Thread: Discussion Lipo "C" Ratings - A Replacement Overdue? View Single Post
Aug 12, 2011, 01:19 PM
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Rugby, UK
Joined Feb 2007
1,011 Posts
Quote:
 Originally Posted by xStatiCa I have looked through this entire thread and still scratching my head trying to figure this out . Can someone post a formula that doesn't assume anyone knows anything about what is going on? I am having a hard time figuring out what some of the numbers represent. Take a battery 6s5000 lipo with IR of each cell being 3,2,3,2,3,2. How do you calculate FOM1 (i have also seen FOM2 as a second method).
OK here goes with the assumption that you not electrically knowledgeable but explanation made as short as possible:-

IR is internal resistance of the cell in milliohms.
Ohms law is Voltage = Current x Resistance
Power (in Watts) = Volts x Amps (also = Current squared x Resistance)

A good cell has low IR so that the heat generated in the cell is low and the pack lasts longer besides delivering higher voltage at a given current.

If a Lipo cell has an internal resistance in milliohms mutiplied by the cell capacity in mAh of 12000 or less we consider it a good lipo cell.
To compare cells FOM = 12000 divided by (Cell in milliohms IR x Cell Capacity in mAh).

In your example the average cell resistance is 2.5milliohms (3+2+3+2+3+2) divided by 6

Therefore FOM = 12000/(2.5 x 5000) = 0.96 so that is a fairly good Lipo.

Notes: The IR cell values you measure are from a charger so that the resolution is only 1 milliohm which cannot be very accurate when measuring such low values. You should also measure at the same temperature to make a valid comparison. ie about 70- 77F ( 20 - 25C) as the IR is very temperature dependant.

Hope that helps.

Wayne