question: Why do devices such as phones take such little time to get to around 80%, and then take ages for the last 20%?
comment: The charge stored in a battery when supplied by a constant voltage is exponential, depending on the type. ELI5: you’re at a public pool on a 105° degree day that only lets people in 1 at a time. They are gonna fill that pool up until they’re shoulder to shoulder, but each person goes to a random open spot in the pool. When the pool has no one in it, people enter from wherever and they can find their spot quickly. The more full the pool gets, the more time it takes for each person to find their designated spot.
passages: Section::::Device test.
  
  Once the front-end process has been completed, the semiconductor devices are subjected to a variety of electrical tests to determine if they function properly. The proportion of devices on the wafer found to perform properly is referred to as the yield. Manufacturers are typically secretive about their yields, but it can be as low as 30%. Process variation is one among many reasons for low yield.
   BULLET::::- Cycle Testing: Mechanical movement is the predominant failure mechanism causing mechanical and electromechanical devices to wear out. For many devices, the wear-out failure point is measured by the number of cycles performed before the device fails, and can be discovered by cycle testing. In cycle testing, a device is cycled as rapidly as practical until it fails. When a collection of these devices are tested, the test will run until 10% of the units fail dangerously.
   It is easier to achieve the higher efficiency levels for higher wattage supplies, so gold and platinum supplies may be less available in consumer level supplies of reasonable capacity for typical desktop machines.
  
  Typical computer power supplies may have power factors as low as 0.5 to 0.6. The higher power factor reduces the peak current draw, reducing load on the circuit or on an uninterruptible power supply.
   Each weapon and component considered vulnerable to the effects of a rapid change in pressure, for example barrels, breech blocks and bolts, will be tested by firing one dry round at a corrected minimum of 25% over pressure and one oiled round at a corrected minimum of 25% over pressure. 25% over pressure means 25% in excess of the Service Pressure (P). The Service Pressure is defined as the mean pressure generated by the Service Cartridge at a temperature of . Such a high pressure proof is conducted with both the weapon and ammunition conditioned to an ambient temperature of .
   Another method of controlling feature creep is to maintain multiple variations of products, where features are kept limited in some variations. Because the ever-growing, ever-expanding addition of new features might exceed available resources, a minimal core "basic" version of a product can be maintained separately, to ensure operation in smaller operating environments. Using the "80/20 Rule" the more basic product variations might support the needs of about "80%" of the users, so they would not be subjected to the complexity (or extra expense) of features requested by the other 20% of users. The extra features are still available, but they have not crept into all versions of the products.
   Leakage current is also any current that flows when the ideal current is zero. Such is the case in electronic assemblies when they are in standby, disabled, or "sleep" mode (standby power). These devices can draw one or two microamperes while in their quiescent state compared to hundreds or thousands of milliamperes while in full operation. These leakage currents are becoming a significant factor to portable device manufacturers because of their undesirable effect on battery run time for the consumer.
  
  Section::::In semiconductors.
   Two main factors of why people buy so much and so often is due to planned and perceived obsolescence. This factor of production was introduced first in the United States and it revolves around the design of products. With these methods, the products are intentionally designed to be discarded after a short amount of time. As of 2012, only 1% of goods purchased were still in use after 6 months.
   Section::::Batteries.:Voltage cut-off in portable electronics.
  
  Some portable equipment does not fully use the low-end voltage spectrum of a battery. The power to the equipment cuts off before a relatively large portion of the battery life has been used.
  
  A high cut-off voltage is more widespread than perhaps assumed. For example, a certain brand of mobile phone that is powered with a single-cell Lithium-ion battery cuts off at 3.3V. The Li‑ion can be discharged to 3V and lower; however, with a discharge to 3.3V (at room temperature), about 92–98% of the capacity is used.
   Since no measurement can be perfect, this method suffers from long-term drift and lack of a reference point: therefore, the SoC must be re-calibrated on a regular basis, such as by resetting the SoC to 100% when a charger determines that the battery is fully charged (using one of the other methods described here).
  
  Section::::Determining SoC.:Combined approaches.
   "The Next Web"s Juan Buis wrote in late November 2016 that "complaints are shared [on Apple's support forums] about various iPhone models turning off when the battery falls below a certain percentage", and blames the iOS 10.1.1 update. He further wrote that "the original post [on support forums] explains that the phone dies when there’s 30 percent charge left, and many others have since replied that they’re experiencing the same." iOS 10.2, released in December 2016, adds a telemetry diagnostic tool that Apple can use to report battery consumption, for diagnosing reported issues with 30% battery shutdowns. In February 2017, Apple released a statement to "TechCrunch", saying that the iOS 10.2.1 update, released on January 23, had reduced the occurrence of battery shutdowns by "more than 80%" on iPhone 6S devices and "over 70%" on iPhone 6 devices. In full, the statement read:
   Further, this power dissipation will remain as long as the overload exists, which means that the devices must be capable of withstanding it for a substantial period. This power dissipation will be substantially less than if no current limiting circuit had been provided. In this technique, beyond the current limit the output voltage will decrease to a value depending on the current limit and load resistance.
  
  Section::::Single power-supply circuits.
  
  An issue with the previous circuit is that Q1 will not be saturated unless its base is biased about 0.5 volts above V.
   Due to inconsistent usage, it is not always clear from the context what a percentage is relative to. When speaking of a "10% rise" or a "10% fall" in a quantity, the usual interpretation is that this is relative to the "initial value" of that quantity. For example, if an item is initially priced at $200 and the price rises 10% (an increase of $20), the new price will be $220. Note that this final price is 110% of the initial price (100% + 10% = 110%).
  
  Some other examples of percent changes:
   For example, an electric space heater may have a resistance of ten ohms, and the wires that supply it may have a resistance of 0.2 ohms, about 2% of the total circuit resistance. This means that approximately 2% of the supplied voltage is lost in the wire itself. An excessive voltage drop may result in the unsatisfactory performance of a space heater and overheating of the wires and connections.
   Changes in percentages can be reported in at least two forms, percentage change and percentage point change. Percentage change represents the relative change between the two quantities as a percentage, while percentage point change is simply the number obtained by subtracting the two percentages.
  
  As an example, suppose that 30% of widgets made in a factory are defective. Six months later, 20% of widgets are defective. The percentage change is %, while the percentage point change is −10 percentage points.
  
  Section::::In computing.
   Section::::Failure modes.
  
  Thyristor manufacturers generally specify a region of safe firing defining acceptable levels of voltage and current for a given operating temperature. The boundary of this region is partly determined by the requirement that the maximum permissible gate power (P), specified for a given trigger pulse duration, is not exceeded.
  
  As well as the usual failure modes due to exceeding voltage, current or power ratings, thyristors have their own particular modes of failure, including:
   Thus, the lower limit of the reference range can be written as 4.4 (90% CI 4.1-4.7) mmol/L.
  
  Likewise, with similar calculations, the upper limit of the reference range can be written as 6.3 (90% CI 6.0-6.6) mmol/L.
  
  These confidence intervals reflect random error, but do not compensate for systematic error, which in this case can arise from, for example, the reference group not having fasted long enough before blood sampling.
   Leakage current has become more and more important as transistor sizes have become smaller and threshold voltage levels lower. A decade ago, dynamic power accounted for approximately two-thirds of the total chip power. The power loss due to leakage currents in contemporary CPUs and SoCs tend to dominate the total power consumption. In the attempt to control the leakage power, high-k metal-gates and power gating have been common methods.
   Percent changes applied sequentially "do not add up" in the usual way. For example, if the 10% increase in price considered earlier (on the $200 item, raising its price to $220) is followed by a 10% decrease in the price (a decrease of $22), the final price will be $198, "not" the original price of $200. The reason for the apparent discrepancy is that the two percent changes (+10% and −10%) are measured relative to "different" quantities ($200 and $220, respectively), and thus do not "cancel out".
   The two errors commonly committed in the early stages of a technology's development are:
  
  BULLET::::- fitting an exponential curve to the first part of the growth curve, and assuming eternal exponential growth
  
  BULLET::::- fitting a linear curve to the first part of the growth curve, and assuming that take-up of the new technology is disappointing
  
  Similarly, in the later stages, the opposite mistakes can be made relating to the possibilities of technology maturity and market saturation.
   EEPROM and flash memory media have individually erasable segments, each of which can be put through a limited number of erase cycles before becoming unreliable. This is usually around 3,000/5,000 cycles but many flash devices have one block with a specially extended life of 100,000+ cycles that can be used by the Flash memory controller to track wear and movement of data across segments. Erasable optical media such as CD-RW and DVD-RW are rated at up to 1,000 cycles (100,000 cycles for DVD-RAM media).
   For a given power supply, efficiency varies depending on how much power is being delivered. Supplies are typically most efficient at between half and three quarters load, much less efficient at low load, and somewhat less efficient at maximum load. Older ATX power supplies were typically 60% to 75% efficient. To qualify for 80 Plus, a power supply must achieve at least 80% efficiency at three specified loads (20%, 50% and 100% of maximum rated power). However, 80 Plus supplies may still be less than 80% efficient at lower loads. For instance, an 80 Plus, 520 watt supply could still be 70% or less efficient at 60 watts (a typical idle power for a desktop computer). Thus it is still important to select a supply with capacity appropriate to the device being powered.
   Maximum limits on fan-out are usually stated for a given logic family or device in the manufacturer's datasheets. These limits assume that the driven devices are members of the same family.
   BULLET::::- India: switched from the EU limits to the US limits for mobile handsets in 2012. Unlike the US, India will not rely solely on SAR measurements provided by manufacturers; random compliance tests are done by a government-run Telecommunication Engineering Center (TEC) SAR Laboratory on handsets and 10% of towers. All handsets must have a hands free mode.
   Section::::Process.:Packaging.
   Client devices in wireless networks may have conflicting requirements for power consumption and communication throughput when in power-save mode. For example, laptop computers may require relatively high communication throughput and may have low sensitivity to power consumption. Therefore, a relatively low DTIM period, for example 1, may be suitable for these devices. Pocket devices, however, may require relatively low communication throughput and may be operated by batteries of relatively low capacity. Therefore, a higher DTIM period, for example 8, may be suitable for pocket devices. But some of these have a medium to high communication throughput, while still having small batteries, so would benefit from a medium DTIM period, such as 4.
  