The Status in RC Batteries gives a very fast overview on the actual real health of the LiPo pack. To make understanding easier, the explanation will be based on a real example - as shown below. Note that not all data is always entered, so some values may not show up.

Battery: 1300mAh 3S1P 45C HobbyKing Nano-Tech
Status: FOM 1.15 @ 39 chrgs | Max I: 32A | chrg. 1.00C | IR ~7.00mΩ | IR σ 2mΩ @ 21°C | Real 32.53C | cap. 100.77%

- FOM tells us that from the expected 30C, we have more than expected. Good.
- 39 charge cycles so far
- Max recommended continuous current draw is 32A
- Battery has been charged with 1C
- IR ~ average shows us 7mΩ per cell. There are way better cells out there. OK, but not excellent either.
- IR σ is the Variance, telling us how many mΩ apart the individual cells are. Remember that the worst cell defines the overall state of the pack. In general, the smaller the σ value the better. Note the temperature at which the IR measurement has been taken is recorded
- Real C is the calculated real C Rating based on the worst cell IR of the pack. - 100.77% of the current that has been charged into the pack has been discharged. usually a higher value than 100% is due to temperature change of the pack during the process


Warning

FOM0.02, MP0.06, IR ~111mΩ σ253mΩ

In case the IR ~/ IR σ are displayed in red color - it means the variance between the cells is higher than the cell average. This is an indication that one cell of the pack is way worse than all others and that this pack can explode if not charged using a Balancer-capable charger. Best would be to retire this pack ASAP!

More detailed explanation on each information type:

• chrgs.: Actuall number of charge/discharge cycles of the Pack. In other words, how many times has this Pack been used.


• FOM: Figure of Merit is a calculation that uses measured cell internal resistance and normalizes it to cell capacity. It is very useful when comparing packs of different sizes and from different manufacturers
  The meaning is as below, taking into account that a FOM=1 means a 30C Cell (able to deliver 30C based on it's internal cell witrh the highest IR.
  • FOM < 1: below average < 30C capable
  • FOM == 1: average - 30C capable
  • FOM > 1: good - More than 30C capable
  • FOM >> 1: outstanding - much more than 30C capable



• MAX I: Regarding the estimate of the "Maximum recommended average current draw to prevent pack damage" - the max current is the square root of (6*capacity/IR)
  
  The calculation of the FOM/Current draw were developed by Mark Forsyth and Wayne Giles. As always we emphasize that this is just a pragmatic rule of thumb based on extensive practical battery testing that seems to reflect practice fairly well over a wide range of LiPos and capacities - although some recent cells like the Turnigy Graphines do better than predicted. It has some theoretical basis in a maximum allowable % of energy dissipated internally (6 Watts/AmpHr) but we don't rely on a theoretical explanation. It is a purely empirical observation that has been surprisingly useful to date.


• Real: Computed value of the Real discharge capability of the pack using the IR of the worst cell in the pack. This is the real discharge capability of the pack.
  Note: In case Real C Value is displayed in orange, it just means that this pack can be used above the recommended Max I!


• cap: percentage of capacity charged and extracted later. This is usually done doing charge/discharge cycles. Around 100% and the pack provides the same amount of capacity as has been put inside.


• chrg: Current used to charge the cells. 1C means that a pack with 1000mAh has been changed using 1A


• IR ~: This is the Average Internal Resistance of all cells. Average value is taking into account how the cells are arranges (Serial or Parallel) and computes an overall average. In the battery details view, the color will change accordingly depending on the overall IR of the pack.
  • Green: IR < = 6mΩ : Excellent
  • Yellow: 6mΩ < IR < = 12mΩ: Good/Reasonable
  • Orange: 12mΩ < IR < = 20mΩ: Aging signs
  • Red: 20mΩ < IR : Read for retirement



• IR σ: The variance of the Internal Resistance tells us how similar the cells internal resistance actually is. The smaller the IR σ, the better. The bigger the difference, and the older the cells probably are. Remember that the overall LiPo pack capability is defined by the worst cell in that same pack!