iPhone batteries are one of the first things that people will notice starting to go out in their smartphone over the course of ownership. According to statistics, the average American holds onto their smartphone for about 22 months before moving on to another one - and most people would prefer to keep their phones for as long as possible (https://www.cnbc.com/2019/05/17/smartphone-users-are-waiting-longer-before-upgrading-heres-why.html)
The Technology The main impediment to people keeping their battery is the first thing to go - the battery. Lithium Ion batteries (or Li-ion) are a long standing technology that nearly all phone manufacturers use. While they are long term stable and relatively harmless, they do suffer from a few issues,
Cycles - Li-ion batteries as a rule normally have a useful life of about 300 to 500 cycles before a notable deterioration begins. A cycle is a full charge up and down, so a charge from 0 to 100 counts as 1 cycle, as well as a charge from 0 to 50 and 50 to 100, would count as one total cycle. This isn't a hard and fast rule, batteries have been replaced as low as 300 cycles for early failure, or records of into the 2000 cycles range. We use diagnostic software in all of our locations to determine health compared to capacity.
Temperature - Li-ion batteries are particularly susceptible to extreme temperatures which can contribute to early failure. A couple times left in a hot car generally isn't enough to cause a problem, but consistent exposure to extreme hot and cold can cause the battery to physically degrade, in the form of expansion.
Research and Findings Having replaced hundred of batteries over the years, there are a few rules of thumb that I have determined, 1. Most people will notice that their battery is not functioning very well once it has hit the 70-75% maximum capacity mark. This is also the point where, at low percentages, the percentage may not be accurate. This often leads to the phone turning off when it still purports to have between 1-10% left. 2. Maximum battery capacity generally degrades at a rate of about 10% per year of use, or about 10% per 350ish cycles. This gives Li-ion batteries a "useful life" of around 3 years, which is a bit further than the average length of time that the average person holds onto their phone for. 3. One the maximum capacity of the battery gets to around the 30-50% range, the battery becomes incapable of maintaining a minimum amount of power to the phone, causing the phone to stay turned off/only work when on cable power. This roughly corresponds to 5-8 years of usage.
In the end, if you're concerned that your battery isn't up to snuff, feel free to drop by. We'll take a look at it and let you know how much longer (on average) you may have. Replacing phone batteries is one of the best ways of increasing the longevity of your devices, reducing electronic waste, and avoiding getting a new device at a carrier store or Apple store.