What's the difference between XX Volt and XX Volt MAX on rechargeable lithium ion batteries?
There is one brand of &quot;20-volt Max&quot; tools that reasonably clearly state on their packaging something to the effect of:

Battery provides 20 volt maximum starting power. Provides 18-volts in use.

As a matter of fact, they'll sell you a simple physical adapter (and some wiring between the contacts) that will allow you to use the &quot;20-volt max&quot; batteries on their older 18-volt line of tools.
TL;DR:
&quot;Max&quot; is 98.475% marketing because &quot;Max&quot; sounds, well, like more than &quot;not-max&quot;. (And 99.238% of all statistics on the internet are made up on the spot.)