Charm's precise mechanics are not 100% certain, but I know this:
I had found some old enchanter posts of people who had done charm tests at varying CHA levels and the results indicated either no change or so small as to be within the (albeit significant) margin of error.
I did a lot of charming on animals on my druid on Al'Kabor, so I have a lot of data from it. Charm duration was ~5.5 minutes average using 20 minute charm spells. This was the case on both green mobs and dark blues. (using animal tash as well) Halfling druids have low CHA.
A charm test (an overnight log with automated casting) I did on Live using a level 25 enchanter with significant CHA on a level 1 decaying skeleton in 2015 resulted in a very similar average duration. (~55 ticks)
Daybreak recently stated that charm (and root, blind, fear) have a minimum 5% chance to break per tick. So that explains why the spell can break on level 1s with max CHA. However if it were 5% chance per tick every tick, it would break far too often compared to the observations. So that leads me to believe that the Daybreak dev omitted another roll that determines if the save throw is even checked. Currently this roll is 25% on TAKP. (total domination AA reduces it)
The Daybreak developer was explaining what charisma does for the player:
Quote:
how much merchants will charge you
bard fizzle chance
Chance of an NPC aggro'ing you when you cast pacify
Chance for SPA 63 to successfully blur your target
Chance for DI spells to heal for their full amount
Chance for SPA 22(charm), 31(mez), 34(confuse), 63(blur) to be resisted, you hit this cap at 200 CHA
Note SPA 3(movement rate?), 20(blind), 22(charm), 99(root) have a minimum 5% chance of breaking every tick regardless of how much CHA you have. (only 22 gets a CHA bonus)
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note: I added the ()s except the last one; that's copied from our code comments that I put above our logic.
So that post says that charm, mez, confuse and blur resist rolls have a CHA modifier when the spells land. Then it suggests (I'm not 100% sure on that, it's a little ambiguous) a bonus for charm tick saves.
I think the most likely conclusion given the available data is that CHA merely helps (if at all) charm hold when the chance to break is above the 5% minimum-- i.e. the NPC's MR is high enough to resist above 5%-- such that if the target is zero MR then CHA does nothing to help. Achieving that minimum is also easily accomplished with tash on the target for common NPCs and tash is not even needed if the NPC is a low dark blue.
Incidentally I made the chance 6% on TAKP since 25% * 5% seemed too good in my simulations vs. the data I had. 25% is a guess anyway and it's possible rounding errors or comparison operators (> vs >= etc) might explain the discrepancy.