texaspyro said:
This test does not depend on knowing the test current, just the load resistance and cell voltage. Note that Rload should include the resistance of the leads to the cell.
Ok now to the test:
Ideally you want a load resistor that will load the cell to around at least 1C. Connect the load resistor to the battery. Give it some time to drain off the surface charge of the cell (monitor the voltage at the cell and when it somewhat stabilizes). Measure the voltage AT THE CELL. Call this Vload. Remove the load and quickly measure the cell voltage. Call this Vcell. Calculate Vdrop = Vcell-Vload. The cell Ir = (Vdrop * Rload) / (Vcell - Vdrop)
Thanks for some added details, I did now specified everything since there is a lot of other thread where I spoke about that with more clarifications.
But there is just some of them I disagree with.
It was proven that DC internal resistance measurement are more precise when doing measurement at TWO different load level than just one load level compared with the no load voltage.
Read on the Battery University, Sandra lab website and A123 technical literature, etc
The problem with single load level is HOW can you ensure that the time you wait for the no load V measurement is the best delay?.. 5 sec, 30 sec, 1 minutes.. etc… The voltage vary in time and increase when you release the load… that add inaccuracy in the measurement.
I have both Ri meter type and the two level load give a lot more accurate and repeatable measurement results.
I was surprised to see that many single load level Ri meter on the market use DIFFERENT time delay for the no load measurement!!!.. How can you ensure wich is good?.. 2 sec?, 15 sec etc… since the voltage you measure is influenced by that determined delay !
The load resistance value is not really important.. it’s the measured current and the corresponding voltage at the cell tab.
You can have a 1 ohm, or a 1.21 ohm etc.. the resistor precision is useless… you only need to draw a current to the cell that is in an appropriate range for the RI measurement. So by measuring just the current you don’t need to care with the resistance loss of the wiring, contact etc…..
For sure the resistor wattage MUST be appropriate and being at least 3 times higher than the power you will draw. To ensure is stay in the spec range
Ex: lifepo4 cell measured first with a 1.1 ohm resistor :… and you measure 2.8A and 3.400V..
That mean your total load resistance is 3.400 / 2.8 = 1.214 ohm…… including every wiring and loss ( you don’t need to care about the real ohm of the resistor !!).. just take the current and corresponding voltage value!
Repeat that two times with a different load and do the math.
Yes The Ri at known C rate is important… but the most important is to know the RI at the C rate YOU will use your battery !
This is the spec you will use… This is what you need to know.
Unless you do some opensource battery test and want to compare with the manufacture official specs.
Doc