PD or Iota for my upgrade?

Salvo wrote:
The output resistance of very type of voltage source (batteries, converters, power supplies, etc) is calculated using differential measurements. This is elementary electronics!

We agree about how to calculate the dynamic impedance of the load. That's not in dispute. However, this was a discussion about whether testing with a steady state resistive load is a legitimate test that is comparable to a battery load. In steady state conditions, one can't ignore the battery voltage, even though the battery is modeled by a zero impedance perfect voltage source in series with a low value resistor having a resistance equal to the dynamic impedance you calculated.

From the point of view of the converter, once it has reached its steady state output voltage, the battery (modeled as zero impedance perfect voltage source plus low value dynamic impedance resistor) looks exactly like the pure resistor. I thought your post was somewhat misleading by calculating only the dynamic resistance part of the battery model and ignoring the steady state load resistance arising from the zero impedance voltage source part of the model.

This is also elementary electronics. The dynamic impedance isn't necessarily the same as the steady state impedance. For determining the DC performance of the converter we care more about steady state than dynamic conditions. We just want to know how much DC current it can supply at what DC voltage, not how many milliseconds it takes for the converter to reach that steady state output voltage/current.

If all you were doing is talking about the few milliseconds before the converter has reached its steady output voltage, i.e., the few milliseconds it takes to converge to the output voltage (or current limit), then I agree with you, the resistive load and battery load look different. The rate at which the converter reaches the steady state voltage it is converging to by the feedback system is slightly different in the two cases for a few milliseconds. But after that time, for the hours while it is actually charging, the two loads are identical.

The battery looks like the resistive load and testing the converter with a resistive load is an accurate and controlled way of testing the DC output capabilities of the converter. The size of the resistive load needed to simulate the battery is not the dynamic resistance you calculated by itself. It is that dynamic resistance, plus a resistance equal to the "ideal" battery voltage of the model divided by the current the converter is putting through it.

If there were some sign that the converter had AC problems and oscillated uncontrollably under some loads, then I'd agree with you that the low dynamic impedance you calculated would be relevant in testing. But, I haven't seen or heard any reports for any converter that it has trouble converging to the output voltage it is trying to reach or goes into any type of AC oscillations. Thus, I think DC steady state testing with a resistive load equal to the output voltage (14.4 volts) divided by the output current we expect the converter to supply is a legitimate test.