I am purchasing a used rex. It is my first 450 electric & I am in the market for some batt's.

I was wondering what the pros/cons of using a 4s pack are? My heli has the standard align 450L/35g drive train & the esc specs say it can handle up to 16v.
So who is running at 14v and why. I have searched around quite a bit & cannot find much info on anyone running these.

Thanks all, Noah

If you run a 4s set up you'll need to reduce the motor kv or swap pinions to accomidate the increase in voltage.
Check out the DarkHorse motor pinion calculator to assist you.
The benefit to 4s is for higher Amp draw from high performance motors, and longer run time.

From an electronics efficiency standpoint, i'll explain how it works so you can better apply the information your self.

When we measure the power output of our motors, we measure it in watts. Watts is the sum of the voltage times the amperage (V x A = W)

If our model is flying on 11 volts, and is pulling 20 amps in a hover (11 x 20) the motor is putting out approximately 220 watts of power.

The important thing to remember about this is that if the PHYSICAL LOAD on the motor does NOT change, amperage will always increase with the voltage. Obviously, the higher the amps you're pulling, the harder you're taxing your electrical system and batteries. The goal is always to keep amp consumption as low as possible while still maintaining the performance you desire.

So, if we decided to simply crank the voltage up to 14 volts without making any gearing changes to our setup, now the helicopter might be pulling 30 amps in a hover, and likewise, be producing 420 watts of power.

With this information, you can go in two directions, efficiency or more power.
If you already know that your helicopter was flying fine on the previously mentioned 220 watts of power, then by dropping your pinion size until you get your amperage draw to around 15 or 16 amps will end up giving you a lower overall power consumption compared to flying on 11 volts, but producing the same power the 11 volt battery was. (14x15 = 210 watts). The end result is happier electronics and longer flight times without a performance sacrifice (minus the extra weight of the battery, but that's for a different topic).

Likewise, if you were happy with your flight times before on 11 volts but wanted more power, you could drop your pinion to get amperage draw to around the previous mentioned 20 amps and then increase voltage to 14 volts to give you a boost to 280 watts output versus 220 without sacrificing any flight time. Or you could cut your flight time shorter all together and go for a massive power increase by using a larger pinion.

This is one of the strongest points of electric flight versus nitro in the sense that it's endlessly adjustable to get the exact performance you want, it just takes some time to get your head wrapped around the logistics of it all .


Note that all my figures in this post are entirely theoretical and are just meant to get the idea across. Theres also other factors that need to be figured in such as battery weight (4 cells are heavier than 3).

Not to mention that 14 volts with a high amp draw is probably highly unsafe on a micro heli. I know guys have run 3500+ headspeeds before but I don't think i'd want to be anywhere near that thing..

credence, You are the man. [8D][X(][8D] Thank you for the great info.... I have a few more questions if you dont mind. & please correct my calculations if they are just plain wrong. Mainly I am not sure but it seems to me that a 2200mAh batt @ 11.1v has a total of 24.4 stored watts.... I am assuming I have this correct and based on this assumtion I figured the following:

Ok Im looking at TP batt specs here. Which are as follows for the 2200mAh packs.
2200 @ 11.1V weighs 170g & the 2200 @ 14.8v weighs 218g.
If 2.2ah*11.1v= 24.4w & 2.2*14.8= 32.6w
Then 170g/24.4w= 7grams per watt for the 3s
& 218g/32.6w= 6.7grams per watt for the 4s

I figure the extra 48grams is about 5-6% weight gain for the average 750g 450 heli. While the extra 8watt gain is about 33% added power to the 3s pack.

So again I am confused if people can add 6% weight and gain 33% power why arent they. can the motor not handle the extra rpms?....
Why are more people not flying the 4s packs. Is this just too much extra weight? Or just too much extra for some people (extreeme 3D'ers)
I am very confused. I would like to invest in about 5 batts this week & unless I am totally missing something it seems like a no brainer to fly 4s packs.

Also, is one a more efficient setup than the other?
Anyone out there flying 4s packs please let me know what you think.

Thank You, Noah

I just bought a 3S, and decided to get a 4S too to try out. I'll let you know how it goes. I'll probably have to swap pinions every time I want to use it.

noguy: wattage is a product of power consumption, not storage. The main reason most people run 3S instead of 4S on a T-REX is simply because the batteries are cheaper. Also, the added weight of an extra cell can have a bigger impact with some power systems than others.

Some motors are more efficient on 3S and others work better on 4S. There's no simple 'cookie cutter' formula. There are many factors to consider. The best thing to do is to decide what flying style you want your bird to be set up for (docile flying + long flight times vs. mad performance + shorter flight times), then find motor, batt, and ESC setups others have tested and pick the one that best fits your expectations.

Good luck!

ct420 is right. You cannot calculate watts based on the batteries capacity rating.

mAH stands for milliamp hours. A milliamp hour is the amount of charge removed from the battery after drawing 1 milliamp of current for 1 hour.

You can do a general calculation on runtime if you take your battery capacity and divide it by your amp draw.

E.g 2200 mAh battery discharged at 2.2 amps (2200 mA) would last one hour. Discharged at 4.4 amps, it would last half an hour.

Here's a quck sample (note you need to use the mA (not Amp) value

2200 mAh / 25000 milliamps (25 Amps) = 0.088

To get the runtime in minutes, simply multiply by 60.

0.088 x 60 minutes = 5.28 minutes

The full formula is: (mAh / C) x 60 = Time in minutes.

Naturally, this formula assumes constant current draw. Since that isn't the case for our models (amp draw varies wildly depending on what you're doing), this basically just gives you a rough estimate on the runtime you can expect. In most cases, the actual runtime will probably be more than what you get using this calculation.


The reason most people don't go to 4S is because of expense. Batteries are more expensive, the controllers are more expensive, and you need a motor that won't cook on the extra voltage as well, and these motors are typically pretty pricey.

I have another question. Forgive me for being such a pest.

If (mAh / C) x 60 = Time in minutes. You are simple divideing the stated capacity of the batt(mah) by amp to leave the time. So it seems to me that based on how many amps you draw decides how long the pack will have output. The formula you gave is a linear function so it would seem to me that i should be able to incorporated into the watt formula. V*A=W. I dont exactly understand why I'm wrong on this.

In your previous post you gave an example of 220w for required power @ hover. If the gearing is adjusted to keep the headspeed the same & the heli requires 220w @ given headspeed. Then couldnt you say:
220w=11.1v*C ; c=19.8amps & 220w=14.8*C ; c=14.9
(2.2ah / 19.8a) * 60 = 6.7min & (2.2ah / 14.9)*60 = 8.9mins 8.9-6.7=2.2 2.2/6.7=.33

It seems that this shows the same 33% gain in power. Whether you gear for more flight power or longer flight time.
Another way I look at it is each cell stores X amount of energy. A 3s pack has 3*Xenergy & a 4s has 4*Xenergy. Should be a 33% gain in stored power(or watts).
Yet with the TP packs I checked out the heli only gains 5-6% weight.

Shouldnt a controller rated for 16v (align35g) handle the 4s fine? It would be running less amps through it @ a higher voltage. & if headspeed it geared to be the same from 3s to 4s, shouldnt the wattage be the same?

I am starting to guess that it must be that most motors are the problem. Would they just spin too fast for their design? What motors are people getting to run well @ 14.8v? I have searched around and found little specs on motors(as far as voltage ratings). Also the only stuff I find online about people running 4s setups is folks running CRAZY headspeeds.
I am looking for normal headspeeds & longer flight times.

Anyways what do you think? Also I really appriciate you guys being so helpful. Everyones input means alot to me. Thanks, Noah

Noah,

I'm definitely no math expert, but from what I can see, your calculations look OK in the sense that they show the batteries relative power output per cell versus relative weight.

I don't think wattage is the term you want to be using though which is probably what's throwing people off.
I'm not completely sure but I don't think you can get a batteries output in wattage without an active load since wattage, as was stated, is a measure of power consumption, not capacity.

The reason i'm not completely sure is that, and this is something that just now occurred to me, technically the 2200 mAh rating assumes a 1 milliamp load to begin with since that's how the capacity ratings are derived, so maybe you can assume a nominal watt output without a physical load just for theory purposes as you have done to gauge relative battery power.

Basically, your original calculation may prove correct, in a round-a-bout sort of way, at least on paper. In the real world it wouldn't apply since you wouldn't be obtaining wattage from the battery end, as the load would be coming from the motor, not the battery.

I don't know the proper answer to this question. Perhaps some one more well versed in electric and battery theory will be able to chime in.

As for the rest of your post, most controllers that ship with these helis are only built for 3 cells, if you've got one capable of 16 volts, then you're golden in that you don't need a new controller. Less expense on your part .

As for motors, my own personal experience in both using and selling brushless motors has been that load current is far more important than voltage, and most motors will work much higher than their mfg. rated voltage, but won't work much higher than their rated loaded amperage.

That said, the big difference between a motor designed for 3 cells versus 4 cells is in mechanical design and general efficiency. On 4 cells, the motor will be spinning quicker, which means the bearings not only need to be rated for a faster speed, but things like the magnets need more attention to ensure they don't dislodge during flight from increased centrifugal force. Cooling is also important since the higher RPM means more overall friction and thus heat buildup. motors designed for higher voltage usually yield more power at that voltage setting too since they're more efficient in that RPM range.

Most motors can be pushed passed their rated voltage without much trouble in my experience. How long they last, though, will vary wildly on manufacturing quality. Even from the same company, some motors are built better than others. Some can be pushed way out of range and operate flawlessly for their lifespan, others will fail after a few flights or simply refuse to work.

Case in point, i've got a motor here that i've been using in plane conversions that was originally rated for only 4 cells. I'm running 6 cells through it (22V), power output is excellent, motor stays only warm to the touch after a flight. No failure so far . Have had to replace the bearings once already though.


Anyways, for your setup, keeping track of headspeed would be the easiest way to do this. Shoot for 2500 - 2800 rpm. You will naturally get a lower draw current using the 14 volt battery versus 11 volt as long as you keep the headspeed within reasonable levels. Lowering the draw will result in prolonged flight time, which means you've got an efficient system.

Daniel, You are the man. Thank you!

You are right about me calling the batterys' stored "power" watts. That is wrong. watts are "consumption" at a given moment. The value from the calculations I gave, I think should be called Wh (watt-hours). Like the kWh (killoWatt-hours) your power company uses to charge us for total "power" used.

I think it should read like this 2.2ah=(2200mAh) 2.2Ah * 11.1V = 24.4Wh & 2.2Ah * 14.8V = 32.6Wh

yes these are all theoretical. Sorta a 'static' value versus the 'dynamic' value which would be the actual Wh's put into flight and not lost in friction, heat, & ect.

I have one 3s pack now & I think Im gonna try a 4s. With your familiarity with bl motors, do you have any motors that you would recomend for a 450 heli? One of the better quality as you described. Also would it be better for me to choose a lower kv motor of said high quality?

Thank You again, Noah

TrexTuning.com has motor evaluations for both 3S and 4S, and a little searching at HeliFreak may yeild more info.
GoodLuck

http://www.trextuning.com/4s-supertest.php


This little comparison here would do a far better job than anything i'd suggest in terms of motors for a 450 heli.

Realistically, for what you want, just about any of the motors on that list would do fine. A surprise to me was that the Align 430L is on that list. I didn't think it was rated for 4 cells.

The Align 430L is a nice motor. I've actually got one here that I used a while back on my X-400. I think it come standard with a lot of the T-rex kits now doesn't it?

It does indeed come standard, my 450 SA came with it. Unfortunately, from that little test, it seems to draw a lot of current while producing only mediocre results. Dont get me wrong, I plan on flying the hell out of it but it definitely leaves some room for improvement. Of course, it was in some pretty esteemed company, Medusa, JGF, etc all are top notch motors and it sure shows in the numbers. JGF 450th might be in my future...

Thank You guys all so much. [sm=thumbup.gif][sm=thumbup.gif][sm=thumbup.gif] All of this info has been really helpful.
& since I am not going to be "stick banging" I think I will like the added run times. I'll post some results when I get to the 4s setup.
Will probably start out with one HXT D9 2200mAh 4S1P 20-30C lipo pack for experimenting. They are only $44. And if I like the results maybe Ill invest in some TP prolites.

Noah