When preparing a technical report or paper, is it better (more readable, more common) to use a space between each value and the units for it, or no space?
With space: 10 mA
Without space: 10mA
If there is a standard in academic typesetting¹ and typesetting in general, it is to use a narrow space, i.e.:
With space: 10 mA; without space: 10mA; with narrow space: 10 mA
The SI Standard says (p. 47):
a space is always used to separate the unit from the number.
However, it does not specify the width of the space. But at least, using no space at all is against this standard.
As for readability, I am not aware of any studies on this, so I can only give you my informed opinion. Consider the following examples (I use a picture to ensure that we are seeing the same thing and to be able to employ justified text):
In the first sentence, no space is used. This makes it take longer to identify the unit as it merges with the numbers, in particular with long numbers.
In the second sentence, a full space is used. Units are now read as individual words because that’s how our brain is trained to read full spaces. As our brain is also trained to process words one by one, it will first read and process 192.34 and then continue with K. In the third sentence, where narrow spaces are used, our brain is more likely to process the 192.34 K in one chunk and we also do not have the same problems as in the first sentence.
Finally, consider the list in the second and third sentence (1 Ω, 2 Ω and 3 Ω). With a full space it becomes more difficult for our brains to group what is written into individual items, as, e.g., the first Ω has almost the same distance from the preceding 1 than from the 2 that follows it. With a narrow space, this is easier.
Thus, using a narrow space is a compromise to have number and unit to be seperated less than words but more than the individual digits of the number.
¹ Some journals do not seem to care to make their publications consistent in this respect.
Different organizations may have different standards, but since you used electrical engineering units as an example, I'll highlight the 2014 IEEE-SA Standards Style Manual:
12.4 Letter symbols
In IEEE standards, letter symbols should be used rather than abbreviations. Letter symbols include symbols for physical quantities (quantity symbols) and symbols for the units in which those quantities are measured (unit symbols). The quantity and its unit can usually be separated by a non-breaking space to avoid unfortunate pagination. (emphasis added)
This document is a treasure trove of best practices for technical writing (in my opinion, at least).
You know the problem is an answer to this might be subjective to current practice and of course editors' preferences. Easy answer is your editor will point out how they want it.
Here's what I did, I opened google scholar and typed in voltage regulation and then scanned the paper entries and you know what there were no space 12V papers, small space 12 V papers and even 12-V papers from a cursory scan of 200 IEEE entries.
So best to ask your journal or conference publication chair.
Unless specified to do this a certain way in the Instructions for Authors, this is not your problem. It is the page editor's responsibility to make sure the paper matches the publiser's or journal's style sheet. Chances are they have some very intelligent software that makes most of this an automated process. Just follow the editorial process and make sure that anything sent to you pre-publication looks acceptable.