Internal Battery Resistance Change During Use
Date: Spring 2012
Does the internal resistance of a battery change if the battery is being discharged?
Yes. Batteries play a lot of messy games with their internal resistance.
The resistance can depend on temperature, percent-of-charge-remaining, recent history of current drain, and battery aging.
Each kind of battery (lithium, alkaline, lead-acid, Ni-Cd, NiMH) has a different set of behaviors.
I have seen a lithium battery, when suddenly asked to push a large load,
allow its output voltage to drop a lot at first, because its internal resistance was high, because the battery was cold.
Then the large current flowing through that internal resistance warmed up the battery,
and the voltage delivered actually increased with time, for a little while.
There are two types of batteries in discussions of electricity, an ideal battery where the voltage remains constant regardless of the current drain, and real batteries.
The output voltage of real batteries does vary according to temperature and current load.
So, yes, the internal resistance of a battery does change if the battery is being discharged.
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Update: June 2012