Tl;dr
My 2015 Macbook Pro Retina shuts off without any warning when the battery discharges to 75%.
According to Coconut Battery
Design Capacity: 56.7%
Made in 4/13/15
Has lasted 3,268 cycles, and the battery status is apparently 'Fair'. Simplo made my battery.
macOS Mojave Experience
I've recently started to notice a decline in my battery when I decided it would be a 'good' idea to upgrade to Mojave. Long story short: it wasn't. I have 2 desktops with dynamic wallpaper (changes throughout the day), and most of the time I leave my Mac asleep (8 hours~) to reopen it, the trackpad is hard: Macbook is dead.
macOS High Sierra Experience
On HS(10.13.6), the battery used to die at around 30-40%, which to me isn't as bad. When I opened it after it's 8 hour break, it shows me the login screen (takes a couple 4-5 seconds to wake up). Note that on Mojave, if it did survive the 8 hour nap then it woke up immediately; and I store my Mac in a cool place, the bottom feels slightly warm. Power nap is off for both situations.
Battery Issues
Recharging boots it up and shows me anywhere from 64% to 75% battery left. I've also noticed a bunch of battery glitches in the top menu, and whenever I charge it'll show me 45% then quickly switch to the 'real' percentage.
Before someone tells me to 'reset your SMC', I will point this out: System Report > Controller returns that there is no controller. I have tried resetting the SMC twice and it did reset (indicated by power adapter turning green then orange) but had no effect on the battery.
I'm not sure about getting a new battery, as I may be getting another Macbook later this year, so blowing $100 on a new battery for a 6 year computer I may not use for a long time is not a financially smart decision.
I have also forgot to mention this, but when my Mac reaches 100%, Coconut Battery says it can still be charged, and it actually can. When I "overcharge" it at 100%, turn off the charger it takes significantly longer to get from 100% to 99% if I were to just stop charging directly when 100% is reached.
So, what can I do so my Mac dies at 56% instead of 70%~?
Best Answer
Typical life expectancy ("Maximum cycle count") is 1,000 cycles. From Apple's webpages: "Your battery is designed to retain up to 80% of its original charge capacity at its maximum cycle count. For best performance, replace your battery when you reach its maximum cycle count."
In short: your battery has been heavily used and needs replacing. I'm surprised that the OS hasn't given you a notice that the battery needs servicing. It just isn't reliably holding charge.
Apple defines one cycle as a complete 100% - 0% discharge; and this also includes partial discharges cumulatively, so a discharge to 75% (before recharging) is 0.25 towards the next cycle.
If the data is to be believed, in six years, you've managed an impressive average of a complete discharge from 100%, then a recharge and another 50% discharge, every day.
There's nothing else that can be done to improve it.