People who don't charge their phone at night... Why?

If you want a short answer, then the answer is that it’s probably a placebo for devices with big batteries, and it’s real for foldables with much less room for packaging.

If you want a long answer (with some speculation), then the answer is that battery management is always a balance between longevity and usability. Let’s use the examples of an iPhone and a Tesla. The iPhone has an average lifetime of 2-4 years (after which 90% of shipped units will likely be recycled / refurbished), while the car has an expected lifetime of 10-20 years. Moreover, the small phone battery can be replaced for $50, while a 78.8 kWH model 3 battery can cost upwards of $10,000 and comes standard with a legally-mandated 120,000 mile warranty (in the USA). It’s very tempting to apply the same battery management strategy to the car as well as the phone. But this would be foolish.

Before we go any further, you need to understand a bit about how to measure the charge of a battery. All lithium chemistries feature a relationship between the voltage of the cell and the amount of current it can sustain. This relationship forms a curve called the “charge curve.” Since we usually try to keep batteries from exploding, charge curves generally start at or slightly above the maximum safe cell voltage (this is the true 100% of a cell), and the available current drops steadily until you reach a cutoff point where it rapidly falls to zero (usually the 0% kept before this cutoff since this region will quickly foul the internal structure of the battery). Since engineers are cautious, we usually back off of true cell range by a few millivolts. You can use some math to derive the available watt-hours of energy left in your cell by measuring the voltage, which is how you derive the percent charge.

Back to the main topic. You already know that cycling the battery causes it to degrade. But there are actually a bunch of things that will degrade a cell, such as age, temperature, and time spent at the extreme ends of the charge curve. Any properly engineered system using rechargeable batteries was designed as a compromise between cell longevity, cell performance, and system cost. In my above example, the car has a long lifespan because engineers tilted the balance in favor of cell longevity at the expense of cell performance (bigger battery pack to allow usable range between 20-80% SOC) and overall cost (bigger pack plus advanced BMS and thermal management). By contrast, the phone is only going to last a few years, and the users want to absolutely maximize the battery life per charge. Moreover, since modern phones can get new batteries cheaply, the OEM is incentivized to favor performance over cost and longevity.

So now we know that a phone should use the battery harder than a car. But phones also have another trick up their sleeve: the charge reported in the UI is often not directly representative of the actual state of charge. For example, a phone can mask some degradation by allowing it to charge deeper into the pack as it gets older (yes, this increases degradation, but it’s less important since the phone is nearing the end of its life). In addition, most phones use AI trickery to learn your schedule, which allows them to maintain the pack at 80% overnight, and then pack electrons in right before you disconnect the charger (note: charging non-LFP chemistries to 100% is generally OK so long as you don’t leave it there for an extended period). And if that isn’t enough fun for you, specialty form factor devices such as foldables are often a lot more aggressive with cell utilization to make up for packaging limitations (you don’t hear much about foldable battery issues because upmarket users generally swap devices more frequently). Now you have enough info to answer one of your two questions: yes, you can leave your phone plugged in, because the engineers who designed it understand the behavior of the average user, and are smart enough to create a device that will fail long after you sold it to get a new one.

So, if you are still reading this far, what about the 85% charge limit? The charge limit feature was rolled out primarily to foldables, which as mentioned previously are designed to get more cell performance at the expense of cell longevity. The fact that they created this feature means their engineers likely weren’t able to satisfactorily achieve a 4-year cell life while also attaining a usable battery performance during daily usage, and created this as a band-aid. So unless you are part of the minority of foldable users, this problem is not your problem, and you can sleep soundly knowing that the hundreds of EE’s who designed your cell phone were able to set up a system that will last longer than you plan to keep your phone.

Note: I can’t really speak much to how Samsung does battery management, so take what you see here with a grain of salt