoke long before 40A . . . but to even get to a 40A demand using a 1450kV motor on 3S would need an 10x4" prop (on a beefier 1450kV motor, naturally). The math is right, but the data plugged in is high, so the estimate of the need is high.



Mike,

I still say the motor is good with a 3S 20C+ pack between 1300 and 2200mAh (positioned to balance the plane), throwing a 8x4 prop (at 14.8A eCalc estimates is a bit over the limit . . . so don't run full throttle for more than 60s at a time and it'll all be fine). Any ESC above 18A will last longer than the motor does.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#10
Henry,

http://www.ecalc.ch

It can be a bit of an unwieldy tool, but with a bit of practice there's a lot it can tell you. It's an estimator, so the numbers it generates should be taken with a grain of salt, but they're generally close enough to get a sense of go/no-go on a setup, run a comparison between known and candidate setups, or just get a feel for trends.