C Rating for Battery Decision Making!


#1

Hello DroneTrest Community

I’m building a quad, but I can’t decide on what battery to choose. I read online that the C rating should be multiplied with the mAh to get the current, but on a thread here about C ratings, it seems as if it’ okay to choose a really high C rated battery. So my problem is my motors need at least 25A to fly and my ESCs 30A, but if I was to choose a battery, what should I choose? If I was to choose a high C rated battery for the same mAh (say with a C*mAh of about 70A), wouldn’t that be like overkill? I mean, would too many amps kill my motor, or will the ESC maybe limit the amps to only what the motor and the ESC can handle (or need)?

I was planning on buying 2200mAh 20C 3s LiPo, but please recommend the best (in terms of battery life and performance).

Thanks


#2

The c rating is just a measure of how fast the battery can release all its energy so you will not burn a motor if it is too high (it’s the voltage that will fry your motors).

Having a too high c battery just makes it heavier so there is definitely a balance, i suggest you read our guide on choosing a battery as it covers everything


#3

Okay, so if I was to choose a battery, it would be one where mAhC=100A? I think this is what I should do because it said that the total current draw of the quadcopter is amp draw per motor * 4, having each motor drawing 25A. I feel as if I’m wrong here because prior to reading this I was suppose to buy a battery with a 40A mAhC rating, and I thought that was enough to support the 25A my motors need individually. I’m just straight up confused…

The way I now understand it is the C rating and the amp discharge rate would NOT destroy the motor, meaning I can have a battery discharge at 100A and neither my motors nor my ESCs will be affected badly.

Help pls!


#4

Yeah you are correct about c rating, and if you are building a racing quad, then yeah you will want around a 100A battery. But remember it’s not mAh * C, but Ah * C to get max continuous current output on your battery.

Also just something to think about, you could get away with less current output on your battery to save weight as many batteries have a burst current so can output more than that for a few seconds, some batteries can even output their burst current for up to 60 seconds which is more then enough for typical flying as you hardly keep full throttle for more than 30 seconds at a time.


#5

Okay thanks a lot!

Does this mean I need at minimum 100A of current? I have 4 25A motors, but at first I thought 40A would be enough to cover the current demand for all because it’s higher that 25A and I just want to be safe.

Last question: Let’s say I want to buy a LiPo. First choice is a 2000mAh 30C battery and the second is a 4000mAh 10C. Which will last longer? I would assume it’s the second one because of the lower current discharge, but I’m not too sure…


#6

Hi
If you posted some more details of your craft - motor size & make , props Frame size, Then perhaps suitable battery could be recommended.
In the meantime.
Yes , if you really require 25amps to each motor , then you require 100 amps.
Please bear in mind that this is almost certainly a maximum.
As to your second question , presume you mean flight time by " last longer "
Sorry but it’s a bit " apples and oranges "
I’m sure if you read Alex’s guides a few times then it will become clear.
In the meantime , (with apologies to electrical engineers ) , you could consider the mah rating as the size of the fuel tank. The C rating is how big the diameter of the fuel pipe leading out of the tank can be.


#7

Okay so that answered the main question here.
The specs of my DIY quadcopter is it is a 30cm x 30cm quadcopter
It uses Racerstar BR 2212 Brushless Motor 2450kV, the ESCs HW30A XXD from Banggood. Props are 6040

In the end it should weigh less than 1 kilo

Oh and weird question: I did some thinking and thought if I was to connect the battery to the components in parallel I could get away with amperage above 25A. Is it possible?
If PDBs are in parallel then I guess my idea might work

Thanks


#8

PDB is parallel.
Not sure I understand your idea.

As for battery , possibly your motors are a bit big.
If you are flying acro / performance you might get up to 100 amps. So a 1500 mah (or higher for longer flight time )75 c battery.
Maybe 2200 mah 50c be best.
If you are flying steady camera platform then a (cheaper ) lower c would be ok.
Sort of an odd combination ( to me ) - those big motors on 6" props.
I’m sure it’ll fly just fine though.


#9

Okay to summarize and maybe close this topic, if I was to choose a battery with a higher C rating for the same mAh, the one with the higher C rating would deplete faster, and for the same C rating, if the mAh was different, the one with the higher mAh would last longer.

The Ah*C rating can be as high as 200A, but my motors will only take what they need, and it will only take that much if it was in burst (but wouldn’t my ESCs, rated at 30A, be destroyed? I know that it’s the V that can destroy, but amps play a role, too…)

And the required amperage for a quadcopter is equal to the current draw of each motor times 4.

But with what uxg said, wouldn’t 1500mAh last a short time, especially with that high C rating?

Final question (sorry…) what is the battery that can support this quad and last at least 20 minutes?


#10

Your first para :
Part 1 , no , presuming they could both handle the output , they would both last the same.
Part 2 , yes , the higher mah would last longer.
( as said before , mah is size of fuel tank )
You really will not be drawing more than 20 amps let alone 30.
Para 3
Yes if each motor draws 10a total be 40 a (quad Copter )
Para 4
About 3 to 6 mins
No you cannot fly 6" props for 20 mins.


#11

I re-read this thread and now I was wondering if I can just have my motors run at about 10A. I think this will help me in finding a cheaper and smaller C rated battery, as a high C would mean a high discharge rate… correct me if I’m wrong…

I’ve been reading around the internet, and I learned that my motors will not always be drawing 25A, its maximum rated current. It may only draw even just 10A. I think this is good news, because this means I can get a battery with a Ah*C of about 60A. But please read on…

I also found this on a site: http://www.modelairplanenews.com/choosing-a-motor-speed-control-battery-pack-taking-the-mystery-out/

It said:

To know how your motor’s operation meets its manufacturer’s specifications, you must know several important numbers: current (amps), voltage (volts) and power (watts). One of the best ways to do this is with a Super Watt-Meter from AstroFlight. Plug this meter between the battery and the ESC, and you’ll be able to monitor what your power system is doing.

This is important for two reasons: first, every motor, ESC and battery operates at a specific maximum allowable current measured in amps. Exceeding their limits could soon the burn out motor, the ESC, or the battery. Even running the system at slightly above the maximum specified current could cause any or all three of your electrical components to fail prematurely. Prop size is among the variables that affect the flow of current through a system. A larger prop will pull a higher current from the battery through the ESC and make the motor run hot. Second, use the meter on your flight system to determine the best prop size to obtain the maximum recommended the motor wattage without overworking all of the electrical components. It allows you to maximize your plane’s power, at least, with that electrical system. The Watt-Meter is a worthwhile investment.

This is where I was worried about the amperage and its role in the quad, in terms of destroying it. With a high C rating the current discharge is way above the rated specs on my parts. So maybe a 2200mah 25C LiPo, like http://www.banggood.com/YW-11_1V-2200MAH-25C-Lipo-Battery-for-RC-Model-p-1014379.html?rmmds=search would suffice, if my idea is correct?

Thanks