Battery Calculations

Afternoon all

I’m after some advice regarding Batteries.

I’ve read in detail some articles that explains what C ratings mean on batteries and how to work out their AMP draw limits.

I currently have a 1550 and 1800 3S battery. Both rated at 75C. I’ve gone for these based on the 25A ESC’s and should give me enough of a limit (or so i think) to be safe flying with no battery issues.

However, i’ve seen posts on drone forums of people using lower C rated batteries and wonder if this is safe or not. If i need to stick to the higher C rated batteries, i’m fine with that, but there are limited capacity options for higher amp draw, where-as lower C rating batteries seem to be largely available.

My setup consists of the following:
ZMR250 frame
Chaos CF2205 2300kv motors
Chaos BHeli-S Dshot 25A ESCs
5045 Props
CC3D Revo (with GPS)
FS-X6B Receiver
Matek PDB-XT60

Based on my setup, what would i be safe using? I would prefer a higher capacity battery where possible, but i think i’m limited to 1800mah on 75C. I’ve seen Turnigy to a 2200mah 45C 3S with a burst rating of 90C, but these seem non-existent in the UK and have to order from China suppliers by the looks of it.

Good questions…

C rating are not normally actuate they are produced by the manufacturer by there own in house tests.
So take them with a pinch of salt, but higher C rating should be better.

With this you can work out the theoretical max amp PULL from the lipo (Your motors, Props PULL amps through your ESC from your lipo) — More on that in a second.

So C Rating X mAh of the lipo. gives you the max amps a lipo can/should/might give.
95C x 1300mAh (95x1.3=123.5Amp)

So when you have a High C rating on a small Capacity lipo compaired to Low C rating on a High Capacity lipo there max amp is the same. Lets look at your lipos you have listed above.

75c x 1550mAh = 116.25 Amps
75c x 1800mAh = 135 Amps
60c x 2200mAh = 132 Amps

AMPs is pulled and Volts are pushed.
So your motor and prop combination are what will pull the Amps, your ESCs have to be rated high enough. That they wont burn out. If you try to pull 40 Amps through an ESC it will (until it overheats and burns out, Some ESCs have current limit protection and will limit the pull).

Pulling more Amps than your lipo is rated for can damage your lipo and cause it to puff and shorten its life.
After flying if your lipo is really warm to hot than that’s a bad sign.

My lipos normally come down a little warm.

Remember Warm Lipos on discharge = Good
Hot Lipos on discharge = Bad
WARM or HOT lipos on Charge = BAD BAD BAD

Your limit on lipos will be more due to weight then its C rating.
Bigger the Lipo, more weight you quad has to carry and the less flight time your get. Its all about balance

Here in the UK we do need a good Lipo supplier cause ordering over seas just hurts.

For a free style quad your better of with a 4 Cell 1300 - 1500mAh 75C + Lipo.

I hope that helps with some of your questions…
Any thing else or more questions please feel free to reply back here.

Thanks for the reply. I understand the battery calc, but couldn’t find anything regarding how to calculate the amp draw for the motors/props/esc’s (either that or i wasn’t looking hard enough).

What benefit would a 4S be over a 3S? I’m unlikely to be doing acrobatics, more or less just lazy flying and possibly taking aerial photos using a GoPro. I could have bought a Mavic or something similar, but i wanted to experience the building aspect of owning a quad :slight_smile:

Try this site.

Lots of motor manufacturers will produce a data sheet to go with there motors and some combinations of props.

3S over 4S more volts… more punch, less voltage sag and longer flying times…

Just build it… its more fun and you get to learn something.

suggest you could use 1800 3s/4s or use 2200 3s with lower C- rating for your quad, but ESC you need over 30A

Can I ask how much your quad weighs without a battery?

Assuming the following:

Max Motor Pull (g): 2720.00
Props: 6045
Motor Max A: 19.5
Max Thrust per Motor (g): 680
ESC Max A: 25
Drone Weight (g): 350.00

From this site alone I would recommend the following 3S batteries:

Brand Capacity Ah Price Constant C Burst C Batt. Weight Con. P:W Bur. P:W
Tattu 1050mAh 1.05 £11.99 75 150 156.4 5.37 5.37
Tattu 1800mAh 1.80 £15.49 45 90 156.2 5.37 5.37
TATTU 1550mAh 1.55 £16.89 75 150 135.6 5.60 5.60
Tattu 1300mAh 1.30 £14.19 75 150 122 5.76 5.76

The reason being that you ‘Constant C’ matches the ‘Burst C’ in the Power:Weight column. In my opinion this is the best way to determine a safe C rating for your battery. This way you won’t experience sag and your overall battery life will not be hindered.

However, Im very new to this game and my numbers may need to be checked by a more experienced person.

Hope it helps anyway.

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And here is my 4S recommendation:

Max Motor Pull (g): 3680.00
Props: 6045
Motor Max A: 22.3
Max Thrust per Motor (g): 920
ESC Max A: 25
Drone Weight (g): 350.00
Brand Capacity Ah Price Constant C Burst C Batt. Weight Con. P:W Bur. P:W
Tattu 75 1.8 £22.17 75 150 198 6.72 6.72
Tattu R-Line 1.8 £32.93 95 190 196 6.74 6.74
Tattu R-Line 1.6 £30.44 95 190 192.6 6.78 6.78
Tattu R-Line V2 1.6 £32.89 100 200 187 6.85 6.85
Tattu 75 1.6 £21.89 75 150 180 6.94 6.94
Tattu R-Line 1.3 £25.93 95 190 164 7.16 7.16
Tattu R-Line V2 1.3 £27.94 100 200 163 7.17 7.17
Tattu 75 1.3 £17.94 75 150 156.4 7.27 7.27

You need a current meter to really work out what your draw is, however, knowing the weight of you setup is essential. Bigger batteries weigh more and it is a balance. The props you choose will also affect current draw, but the way you fly will too. One of my quads I can only but 2 bladed props on that are not bull nose because I chose the wrong motors for the weight of the quad. If I smash the throttle it will draw 160A. The ESCs are a little undersized but handle the burst rating and I occasionally fly like a loon.
Gentle cruising like you say you do, you should be able to get away with a 2200mAh 3s with a 45C rating. These are pretty cheap too