Calculating Sag voltage ?

[jk1]

Is their a way to calcutae what the sag voltage would be if you know the C rate of the cells and how many AH capacity you have ?

I know sometimes you find discharge graphs of 1 cell, that shows voltage SAG per C rate load, but how do you use that when you have a large capacity pack ? it would be less sag the more cells in parallel you have ?

 

[izeman]

Is their a way to calcutae what the sag voltage would be if you know the C rate of the cells and how many AH capacity you have ?
I know sometimes you find discharge graphs of 1 cell, that shows voltage SAG per C rate load, but how do you use that when you have a large capacity pack ? it would be less sag the more cells in parallel you have ?

no. there is not. not from C-rate and capacity. what you need to know is IR - internal resistance of the cells/pack.

IR = (Vunloaded - Vloaded) / Current
if you reshape the calculation you can calculate your deltaV (or voltage sag) at a given current for a given cell with known internal resistance.

if you know the voltage sag of a single cell at a determined current you can easily calculate the sag of the big pack. let's make an example:

voltage sag of a single cell (2500mAh) at 3.5v and 1A is 0.3V (taken from the graph)
you now have a 20s10p pack and you will need 40A @70V peak. so it's 40 x 0.3V / 10p = 1.2V total sag.

rem: numbers are just imaginary and depend on chemistry, age of cells, voltage, temperature, etc ...

 

[dogman dan]

Nice, great post. I was just going to say you don't know what the c rate is, till you know the internal resistance.

The c rate may not be what they told you, and in any case, it changes over time, getting less and less. This is why we like to oversize the pack enough to run at a fraction of stated c rates.