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.