Can someone please explain how this simple circuit:
can generate this waveform?
That's an almost dead 12v SLA, a 50v 3A diode, and the scope is dc coupled. When I flip the diode it conducts and displays normally. The frequency looked suspicious, but it does not equal my mains frequency which is 59.56
Just picking up 60 Hz noise from the surrounding lines. The battery/diode has nothing to do with it per se, except acting as a slightly better antenna.
And you can't rely on the frequency value as measured by the scope. It is on the ballpark of the 60 Hz, it is mains.
Ok, got it.
As a follow-up, what I was preparing to do was protect my benchtop power supply while I reverse charged this battery. But then I realized there is no way to do that, is there? You can use a diode to forward charge a battery, but how do you protect the power supply for reverse charging? (This is an experiment in desulfation)
No, your power supply need to be able to sink current. Many supplies can do this, but usually for a limited amount of time.