Formula for determining Battery size needed
 

Does anyone know the formula for determining battery size needed for a watt / amp solution? for example I was running a 3000 5 S 40 C with a Hoffman magnetics EDF motor and a 150 amp ESC. The motor puffed my battery. A fella at the field asked me how many amps I was pulling and I told him 125 amps. He quickly did some math in his head and said I needed at least a 3300 battery. How did he figure that out? I didn't get a chance to ask him and haven't seen him since.

RE: Formula for determining Battery size needed
 

Theoretically your 3000mAh 40C pack should be able to deliver 120A (3000mA x 40 = 120A)... in practice one should ask a pack for more than ~80%.........so, about 96A max.

The calculation to get 125A goes like this 125A/40 = 3175mAh... but in practice (using the 80% rule) you'd probably need 125A/40 x 1.25 = ~4000mAh

Just to add in some other examples to demonstrate the method:

1000mAh 20c pack....thus rated for 20A... in practice (80% rule again) = 16A max

1500mAh 25C pack... rated for 37.5A.....in practice (80%) = 30A max

RE: Formula for determining Battery size needed
 

Putting aside all of the rules and general rules of thumb and inaccurate labels.

If you require an actual 125 amps then 125/C (discharge rating)= capacity (size) OR 125/capacity = "C" discharge rating

If you had a 100C pack then the size needs to be 1250 mah at a minimum. If you had a 10C pack then you need a 12500 mah pack.
If you are stuck with certain size capacity requirements of 3000-3500 mah then you need actual 36-42C packs.

I don't fly edf , but if they fly at a fairly constant power level then your desired flight time is important because that determines the minimum "C" rating and eliminate a variable.

RE: Formula for determining Battery size needed
 

Thank you sir!