Battery pack resistance is an important parameter to calculate and understand.
The DCIR of a cell is the Direct Current Internal Resistance. This is the resistance in charge and discharge to a direct current demand applied across the terminals.
If we connect cells in parallel and series, the estimation of the total resultant resistance is quite simple. We use the standard calculations as we would for resistors in parallel and series.
We can just use the equations for resistors. However, when we look at cells in parallel and series it appears more complex.
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If we assume that all of the resistances are equal to R1 we can estimate the total pack resistance based on the number of cells in series (s) and parallel (p):
OK, a very simplified case, but it gets you an estimate.
What you need to add to this is the resistance for all of the other parts in the system:
- busbars
- contactors
- joints
- connectors
- fuses
- current shunts
The total of this other resistance normally falls somewhere between 0.01 and 0.05 Ohms. For small low voltage packs this can be ignored, but worth checking on your final design and measuring.
In the battery pack database we estimate the pack resistance where we know the cell configuration and cell resistance.
This plot shows the 10s pulse power resistance plotted versus pack total energy.
The expectation is that resistance should decrease with increasing pack size.
Hello Nigel,
Thanks for this post.
I think the Total pack resistance (RT) = (s/p)*R1, and not “s/(p*R1)”. Please correct me if I am wrong.
you are absolutely correct, thank you, Nigel