question: Why do phone batteries degrade over time such that they go from 100% to 1% very quickly, but can continue to run at 1% for hours on end?
comment: Nearly anything that uses fuel has a "reserve", an amount below empty that's there in case you plan badly. Cars have the same thing with their fuel gages. You can get a fair amount of distance out of it when the gauge says E.
passages: When exposed to high temperatures, the lithium-ion batteries in smartphones are easily damaged and can fail faster than expected, in addition to letting the device run out of battery too often. Debris and other contaminants that enter through small cracks in the phone can also infringe on smartphone life expectancy. One of the most common factors that causes smartphones and other electronic devices to die quickly is physical impact and breakage, which can severely damage the internal pieces.
  
  Section::::Examples.
   Section::::Hardware.:Battery.
  
  The average phone battery lasts 2–3 years at best. Many of the wireless devices use a Lithium-Ion (Li-Ion) battery, which charges 500-2500 times, depending on how users take care of the battery and the charging techniques used. It is only natural for these rechargeable batteries to chemically age, which is why the performance of the battery when used for a year or two will begin to deteriorate. Battery life can be extended by draining it regularly, not overcharging it, and keeping it away from heat.
  
  Section::::Hardware.:SIM card.
   BULLET::::- In South Africa the code on a battery to indicate production date is part of the casing and cast into the bottom left of the cover. The code is Year and week number. (YYWW) e.g. 1336 is for week 36 in the year 2013.
  
  Section::::Use and maintenance.
  
  Excess heat is a main cause of battery failures, as when the electrolyte evaporates due to high temperatures, decreasing the effective surface area of the plates exposed to the electrolyte, and leading to sulfation. Grid corrosion rates increase with temperature. Also low temperatures can lead to battery failure. 
   Most modern cell phones, laptops, and most electric vehicles use Lithium-ion batteries. These batteries last longest if the battery is frequently charged; fully discharging the cells will degrade their capacity relatively quickly, but most such batteries are used in equipment which can sense the approach of fill discharge and discontinue equipment use. When stored after charging, lithium battery cells degrade more while fully charged than if they are only 40-50% charged. As with all battery types, degradation also occurs faster at higher temperatures. Degradation in lithium-ion batteries is caused by an increased internal battery resistance often due to cell oxidation. This decreases the efficiency of the battery, resulting in less net current available to be drawn from the battery. However, if Li-ION cells are discharged below a certain voltage a chemical reaction occurs that make them dangerous if recharged, which is why many such batteries in consumer goods now have an "electronic fuse" that permanently disables them if the voltage falls below a set level. The electronic fuse circuitry draws a small amount of current from the battery, which means that if a laptop battery is left for a long time without charging it, and with a very low initial state of charge, the battery may be permanently destroyed.
   Batteries also have a small amount of internal resistance that will discharge the battery even when it is disconnected. If a battery is left disconnected, any internal charge will drain away slowly and eventually reach the critical point. From then on the film will develop and thicken. This is the reason batteries will be found to charge poorly or not at all if left in storage for a long period of time.
  
  Section::::Chargers and sulfation.
   Some deterioration occurs on each charge–discharge cycle. Degradation usually occurs because electrolyte migrates away from the electrodes or because active material detaches from the electrodes. Low-capacity NiMH batteries (1,700–2,000 mA·h) can be charged some 1,000 times, whereas high-capacity NiMH batteries (above 2,500 mA·h) last about 500 cycles. NiCd batteries tend to be rated for 1,000 cycles before their internal resistance permanently increases beyond usable values.
  
  Section::::Lifetime.:Charge/discharge speed.
  
  Fast charging increases component changes, shortening battery lifespan.
  
  Section::::Lifetime.:Overcharging.
  
  If a charger cannot detect when the battery is fully charged then overcharging is likely, damaging it.
  
  Section::::Lifetime.:Memory effect.
   Old rechargeable batteries self-discharge more rapidly than disposable alkaline batteries, especially nickel-based batteries; a freshly charged nickel cadmium (NiCd) battery loses 10% of its charge in the first 24 hours, and thereafter discharges at a rate of about 10% a month. However, newer low self-discharge nickel metal hydride (NiMH) batteries and modern lithium designs display a lower self-discharge rate (but still higher than for primary batteries).
  
  Section::::Lifetime.:Corrosion.
  
  Internal parts may corrode and fail, or the active materials may be slowly converted to inactive forms.
  
  Section::::Lifetime.:Physical component changes.
   In December 2016, Apple revealed new details about the issue, stating that the affected devices contained a "battery component that was exposed to controlled ambient air longer than it should have been before being assembled into battery packs". The statement also said that customers "outside of the affected range" may also experience unexpected shutdowns, though clarifying that some shutdowns can occur under "normal conditions" because the iPhone tries to "protect its electronics". They provided a highly discounted battery replacement to customers for a year. Apple also said that an upcoming update to the iOS operating system would include additional diagnostic tools to help Apple find more information about different kinds of battery issues.
   The safety circuit determines that a pack has been used when its voltage falls below a threshold, or if it detects a minimum number of pulses. Testing a pack is not normally sufficient to cause it to become "used", but normal operation should make it "used" around the time 1% of its capacity has been depleted. The safety circuit causes the pack to become "depleted" when it falls below its end-of-life voltage and turns off. A used pack will continue to function normally, but a depleted pack can no longer be used.
   If the battery is stored or repeatedly operated in this partially charged state for an extended period, the film will slowly crystallize into a solid. This process of "sulfation" takes time, so it only has a chance to build to significant levels if the battery is repeatedly discharged below this critical level. There are numerous other conditions that can lead to the same problem developing.
   A company spokesperson stated that it had received 35 reports of battery failure, which "account for less than 0.1 percent of the entire volume sold". Samsung stated that the hazard was limited to a small fraction of phones manufactured, and released a tool on its website on 19 September to identify affected units by their unique IMEI numbers.
   BULLET::::- Sometimes, the display lights up for a few seconds and then goes out again. This happens when the phone battery depletes by one "bar", which may be seen as pointless as the lights coming on would increase drain on the battery. This was meant to be an early warning for low battery.
  
  BULLET::::- Sometimes, for no apparent reason, an image or video clip is copied.
   Users started reporting battery drain issues after the February security update. The battery drain become worse after the April security updates. Google is looking for a fix.
  
  Users are reporting proximity sensor issues. Google is aware of the issue and looking for a fix.
  
  Some users are having issues making calls and receiving MMS messages. Google claims these issues are fixed but users are still complaining about them.
   Complete discharge of multi-cell packs can cause reverse polarity in one or more cells, which can permanently damage them. This situation can occur in the common arrangement of four AA cells in series in a digital camera, where one completely discharges before the others due to small differences in capacity among the cells. When this happens, the good cells start to drive the discharged cell into reverse polarity (i.e. positive anode/negative cathode). Some cameras, GPS receivers and PDAs detect the safe end-of-discharge voltage of the series cells and perform an auto-shutdown, but devices such as flashlights and some toys do not.
   The deep discharge testing is itself damaging to batteries due to the chemicals in the discharged battery starting to crystallize into highly stable molecular shapes that will not re-dissolve when the battery is recharged, permanently reducing charge capacity. In lead acid batteries this is known as sulfation but also affects other types such as nickel cadmium batteries and lithium batteries. Therefore, it is commonly recommended that rundown tests be performed infrequently, such as every six months to a year.
  
  Section::::Batteries.:Testing of strings of batteries/cells.
   BULLET::::- Lifetime – Around 60,000 hours. For PEM fuel cell units, which shut down at night, this equates to an estimated lifetime of between ten and fifteen years.
  
  Section::::Uses.
   Section::::Lifetime.:Self-discharge.
  
  Disposable batteries typically lose 8 to 20 percent of their original charge per year when stored at room temperature (20–30 °C). This is known as the "self-discharge" rate, and is due to non-current-producing "side" chemical reactions that occur within the cell even when no load is applied. The rate of side reactions is reduced for batteries stored at lower temperatures, although some can be damaged by freezing.
   An equalization charge cycle, having a higher voltage-low current profile, may be occasionally utilized to partially reverse a battery sulfation condition; some SLA 'smart' battery chargers either manually or automatically perform equalization cycles at infrequent times.
  
  Some designs can be fast charged (1 hour) at high rates. Sustained charging at 2.7 V per cell will damage the cells. Constant-current overcharging at high rates (rates faster than restoring the rated capacity in three hours) will exceed the capacity of the cell to recombine hydrogen and oxygen.
  
  Section::::History.
   In November 2016, Apple announced that a "very small number" of iPhone 6S devices manufactured between September and October 2015 have faulty batteries that unexpectedly shut down. While Apple noted that the battery problems were "not a safety issue", it announced a battery replacement program for affected devices. Customers with affected devices, which span "a limited serial number range", were able to check their device's serial number on Apple's website, and, if affected, receive a battery replacement free of charge at Apple Stores or authorized Apple Service Providers.
   On April 28, 2014, Apple initiated an out of warranty recall program to replace any failing power buttons of iPhone 5 models which were manufactured prior to March 2013 at no cost. On August 23, 2014, Apple announced a program to replace batteries of iPhone 5 models that "may suddenly experience shorter battery life or need to be charged more frequently" which were sold between September 2012 and January 2013.
  
  Section::::History.:Litigation.
   The lithium iron phosphate battery Sony Fortelion has after 10,000 cycles at 100% discharge level still a residual capacity of 71%. This accumulator is since 2009 on the market.
  
  Used in solar batteries Lithium-ion batteries have partly a very high cycle resistance of more than 10,000 charge and discharge cycles and a long service life of up to 20 years.
   The computer industry's drive to increase battery capacity can test the limits of sensitive components such as the membrane separator, a polyethylene or polypropylene film that is only 20–25 µm thick. The energy density of lithium batteries has more than doubled since they were introduced in 1991. When the battery is made to contain more material, the separator can undergo stress.
  
  Section::::Safety issues and regulation.:Rapid-discharge problems.
   On December 28, 2017, amidst many complaints about older iPhone models slowing down when new ones are released, Apple released a communication to its customers on its website, acknowledging the effect that old batteries have on the iPhone's performance. The company offered $29 battery replacements as a solution.
  
  Section::::Hardware.:Camera.
   All rechargeable batteries have a finite lifespan and will slowly lose storage capacity as they age due to secondary chemical reactions within the battery whether it is used or not. Some cells may fail sooner than others, but the effect is to reduce the voltage of the battery. Lithium-based batteries have one of the longest idle lives of any construction. Unfortunately the number of operational cycles is still quite low at approximately 400-1200 complete charge/discharge cycles. The lifetime of lithium batteries decreases at higher temperature and states of charge (SoC), whether used or not; maximum life of lithium cells when not in use(storage) is achieved by refrigerating (without freezing) charged to 30%-50% SoC. To prevent overdischarge battery should be brought back to room temperature and recharged to 50% SoC once every six months or once per year
   Section::::Hardware.:Battery.
  
  By the end of 2017, smartphone battery life has become generally adequate; however, earlier smartphone battery life was poor due to the weak batteries that could not handle the significant power requirements of the smartphones' computer systems and color screens.
  
raw_presuppositions: Batteries going from 1% to 0% should be as quick as from 100% to 99%.
raw_corrections: Battery at 1% is more of an estimate and there is a reserve after that.