Astrum I agree with you, plasma batteries are almost only visible on ships produced between the 35 and the 37 millennium, the problem, even as our technologist, is that the adeptus mechanicus is to produce them , or the world which produced them, to disappear, more the rare ships which always have this technology are put in reserve for the protected ones, moreover it is necessary to quote that gene of ship produced since the beginning of the Imperial have defects , for example, the class of large cruiser created on the pleans of the East Terminus, have a defect of shield Gellar, like many model of the time, which already creates a deficit in size.
https://wh40k.lexicanum.com/wiki/Imperial_Navy_Vessels_(List)
(above many Imperium ships are quoted and explained in detail, this is an exellent site)
@Inquisitor_Marcus_Corvinus
What order of the Inquisition are you from?