Battery capacity wear-down, and its relationship to charging practices As an Android user with many devices I'm usually not very concerned about battery, but Nexus 6P with Snapdragon 810 is somewhat notorious for its short battery life, so it makes me start to actually observe and care about it.

Even before that, I have a plug-in type USB voltage/current meter like this one below, that also measures the mAh that passed through:



With this I measure the full capacity (i.e. from the moment the device automatically powers off to it shows 100% and the meter shows no current drawn) of the battery, and compare the value to designed value from time to time.

Lately my Nexus 6P just went over its 1-year mark, and I did this measurement, which shows that the capacity shrank a considerable 400mAh from when I just got it. Before that I also checked my Nexus 4 which when retired was on its 2.5th year, and it shrank a whopping 600mAh, 28.6% of the design value.

Nexus 6P comes with USB-PD fast charging rated at 5V/3A, and it's not unheard of that fast charging will "wear down the battery life more quickly", but I've never seen any concrete proof (e.g. side-by-side comparison) that came with the claim.

This question also talks about whether the battery should be charged/discharged from a certain percentage, as a technique to increase battery lifespan. I often keep the device plugged for long, and charge the device below 15%, so I'm quite curious about the technique. The concept explained there, however, is mainly discharging cycle, which is not what I care much about, since I replace batteries after the device retires. What matters to me is the decrease in capacity, which directly affects day-to-day use in terms of battery life.

So my question boils down to these:


Does battery discharge cycle have any relationship with battery capacity?
What factors could affect battery capacity wear-down?
Firstly, it appears that discharge cycle has been misunderstood. It covers both charge and discharge and is a metric to cover battery life. (See this for more details Android definition of charge cycle?).

Ideal charge / discharge limits in the question Ideal charging / discharging percentage for maximum battery life? discusses how to increase the number of such cycles and thereby life of battery



  Does battery discharge cycle have any relationship with battery capacity?


Yes

The more you use the battery , more the number of discharge cycles ( which covers both charging and discharging as explained earlier) and drop in battery capacity as illustrated in this figure, which shows capacity drop as the number of cycles increase for Li Ion batteries of different capacities



Source


  What factors could affect battery capacity wear-down?



Cycling or the amount of use in terms of charge cycles covered above
Elevated temperature : 


Physically exposing the device to high temperatures ( like on a dashboard of car on a sunny day) or 
Increase in internal temperature, by 
keeping a cell at a high charge voltage (hence the reference to ideal voltages, which doesn't permit build up of temperature )




  Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling



Aging: This source at the end talks of the chemical reasons that cause aging. Without getting into the chemistry of it, what it means is that the capacity of battery starts decreasing from the day it is manufactured, represented by the active zone in figure




Additional Information


How to Prolong Lithium-based Batteries
Charging tag wiki, especially the General and Maintenance section for good practices
Some OEMs at times provide a battery of higher capacity than rated - this would partially offset aging effects. See  Honor 6 with 3100mAh battery against 3000 mAh rating


Edit  Accubattery Pro gives you an indication of estimated Vs actual capacity as shown below. I have no idea how accurate it is but surely can be indicative