Yeah I am not gonna lie I am pretty disappointed with how fast the battery degrades with the iPhone 15 pro max models. Mine is a launch day phone with the battery originally being new in August 2023 and first usage being in September. My iPhone 14 pro max battery health had barely degraded by the time I traded it in for the 15 pro max. I believe this time last year my iPhone 14 pro max was still at 97% battery health after almost a year of usage.
My launch day 15 pro max was just at 94% last week just dropped to 93% yesterday. I don't even consider myself a heavy phone user and only have 209 cycles. I used low battery mode yesterday when it hit under 30% charge apparently this damages the battery and degrades it instantly I guess cause my battery health dropped from 94 to 93 like that.
I am not upgrading this year since I upgraded back to back with the 14 pro max and 15 pro max. I do have AppleCare so I am stuck waiting until the battery health drops to 79% so they can replace it. But given how fast this is degrading now, that probably will happen in the next month or two lol.
Also, to be clear overall, I am happy with the iPhone 15 Pro Max and I do make it a full 8 hours or more a day on a single charge. Just disappointed with how fast the battery health has degraded.