how do i calculate the required ‘C’ value of my quad battery.

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i’m a first time builder, new to the sport.  I’m looking to build a general purpose/explorer type quad with a high enough thrust to weight ratio to be, ‘athletic’, though not a racer.  currently figuring an ‘all up’ weight of about 1500 gr. with available thrust of 4000gr. – 5200gr.  I don’t know how to figure the required ‘C’ value from the motor amp draw values.  please help!

thanks.

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HI Jimmy,  What it comes down to is the amp draw of your motor/prop combination.  The best way to figure this out is to find some thrust tests for your motor/prop config.

for example, if we were going to use Cobra 2208 2000kv motors, we would look at this thrust data:

 

Typically a good place to start is to determine the total weight of the quad and then look at the thrust data to make sure that you have about 4x the weight of the quad in thrust.  It looks like you did ok with that calculation, and if using these motors with 4s, we would want to go with a 6×4.5 prop.  That would give us 982 of thrust per motor and about 3928g of total thrust.  These motors might not be the best choice for such a heavy quad as with a 6×4.5 prop you are really pushing them, but for the purposes of making an example we will stick with these.

You can see from the data that the amp draw is about 20A with these props, so we can use that information to examine C-Rating.

 
This article in our blog explains C-rating in more detail
The above post will give you more info than I can here.

 
Calculations:
We would be looking for a 80A output at full throttle with 6×4.5 props on Cobra 2208 2000KV motors on 4S.

So if we do the calculations, first we would have to find a battery:

let’s try a 1300 45C Lumenier Battery

1300MA-A = 1.3A

Calculate the Discharge rate amps x crating 1.3Ax45C=58.5A

So this battery wouldn’t work

 

We either need to up the C-rating, or up the capacity.

so we could choose a 45c large capacity battery, or a hi c, low capacity battery.

 

A Tattu 1300 75C battery would work on paper, but battery c-ratings aren’t always the most accurate, and to be safe, I would cut the advertised C-rating down by 1/3 to get the most accurate and safe result.

Tattu 1300 75c battery

2/3 of the c rating for accuracy…50C

1.3×50=65 Amps…. still not enough.

 

So lets see, how about a 2000mah 45C battery.

2/3 of the the c rating for accuracy…. 30C

2000mah=2amp

2amp x 30C= 60 Amp.

Still not there,

 

so let’s try an 1800mah 75C tattu

so 1.8a x 50C (a safe estimation of actual c rating)

90 Amps…….Bingo.

So the best battery to use would be the 1800mah 75C Tattu in this instance.

 
Conclusion:
The challenge when choosing the correct battery is to find a good lightweight battery that works with your setup, and also make sure that you have enough MAH for good flight times.  It would have been very easy to use a much larger battery to meet our amp requirements, and this example is in trying to find the smallest that will work for our setup.

 

Good luck!

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