Punx0r said:
Cell DC IR can be calculated from sag via V = IR just fine! It's not complicated - quite simple
Yes, you can reverse a cell in a battery, but this is not really sag. Maximum sag possible on a single cell cannot exceed the cell's open-circuit voltage. So you cannot have 5V of sag on a 4.2V cell. The reality is a 100mohm, 4.2V cell cannot source 50A, so there is no paradox.
Its not accurate. If you are "just fine" with a lack of accuracy, well good for you. I am not. Fact is, V=IR is inacurate here.
Temp? SOC? Alot more go into it. I guess teh IR is the same on a hot day or a cold day, or the cell clearly comes with 2 "IRs", one for hot days and one for cold. Right? (IR varies dramatically with temperature in a way a resistor does not).
"Importantly, IR is a measured number not an intrinsic property, so that different methods of measuring IR give slightly different numbers."
"It just behaves roughly as though it was and that makes it easy to make some predictions about battery performance without having to analyse the electro-chemistry of the internal workings in awful detail. The equations involved and their solution is not the stuff ordinary folks want to be involved in."
"1) Comparing IR values is only valid if they are measured the same way
2) Comparing IR values is only valid if they are measured at the same temperature"
"Overall, precise prediction of cell performance from a simple measurement of IR is probably too much to ask.
However it is very much better than what we currently have and is easy to do and understand for the average punter. It provides good guidance based on a fair history of practical controlled test results. Provided you use the same measuring tool each time and note the test temperature it is a reliable comparative method."
This shit was written years ago,from a scientific standpoing of DIY study by people who have been doing it for years. .... and if you want to find the reference just google the quote. That will tell you much more than I have the heart to on a random online forum. Fact of the matter is one way, on a powerlab, you get one reading, and another, on a Icharger,you get another, and yet again, you do it manually, and get a third different number, ... Which do you trust? Lol. The one that makes the most output? The one that makes the least? The average? Which? Hmmmm.... Do you trust your multimeter or a high end Powerlab? One is obviously wrong.. and you are the scientist, with empirical data.. You have to choose one measurement...
So.. are we talking about "accuracy" or "methodology" here? Is this empirical data or not? Cause if it is not, well hell, you all good with that "voltage" number you got at the end of your figuring.
6(w)*capacity/IR.
Thats all you need to keep track of. IMO. That, and or if your cell is 25+mOh, you got a problem.
Cell DC IR cannot be calculated by V=Ir "sag".... "just fine"... Nope. Not if you want accuracy.
"Sag"... Lol.