What are the formulas to use when determing a brushless setup?

[Gee]

I have some more of the same questions. If you search the forum you would find I have asked similar ones. This time it a new approach. Just the facts. There must be some basic formulas for matching components for a brushless setup. I know before the plug and play systems now day they were used. Not by me but I heard you guy’s punching your calculator buttons when I would as about it.

I’ve had people try to explain to me how I got here and why I am going east instead of west due to certain factors beyond my control. I am better off not knowing. I am fine being told 1 + 1 = 2. I can live with that and don’t need to know why. Just that that is the way its done is fine with me. So here’s a couple of question that hopefully someone can type in what formula is used with out referencing Ben Franklin, who the jokester was that tied Ben's key to the kite string, and we all know why he was out there that dark and stormy night. Simply trying to retrieve the key to his liquor cabinet. A warm brandy nightcap to help with the chill of crawling between the cold sheet for the night? Maybe it was a shot of courage, not really sure what Mrs. Franklin looked like.


Alright, so here's what I know.
Watts = amps * volts


I get confused between watts and amps so let’s just not go there and concentrate on the formulas and info needed to get to point A.

Here the info I gather to use for examples jsut a basic MT basher gearing for 35-40mph.

ESC – 6s, 22.2v max – 120amps maxx

Neu 1515 1y - 2200kv

4s – 14.8v – 4000mah – 25c lipo


Does this formula tell me the continues amp draw the lipo is capable of?
mAh/1000=Ah (e.g. 4000mAh/1000=4Ah)

Ah x c rating = amps (e.g. 4Ah x 25c=100amps)



Alright, using the information on the esc and motor above. What formula(s) could I use to determine the minimum ratings for a lipo for the
C-rating
Voltage


Now doing the same with the Motor and lipo I listed above. What is the formula to come up with to determine esc minimum specs?


Then there would be the formula for determine a motor using the known specs for the esc and lipo I have listed.
Had to edit this due to remember the formula for RPM

volts * kv = rpm (e.g. 14.8v x 2200kv = 32560)



This is what got the confusion ball rolling this time…

I read the little insert that comes with the Monster and it didn’t make real sense to me. Hopefully help with the above formulas will help me get it. But the insert has this on it in part:

Do the Math:
Not all lipos are created equal……..
………….suitable for a brushless application.


Use the continuous rating only. We suggest that you never run the system at more the 80% of the max continuous rating for the best reliability. Using insufficient battery packs will short the life of the controller

Example:
5,000mAh pack = 5Ah
5x20c = 100 amps
100 x 80% = 80 amps maxx continuous


What do they mean by system? Are they referring to the system being the battery? The system being the ESC, Motor, and Battery all together. Maybe the system is the ESC and motor with the battery not being included.

Thanks for the help!

[BrianG]

The formulas for determining everything could get quite messy. There are a lot of variables to take into consideration that aren't quantifiable (like drive style, terrain, etc), and the ones that are quantifiable may be difficult to measure for most people (wind resistance, drivetrain drag, etc). So, the best one can do is estimate based on experience and maybe a little math. But without every single variable, even some of the math uses fudge factors.

By system, they mean running the BL system at more than 80% of the max battery rating. So, if you calculate average current draw via runtime and battery capacity, and then estimate burst values (usually ~6-7x average), you can then compute the battery needed for 80% "rule". I like to have a battery where the continuous rating of the battery is equal or more than the brushless motor's burst draw.

So, let's say you have a system where you are drawing an average of 20A with 120A estimated bursts. 120A / 80% = 150A. If you have a 5Ah battery, you then need a C rating of 30. Of course, that assumes the battery C rating and capacity figures are true, which opens up a whole new can of worms.

Sometime soon (hopefully very soon), there will be an easier way to help everyone pick the right components without fear of damaging something due to improper component selection. Just keep an eye on the RCM main site and it will happen.

[lincpimp]

Unfortunately numbers are not the strong suit of this hobby. The mfg ratings are all over the place, and cannot be directly compared. So experience is the best bet. I know that a 10-12lb MT running 4s lipo geared for 40mph needs 150amps cont capable lipo. Go up to 6s and you can get away with 120amps cont. As Brian says there are just too many variables, so no set formulas...

[Finnster]

Personally, I like to work backwards from my expected wattage, and design from there.
Knowing how much wattage you need is the tricky part, but with some experience or reading Eagletree threads for similar trucks, you can get a ballpark.

For a 1/8th scale MT or truggy of average weight, to get 40ish mph, you will need 1600-1800W peak. To get to 45-50, you need 1800-2100W. The "at speed" wattage will always be low, and you will spend most of your driving accelerating, ie mostly peak wattage, so I don't worry about that.

So, for 35-40 mph in your example, lets figure 1700W.
Now just decide what voltage you would like to run at.
4s? I figure good lipos will hold 3.5V/cell at peak. So, since W=V*A => A=W/V:

A=1700W/(4*3.5V)=1700W/14V=121A peak.

Always overdesign, so given CC's advice of 80%, simply devide 121A/80%=150A (OTOH 121/.8) Just like Linc said.

If you want to be weird and run 7s? 1700W/(7*3.5V)=70A 12s? 1700/(12*3.5)=40A

Ok... So what motor? I like to run my gearing around 13:1 or so, and motors around 28K-32K. Some people like to wind the hell out of the motor, which is fine for big speeds, but doesn't seem to make eff setups for lower speed apps, as you have to lower the gearing alot. 20:1 in a truggy is kinda dumb IMO. Gearing should be independent of kv and voltage specs.

So, figure 30000rpm / (#cells*3.5V) = ballpark Kv.
4S: 30000rpm/(4*3.5V)= 2150kv.

You can do this the opposite way to guess gearing from desired rpm, but you should have some idea of where good gearing ranges should be. 11:1 to 17:1 is a normal range for trucks.

So you got cell count, and desired amps, and the Kv picked out. For batts, use the continous rating versus what you figured your peak amps to be. You will mostly be either peak or off throttle when driving, so only worry about this.

Batts:
For runtime.. You can use needed C ratings as a relative measure to guess runtime. If you design your setup to require 40C batts, runtime will be short. At 30/25C you will get an avg runtime and 15/20C will give you long runtimes. For racing you probably want to size the lipo figuring 30C values or so (depending on race length.) For bashing I would figure ~20C or so where you don't worry about overly heavy batts.
Ie. need 150A batts. If bashing, use 20C, 150/20=7.5 7.5Ah batts will give long bash sessions. If racing, 150/30=5. 5Ah batts should make a 10min race.

For 4S we said 150A was our expected draw. Ensure the lipos you choose have a continous rating close to this. For 5Ah batts, you need 30C min. For 8Ah, you need less than 20C. There is some play in this. Its not like 5Ah 25C batts won't work, but 20C cont/30C burst will be pushing it. If they are batts like Neus and Hyperions and whatnot that have much higher burst ratings, ie 25C/50C, a continous draw will be fine. If they are chitty batteries like MA, better go up in C or Ah ratings for safety. Actually, if they are MA batts, they lie about C ratings so much, just go up in Ah, then kick yourself in the nuts for buying such expensive, but chitty batts. ;)

At the end, this gets you close enough where the final tweeking can be done with gearing to dial the setup in. You can always cheat the numbers in certain ways depending on your app. Light truck, easy race track, and/or short racing, you can drop the batt size down given enough C. If you have a hvy truck, long races, and/or aggressive driving, figure bigger batts/more amps/higher C etc.

[Gee]

Damn foiled again. You guys are going to give up one of those formula someday and I am going ot be there to see it.

Okay, so there isn't a way to accuratlely know the burst of your setup without some data recording program. So you estimated the burst?

I am also starting grasp the higher voltage concept. If you have the controller to let you run the higher voltage it pays off. It was nice when I though 4s was all I needed.

Does the C rating have any direct relation to the lenth of runs you get. For the sake of arguement everything is the same except the batteries c-rating. I read somebody said that the lower c rating don't do well with the burst of power. That takes it toll on the battery and will either hit the LVC sooner or the Life Cycle of the battery ends sooner?

Thanks for the answers. I understand all the factors that play into getting accurate information. I also know in a sense what batteries my setups need but was looking for a way to detemine a little better what I could use when just going for a walk in the rain and on the other side of the spektrum chasing cars on the interstate. Like running a rustler or LS10 I shouldn't need a 30c battery when they are geared for the Sunday drive but the G3R and Sav XL that are geared more to push the limits do. Most of the hobby is built around trying it, only limited by your creativity and mostly your wallot. I would of bunch of battieries just ot see how they work if the wallot was up for it.

Brian. I've gone though your calculators and info looking for anwers. Seems most everything is there except a battereis mah or c-rating. Is there a way to take a some of the data from one of them have a more scientific way of saying I need at least a 3300 25c lipo even if it is a broad range and the c-factors for alot of lipos are whacked it would help a person out. They have a lot of lipos to choose from these days. I didn't have this much of a problem wheh it was just a couple companies and only a few lipos to choose from.

I just found the calculator for Battery Power Requirements for R/C Vehicle. Hate to ask how long ago that was added.



Thanks finnster. It made sense, but I am going to read it another ten times to make sure and see what I come up with.

[BrianG]

Currently, there is no page that "suggests" battery and C rating for a setup. Part of that is because it was never designed that way, part is for liability, and part is because there is a project I'm working on with Mike, and I cannot say any more.

Furthermore, suggesting a mAh/C rate battery relies heavily on the fact that certain manufacturers are honest, which isn't always the case. Even though they may rate a pack for 5Ah @ 30C doesn't mean it will live up to it in the real world. So, someone may choose one of those packs and still have problems. Unfortunately, the only way to know for sure is to rely on other users' findings.

Finnster's method of using a power figure is perfectly fine, you just have to know around what a specific setup will require up front, and that takes experience which noobs don't have.

[Finnster]

I'd say there are good rules of thumb, rather than good rules.

Just like cars and hp/lb. You could do the same thing w/ watts/lb for BL cars.
20hp/lb is a normal commuter car. 10hp/lb is a fast car.

Say 170W/lb is good for 40mph. 2500w in a 10lb car is going to be really freaking fast.
Got a 20lb Baja? How does ~3500W sound for 40ish? 5000W is prolly a real fast baja.

I had 1200W+ in my CRT.5 when I was braking 60mph. This was ~280W/lb.

[BrianG]

Yup, there is definitly merit in what you say Finn. For the project I'm doing, I use this method somewhat, with a few differences.

[lincpimp]

I would say that a higher c rated pack will not drop voltage as much under load (even if the lower c rated pack can handle the load) and you will notice better performance. Not sure if runtimes will be better, but battery cycle life will be better (if both packs have similar cycle life to start with).

Using the best performing battery you can find that fits the size/weight requirements is always a good idea. I just picked up a 6s 5000mah 40c zippy from HC, and will be interested to see how it does. If it can power my big 6s trucks as well as the flightpower 4500 30c (it should) then I will be a happy camper! It is a bit big for the muggy, but should fit the lst fine. Which is good as I only have 1 4500 6s pack since I sold you the 2 4500 3s packs!

[Finnster]

NP Gee.. I may go back and edit it a bit more to clarify a few things. It can be hard to explain thigns clearly and concisely.

To add to what Linc said, I too would get as high as C rating as I can get. However, I would not design the system to require a really high C rating unless I was being really tight on weight (for racing or max performance, etc.) and was ok with shorter runtimes.

For your example: is a 25C 3300mah batt good enough?
Ok, you are talking a 4S setup
W=V*A => (4S*3.5v/cell)*(3.3Ah*25C)= 14V*82.5A=1155W

You can expect that batt of delivering ~1200W reliably. Now is that enough power for your truck? Is it a 10lb E-Revo?
=> 1200W/10lb=120W/lb.
Sounds a bit light to me. So you will have to settle for low speed, or torturing your lipo by forcing more amps out of it than its happy making.

I tried to express some methodology in a way that can be adapted to any application.
Take that same batt, and say we want it in a 12S setup for a massive 45mph 17lb Cen GST.
(12s*3.5v/cell)*(3.3*25)=42V*82.5A=3465W

Sounds not bad, but good enough..? what about runtime?
We want 45mph out of it, its an extra large truck with large tires and heavy drivetrain.. lets say we need 200W/lb* to do what we want, and make sure we have some headroom. (*This is a totally made up number, I'm guessing more accurate figures are lower. I'm just taking these from the above post.)

200W/lb*17lb=3400W

Oh.. right on the edge then of what we want. This setup would require 25C batts then. We can prolly predict an avg runtime.
Say we wanted a longer runtime, screw the weight. We said our motor needs 82.5A, so if we use a factor of 20 instead of 25, where previously we figured:
--> 3.3Ah= (required amps, ie 82.5A)/25C
we instead say: X Ah= 82.5A/20C=4.1Ah
So 12S of ~4Ah batts should give us a much longer runtime.
Note: I am not saying don't buy 25C batts and you have to use 20Cs, we're just using relative rules of thumb.

At the end of the day, we can be pretty confident of a setup that can acheive at least 45mph for a decent runtime and not get too hot and overstress the batts. Maybe this will do 50+mph in the end, but that's fine. Better to aim too high than low. If you put the batts together and the wieght seems too much, scale down a bit, but be prepared to maybe drop the speed targets a bit or stress the batts a bit more.

You never really know until its all put together and running, but at least its some way to put you in the ballpark of what you want w/o buying something way over/underpowered or overly $ for what you need.