Juggernaut Unrelenting Passive

Can someone explain how the dmg/endurance charge works ?
I can have up to 5 endurance charges but the dmg increase is not even close to 30%.

Also the Slaughter passive from the axe dmg nodes , do I see an icon if onslaught procs ? Because I have yet to see any difference with that one too.

The endurance damage buff is "increased damage" not "more damage" modifier

Yes, onslaught should have a buff icon up top. Looks like a man holding an axe above his head or something like that.