LIPO and internal resistance

[MartinJ]

Hi all.

As I continue to sink more money into this hobbie I have now acquired an Icharger 206b which, as you all know, can measure the internal resistance of each cell and the entire pack. Problem is that I don't know what is a good reading or a bad reading. Also, as tha pack ages will the IR change to give me an idea how much more life is in it?

i am currently using 3 cell packs if that makes any difference.


Thanks for any help.

Martin

[GIXERMAN]

From what I've heard in previous posts the lower the better, and all cells roughly the same. As packs get older i would say resistance will go up.

align 5200 40c 6s 2.0-25 ohm per cell
Thunder power 1.5-2.0 oohm one measured on pl6-pl8

[MarkG_R1]

LIPO and internal resistance

Pinched from errr.......somewhere else



a) Packs that are as good as new: between 0 and 5 milliOhm/cell
b) Packs that have been used dozens of times, but still feeling fairly good: between 5 and 10 milliOhm/cell
c) Packs that have been used even more, and start to feel weak: between 10 and 20 milliOhm/cell
d) Packs that almost won't hold your heli in the air anymore: over 20-25 milliOhm/cell

[Mark_T]

Probably more than you ever wanted to know, but a good summary on the subject can be found here - http://www.maac.ca/docs/2013/lipo_ba...__rev3_toc.pdf

Do remember that IR readings vary considerably as the temperature of the packs change so only compare at the same ambient temperature.

Also note that larger capacity packs will tend to have lower IR values than smaller packs, this doesn't mean the smaller packs with higher IR values are worse than your larger packs with low values.

Don't compare the IR figures you see on one charger with those from another, they do not seem to be well calibrated, just use the figures to monitor the change over time on your own packs.

[cjcj1949]

The resistance goes down as the size of the pack in mah goes up. So a 2000mah cell is good at below 3milliohms ok at 5 and and getting doubtful at 10. If you had 200mah cells the readings would be 30,50 and 100. The ir figures come from the drop in volts at a given current. For the same C rating a cell of double the capacity gives twice the current at the same voltage drop. For those who know a bit about electrics resistance is volts divided by current(amps). For internal resistance it's change in volts(volts drop) at a given current.

As Mark says it does depend on temperature a lot and the best guide is to look for a difference between cells. The Wayne Giles Meter (from electriflyer) gives useful results. My icharger 106B gives odd results on some cells. The Wayne Giles Meter does cost about the same as an icharger 106B.

[Mike00top]

If your unsure as to an IR measurement, just compare your own cells you own, not many chargers are that accurate, and only the best test gear will give spot on readings, so just compare, base this on your newest and best perfoming cells and the oldest and worst you have, then knock up a rough scale

my mickeymouse charger (turnigy) suggests a good cell is around 5 - 10 a average cell around 15 - 20 and poorer cells about 30 - 50

i do have some little 1300mah 3s that read 75 and work a treat though,

its not worth getting to hung up about it, equally important is the fact that the cells gives up its energy in a consistent manner, without puffing badly or odd cells very low or high at the end of a flight, a good way of telling if a pack is still ok , is to hook it up to a heavy load, if the cell can take it, i use a car headlamp bulb while checking the voltage checker A) when full charged and B) when at about 3.7v per cell, this tells if all cells are loading in a similar fashion, if the volt drop is roughly the same and not too bad, add that to the rough guide of charger IR, if all's ok i carry on and use em !

[dieselcarb]

less resistance the better (this means less voltage sag/drop under high load) basically more performance if ya get a drastic reduction in performance to what you once remember then yes it probly getting tired. I personally wouldt worry unless it starts swelling up