I believe 18V and 20V lithium batteries are two ways of talking about the same thing (5 lithium cells in series, making up the battery). One refers to the nominal voltage of the pack, and the other refers to the maximum voltage of the pack. I think certain manufacturers (DeWalt, and possibly others) started using the max voltage as a marketing tactic.
Thanks that’s good to know. I’ve got a Dyson V8 and I also have a fair sized collection of Bosch blue 18V stuff. But my point was really that DeWalt 20V and Bosch 18V (for example) is the same number of cells and same pack voltage at different points in the charge cycle. It’s just that one is marketed differently.