Originally Posted by Connexxion
what values should we look for?
I.e. what IR value per cell (+/-)would you call:
And what temperature should we do the measurements?
Obviously we are looking for the lowest value of IR for the best results; ie lowest voltage drop and coolest lipo.
It is difficult to just spout numbers as it is a function of the cell capacity and seems a linear function. Thus a very good 2200 cell is about 3-4 milliohms at 25deg.cent whilst an average is about 6 - 7milliohms and a poor one 8+.
So for smaller or larger cell capacities you should multiply these numbers by 2200 and divide the result by the cell capacity in mAh.
This only a crude rule of thumb and lots of people will disagree with my figures.
There was a good discussion triggered by Mark Forsyth's suggestion of replacing the completely discredited "C" ratings with a Figure of Merit based on the IR values. It is at:
John Julian has also posted a discussion thread in which there is load of useful information at :
Mark has a calculator for determining REAL "C" values from IR figures at:
I must say that I am surprised that these have not attracted a lot more interest by modellers who wish to find out the real capablities of their lipos rather than believing the cynical "C" ratings claimed by dishonest marketeers.
Perhaps people are put off a little by the amount of information there and the effort to flog through it all, but nothing is acheived without some effort and you can jump straight to the calculator if you wish.
Anybody with a decent charger capable of reading IR, or a meter can now get so much nearer to the truth about their lipos with just a little effort.
Re your question about temperature, I used to use 25degC but am now changing to 22degC to fall in line with the calculator. Remember it is most important to allow the pack to settle for several hours in a 22degC ambient.