PD or Iota for my upgrade?
I'm thinking about a converter and battery upgrade. The batteries will be wet cell lead acid 6 volt Possible T-105s or the 230AH, EGC2 from Sam's club. I'll probably go for four. (Does nayone have...
Forum Discussion
Salvo wrote:
What's the battery resistance when it is providing 10A at 12.5V, and 11A at 12.55A?
I realized I didn't answer this.
By your method, it would be R = 12.5V / 10A = 1.2 ohm!!!
That's absurd. Given the data, the resistance is 50 m ohm.
It's not absurd. The battery looks like it has a DC resistance of
R = 12.5V / 10A = 1.2 ohm
when the converter is supplying 10A to a battery that has reached a charge of 12.5V.
It looks like a DC resistance of
R = 12.55V / 11A = 1.14 ohm if the the converter is supplying 11A to a battery that has reached a charge of 12.55V.
Those are the steady state impedance values for the two different steady state conditions. Those are the values one would use for testing the DC output capabilities of a converter. The dynamic impedance right around 12.5V at that charge level is as you say, 50 milliohm.
Suppose we look later, when the battery is nearly fully charged and the current has dropped to 1A at 14.4V? Now the DC steady state performance would be modeled as a 14.4 ohm resistor. What would you use to model the dynamic impedance? Increasing the output voltage to 14.4 volts from around 12.5V has caused a decrease in current from 10A to only 1A! That's a negative dynamic resistance!
My point is that the model of a battery as a voltage plus resistor makes sense only around a relatively small range at a single voltage for a short period of time. As the battery charges, both the DC model resistance and your AC model dynamic resistance can change drastically.
None of that matters if all you want to know is how much current the charger will supply at each voltage, and for that, it is the DC model resistance that is important, not the AC dynamic impedance.
And on the question of resistance vs. impedance - they are the same for steady state conditions and for purely resistive circuits having no capacitance or inductance. Batteries aren't perfectly resistive, but they are close enough and it doesn't matter for steady state conditions anyway, so if you are more comfortable with "resistance" stick that in to replace "impedance" in my comments. :)
