E-Flight Power Sources Ask questions or read about power sources as they specifically relate to e-flight including Lithium-ion, Li-Poly, Nimh and Nicad battery packs.

Formula for determining Battery size needed

Reply

Old 10-19-2012, 07:36 AM
  #1  
RCMaister
Junior Member
Thread Starter
 
Join Date: Oct 2012
Location: Atlanta, GA
Posts: 8
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
RCMaister is offline  
Reply With Quote
Old 10-19-2012, 10:19 AM
  #2  
Dr Kiwi
Senior Member
 
Join Date: Dec 2002
Location: Chattanooga, TN
Posts: 2,198
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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
Dr Kiwi is offline  
Reply With Quote
Old 10-19-2012, 02:29 PM
  #3  
guver
Senior Member
My Feedback: (325)
 
Join Date: Mar 2003
Location: LaGrange, OH
Posts: 14,857
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default 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.
guver is offline  
Reply With Quote
Old 10-19-2012, 02:38 PM
  #4  
RCMaister
Junior Member
Thread Starter
 
Join Date: Oct 2012
Location: Atlanta, GA
Posts: 8
Gallery
My Gallery
Models
My Models
Ratings
My Feedback
Default RE: Formula for determining Battery size needed

Thank you sir!
RCMaister is offline  
Reply With Quote

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off


Thread Tools
Search this Thread

Contact Us Archive Advertising Cookie Policy Privacy Statement Terms of Service