Direct Current

I presume we are talking about smoothed Direct Current [DC] as opposed to unsmoothed DC?

Take a look at this link when you get a chance. I think it will help answer your question.

http://www.kpsec.freeuk.com/acdc.htm

Because frequency is a product of alternating current which changes poles (depending on what part of the world you live in..) 50 or 60 times per second. So it cycles from positive to negative quite frequently! The amount of cycles are called "Hertz" named after a scientist. DC doesn't change poles (from minus to plus and again). So technically, the frequency would be 1 when you complete the circuit, but unless it's part of a cycle, it wouldn't have a frequency. There are prefixes for other types of "cycling" electrical energy. This energy is recognized by the uses for it: Kilo hertz Mega Hertz and Giga Hertz.

True DC current - i.e. from a battery is a simple straight line and therefore arguably has zero frequency.

Direct current which is rectified from AC will have a ripple waveform with a magnitude depending upon the effectiveness of the rectification and smoothing.

In theory 100% smoothing can be calculated and achieved but I remain to be convinced that it can be acheived in practice so I suggest that there will always be a ripple of a very small magnitude which mirrors the rectified AC frequency.

If the frequency of recharging the batteries in a domestic electrical torch is once-per-month, is that the same as 0.37 microhertz?