question: Why will my phone battery drop from 80% to 70% very quickly, but stay at 70% for a long time.
comment: Lots of phones (possibly all of them) use an ADC (analog to digital converter) to measure battery voltage, then the CPU says "oh hey at this battery voltage the battery is has x% of it's useable charge left". Voltage of a battery slowly drops as the charge of the battery is used. They also degrade slowly over time, so whatever piece of code translates that voltage into percentage which is now different due to degredation, will read the wrong value. As to why it stays at a certain value for a long time, the voltage doesn't reduce linearly (at the same rate always) on a battery, so it's really more of a software issue where they only factor on that voltage and not time on etc.
passages: "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:
   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.
  
  On December 28, 2017, Apple decreased the price of an out-of-warranty battery replacement for iPhone 6 or later from $79 to $29 for one year until December 31, 2018, and promised an iOS update that would provide more information about an iPhone's battery health which eventually turned out to be iOS 11.3.
  
  Section::::See also.
  
  BULLET::::- List of iOS devices
  
  BULLET::::- History of iPhone
   Fast Dormancy, or FD, is a mobile technology feature designed to reduce battery consumption and network utilization between mobile devices and their respective carrier networks during periods of data inactivity. It is currently implemented by monitoring the activity level of the mobile device and setting it to a control channel and power state appropriate for its level of activity. Each state results in different power consumption and network throughput while allowing a consistent data connection to be maintained. It is standardized by the 3rd Generation Partnership Project (3GPP) and implemented by most mobile carriers as a standard feature. Fast Dormancy is required to be supported by both the device and the carrier network in order for it to function.
   In use, the voltage across the terminals of a disposable battery driving a load decreases until it drops too low to be useful; this is largely due to an increase in internal resistance rather than a drop in the voltage of the equivalent source.
  
  In rechargeable lithium polymer batteries, the internal resistance is largely independent of the state of charge but increases as the battery ages; thus, it is a good indicator of expected life.
  
  Section::::See also.
  
  BULLET::::- Norton's theorem
  
  Section::::References.
  
  BULLET::::- Student Reference Manual for Electronic Instrumentation Laboratories (2nd Edition) - Stanley Wolf & Richard F.M. Smith
   Where E is the rated battery energy capacity and SOD (Vi) is the battery state of discharge at voltage Vi and P is the average power consumption in the time interval t1 and t2. The state of discharge can be estimated using look up table where the relationship between present voltage and SOD is captured. Determining the energy is also an issue because the energy is changing as the battery gets old. The new batteries have the total energy written on their back but the value can’t be true for all time. It can estimate the energy at highest and lowest discharge rate to decrease the error. The internal resistance also has significant impact on the discharged current. To decrease the effect of internal resistance all the phone components can be switched to their lowest power modes to minimize the discharge current when taking a voltage reading. Finally, this method uses a piece-wise linear function to model the non-linear relationship between SOF and battery voltage.
   A common process often discribed to memory effect is voltage depression. In this case, the peak voltage of the battery drops more quickly than normal as it is used, even though the total energy remains almost the same. In modern electronic equipment that monitors the voltage to indicate battery charge, the battery appears to be draining very quickly. To the user, it appears the battery is not holding its full charge, which seems similar to memory effect. This is a common problem with high-load devices such as digital cameras and cell phones.
   Battery consumption of the phones is a factor that must be taken into account when deploying beacon enabled apps. A recent report has shown that
  
  older phones tend to draw more battery power in the vicinity of iBeacons, while the newer phones can be more efficient in the same environment. In addition to the time spent by the phone scanning, number of scans and number of beacons in the vicinity are also significant factors for battery drain, as pointed out by the Aislelabs report. 
   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.
   A BMS may monitor the state of the battery as represented by various items, such as:
  
  BULLET::::- Voltage: total voltage, voltages of individual cells, minimum and maximum cell voltage or voltage of periodic taps
  
  BULLET::::- Temperature: average temperature, coolant intake temperature, coolant output temperature, or temperatures of individual cells
  
  BULLET::::- State of charge (SOC) or depth of discharge (DOD), to indicate the charge level of the battery
  
  BULLET::::- State of health (SOH), a variously-defined measurement of the remaining capacity of the battery as % of the original capacity
   BULLET::::- Capacity in mAh: mAh stands for milli Ampere-hour and measures the amount of power flow that can be supplied by a certain power bank at a specific voltage. Many manufacturers rate their products at 3.7 V, the voltage of cell(s) inside. Since USB outputs at 5 V, calculations at this voltage will yield a lower mAh number. For example, a battery pack advertised with a 3000 mAh capacity (at 3.7 V) will produce 2220 mAh at 5 V. Power losses due to efficiency of the charging circuitry also occur.
   The sodium-potassium pump is relatively slow in operation. If a cell were initialized with equal concentrations of sodium and potassium everywhere, it would take hours for the pump to establish equilibrium. The pump operates constantly, but becomes progressively less efficient as the concentrations of sodium and potassium available for pumping are reduced.
   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.
   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.
   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.
   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.
   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.
   Capacity loss
  
  Capacity loss or capacity fading is a phenomenon observed in rechargeable battery usage where the amount of charge a battery can deliver at the rated voltage decreases with use.
  
  In 2003 it was reported the typical range of capacity loss in lithium-ion batteries after 500 charging and discharging cycles varied from 12.4% to 24.1%, giving an average capacity loss per cycle range of 0.025–0.048% per cycle.
  
  Section::::Stress factors.
  
  Capacity fading in Li-ion batteries occurs by a multitude of stress factors, including ambient temperature, discharge C-rate, and state of charge (SOC).
   Battery consumption of the phones is a factor that must be taken into account when deploying beacon-enabled apps. A recent report has shown that
  
  older phones tend to draw more battery in the vicinity of iBeacons, while the newer phones can be more efficient in the same environment. In addition to the time spent by the phone scanning, number of scans and number of beacons in the vicinity are also significant factors for battery drain, as pointed out by the Aislelabs report. 
   Several representative examples of different α and corresponding k are tabulated below:
  
  Peukert's law becomes a key issue in a battery electric vehicle, where batteries rated, for example, at a 20-hour discharge time are used at a much shorter discharge time of about 1 hour.
  
  At high load currents the internal resistance of a real battery dissipates significant power, reducing the power (watts) available to the load in addition to the Peukert reduction, delivering less capacity than the simple power law equation predicts.
   A patent filed by the corporation, published in late July 2013, revealed the development of a new iPhone battery system that uses location data in combination with data on the user's habits to moderate the handsets power settings accordingly. Apple is working towards a power management system that will provide features such as the ability to estimate the length of time a user will be away from a power source to modify energy usage and a detection function that adjusts the charging rate to best suit the type of power source that is being used.
  
  Section::::Hardware.:Battery.:Controversy.
   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.
   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 voltage of a LiPo cell depends on its chemistry and varies from about 2.7–3.0 V (discharged) to about 4.2 V (fully charged), for cells based on lithium-metal-oxides (such as LiCoO); this compares to 1.8–2.0 V (discharged) to 3.6–3.8 V (charged) for those based on lithium-iron-phosphate (LiFePO).
  
  The exact voltage ratings should be specified in product data sheets, with the understanding that the cells should be protected by an electronic circuit that won't allow them to overcharge nor over-discharge under use.
   A patent filed in July 2013 revealed the development of a new iPhone battery system that uses location data in combination with data on the user's habits to moderate the handsets power settings accordingly. Apple is working towards a power management system that will provide features such as the ability of the iPhone to estimate the length of time a user will be away from a power source to modify energy usage and a detection function that adjusts the charging rate to best suit the type of power source that is being used.
   For example, for a battery with a capacity of 500 mAh, a discharge rate of 5000 mA (i.e., 5 A) corresponds to a C-rate of 10 (per hour), meaning that such a current can discharge 10 such batteries in one hour. Likewise, for the same battery a charge current of 250 mA corresponds to a C-rate of 1/2 (per hour), meaning that this current will increase the state of charge of this battery by 50% in one hour.
  