ajw22
Well-known member
- Joined
- Nov 16, 2018
- Messages
- 511
I should have known this, and it's so blindingly obvious now, but I simply never questioned it. I apologize if this isn't news to anyone, but...
I always thought that the IR value of a cell is a stable fixed number that only changes with age. A convenient value that an IR tester spits out in seconds, from which the approximate age and health of a cell can be derived.
Not so fast... it also changes significantly with SoC and temperature!
All those times the camera refused to work in the cold? => Because the battery IR gets high when cold!
Camera coming back to life when you rub the battery warm? => Because the IR gets low when warm!
When the cordless drill loses that "Umpf" towards the end? => Because the battery IR gets high as SoC drops! (and the lower voltage)
So if any of you tested IR of nearly empty cells in a cold place... you might want to retest those cells after charging and warming up.
Here's a nice graph, link to paper below. Even a brand new cell could show 100+mOhm of resistance when at 10C and less than 10%SoC!

www.semanticscholar.org
I always thought that the IR value of a cell is a stable fixed number that only changes with age. A convenient value that an IR tester spits out in seconds, from which the approximate age and health of a cell can be derived.
Not so fast... it also changes significantly with SoC and temperature!
All those times the camera refused to work in the cold? => Because the battery IR gets high when cold!
Camera coming back to life when you rub the battery warm? => Because the IR gets low when warm!
When the cordless drill loses that "Umpf" towards the end? => Because the battery IR gets high as SoC drops! (and the lower voltage)
So if any of you tested IR of nearly empty cells in a cold place... you might want to retest those cells after charging and warming up.
Here's a nice graph, link to paper below. Even a brand new cell could show 100+mOhm of resistance when at 10C and less than 10%SoC!


Figure 9 from The impact of battery operating temperature and state of charge on the lithium-ion battery internal resistance | Semantic Scholar
Figure 9: for different temperatures charging current of 1C [10]. - "The impact of battery operating temperature and state of charge on the lithium-ion battery internal resistance"
