While we're making requests to Dave, I'd like to see the voltage waveform of those oscillations on the scope when the battery is completely dead -- does it look like deliberate, programmed hiccup mode type thing, or is it more sinusoidal/unstable-control-loop appearance? Does it happen precisely once a second, or is it an evidently random number? See below for why I think it may be deliberate.
Hi Dave: How about putting a cap (+/- a resistor in series) to damp the oscillation at the end. Initial thought is that it should not effect the curve other than to smooth the end. Yes/No? Cheers, Mark (same post @ YT)
Adding capacitance into a control loop doesn't always reduce oscillation; often it'll make it worse.
In particular, what a dummy load is supposed to do when the runaway drop in voltage occurs is interesting to ponder -- a CP load will legitimately keep on increasing its current draw as the voltage drops, all the way down to the point where the dummy load becomes practically a short circuit. (And for the record, a capacitor wouldn't help with this at all). So, does the dummy load just stay a short-circuit forever? It's an inherently divergent system, is my point -- even with a fresh battery, there are two ways to dissipate 100mW in your load: draw 1.5V at 66 mA, or draw 0.033V at 3A (or something along those lines, I = battery's short circuit current and V = 100mW / I). So "draw 100mW" is actually an inherently ambiguous thing to request of a dummy load, and you can only expect it to find a local solution, not the local solution you were expecting.
With all this in mind, I wouldn't be surprised if the "oscillation" that we see is actually deliberate programming that notices that it has become a short circuit which no power supply can ever expect to deliver any significant power into, so it drops current to zero for a bit to let the DUT recover, and then re-starts regulation in the hope that it finds the non-short-circuit local solution. Otherwise, the load would just continue presenting a short circuit forever, even if you swapped in a brand new battery, which would be a very confusing and unintended behaviour (even if it was legitimately drawing 100mW via the short circuit).
And obviously in the case of a dead battery, it keeps on arriving back at the short circuit solution, so it keeps on pulling back and trying to find the other solution in an endless loop. That's my theory, anyway.