Quote:
Originally Posted by eq1
So, when I discharged this 5th time I was more careful about documenting initial voltage and voltage under a 3.5 amp load - at 1 minute for each stick. From what I've read I can calculate the sticks' internal resistances by subtracting the loaded voltage from the initial voltage and dividing the result by the current, 3.5A. For example: 8 volts initial, 7.5 volts under 3.5A load at 1 minute, divided by 3.5, which produces 0.143...
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Yes.
Just keep in mind that 1 minute delay is not needed... just the time it takes the discharge current to stabilize to a given value... usually a second or so... just be as consistent in you testing as you reasonably can.
Quote:
Originally Posted by eq1
My question is, what are the units for that 0.143? 0.143 ohms?
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Ohms law V=IR is the ratio
where V is in Volts, I is Amps, R is Ohms.
Ohms law is just a ratio relationship ... meaning it obeys rules of algebra ... so I can divide both sides by 1000 to get milliVolts, milliAmps, milliOhms.... etc... and the ratio holds true... you just have to have all terms in the same scale for the ratio to work... meaning if you use milliVolts the ratio only holds true if you also use milliAmps and milliOhms... even if you convert to a different scale after using the ratio.
Quote:
Originally Posted by eq1
I've looked all over and tried to figure it out, but I haven't found anything definitive. I have a Panasonic technical sheet that lists 2 milliohms as the 'internal impedance' of a single cell (12 milliohms for 6 cells I presume),
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Plus any influence of the connections between the cells.
Quote:
Originally Posted by eq1
but the conditions are a little different, such as a 'current' of 1000hz. I don't understand that.
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That is referring to an AC test of resistance not the DC test.
The DC test makes a few assumptions:
- Insignificant capacitance.
- Insignificant change in SoC during the time taken to perform the test.
- Significant Electro-Chemical capacity to power the test.
When these assumptions are reasonably in question the more accurate AC test is used... The AC test does not require these assumptions... which is why battery manufactures often use the AC test method instead of the DC test method.
However ... sense the DC test method is significantly cheaper and easier ... if you can establish those 3 assumptions... and you can be reasonably consistent in your testing methods ... then the DC method can give reasonably accurate results for less effort and money.