question: why do car batteries lose their ability to be charged after sitting unused for a while?
comment: I believe it’s because of sulphation in the battery plates. Sulphur crystals form in some of the plates, and this damages/degrades the plates. The sulphation occurs as the battery discharges normally. When you leave a lead-acid battery to sit unused/uncharged it starts to slowly discharge and undergo this process. The damage/sulphation is proportional to the level of neglect, and even in normal use there will be a degree of this degradation with successive discharge/recharge cycles. If not maintained properly, after a period of time the battery will reach a point where the sulphation damage is so bad that the battery (at best) can only be partially recovered. Usually, by this point, it is simply unserviceable and a new one is required.
passages: In the past, auto batteries required regular inspection and maintenance to replace water that was decomposed during operation of the battery. "Low-maintenance" (sometimes called "zero-maintenance") batteries use a different alloy for the plate elements, reducing the amount of water decomposed on charging. A modern battery may not require additional water over its useful life; some types eliminate the individual filler caps for each cell. A weakness of these batteries is that they are very intolerant of deep discharge, for example when the car battery is completely drained by leaving the lights on. This coats the lead plate electrodes with sulfate deposits and can reduce the battery's lifespan by a third or more. 
   Automotive lead–acid rechargeable batteries must endure stress due to vibration, shock, and temperature range. Because of these stresses and sulfation of their lead plates, few automotive batteries last beyond six years of regular use. Automotive starting batteries have many thin plates to maximize current. In general, the thicker the plates the longer the life. They are typically discharged only slightly before recharge.
   If batteries are used repeatedly even without mistreatment, they lose capacity as the number of charge cycles increases, until they are eventually considered to have reached the end of their useful life. Different battery systems have differing mechanisms for wearing out. For example, in lead-acid batteries, not all the active material is restored to the plates on each charge/discharge cycle; eventually enough material is lost that the battery capacity is reduced. In lithium-ion types, especially on deep discharge, some reactive lithium metal can be formed on charging, which is no longer available to participate in the next discharge cycle. Sealed batteries may lose moisture from their liquid electrolyte, especially if overcharged or operated at high temperature. This reduces the cycling life.
   As with all lithium-ion batteries, electric vehicle batteries may degrade over long periods of time, especially if they are frequently overcharged, however, this may take at least several years before being noticeable.
  
  However, Nissan stated in 2015 that thus far only 0.01 percent of batteries had to be replaced because of failures or problems, and then only because of externally inflicted damage. The vehicles that had already covered more than , have no problems with the battery.
  
  Section::::Batteries.:Future.
  
  BULLET::::- Autonomous park-and-charge
   If a battery has been completely discharged (e.g. the car lights were left on overnight) and next is given a fast charge for only a few minutes, then during the short charging time it develops only a charge near the interface. The battery voltage may rise to be close to the charger voltage so that the charging current decreases significantly. After a few hours this interface charge will spread to the volume of the electrode and electrolyte, leading to an interface charge so low that it may be insufficient to start a car.
   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.
   Some battery manufacturers include a built-in hydrometer to show the state of charge of the battery.
  
  The primary wear-out mechanism is the shedding of active material from the battery plates, which accumulates at the bottom of the cells and which may eventually short-circuit the plates. This can be substantially reduced by enclosing one set of plates in plastic separator bags, made from a permeable material. This allows the electrolyte and ions to pass through but keeps the sludge build up from bridging the plates.
  
  Section::::Environmental impact.
   Motor vehicles, such as boats, RVs, ATVs, motorcycles, cars, trucks, etc have used lead–acid batteries. These batteries employ a sulfuric acid electrolyte and can generally be charged and discharged without exhibiting memory effect, though sulfation (a chemical reaction in the battery which deposits a layer of sulfates on the lead) will occur over time. Typically sulfated batteries are simply replaced with new batteries, and the old ones recycled. Lead–acid batteries will experience substantially longer life when a maintenance charger is used to "float charge" the battery. This prevents the battery from ever being below 100% charge, preventing sulfate from forming. Proper temperature compensated float voltage should be used to achieve the best results.
   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. 
   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.
   NiCd cells, if used in a particular repetitive manner, may show a decrease in capacity called "memory effect". The effect can be avoided with simple practices. NiMH cells, although similar in chemistry, suffer less from memory effect.
  
  Section::::Lifetime.:Environmental conditions.
   Batteries have a finite number of charging cycles, as well as a shelf-life, therefore using vehicles as grid storage can impact battery longevity. Studies that cycle batteries two or more times per day have shown large decreases in capacity and greatly shortened life. However, battery capacity is a complex function of factors such as battery chemistry, charging and discharging rate, temperature, state of charge and age. Most studies with slower discharge rates show only a few percent of additional degradation while one study has suggested that using vehicles for grid storage could improve longevity.
   Which electrical practices, and so which charger, are best suited for use depending entirely on the type of battery. NiCd cells must be fully discharged occasionally, or else the battery loses capacity over time due to a phenomenon known as "memory effect." Once a month (perhaps once every 30 charges) is sometimes recommended. This extends the life of the battery since memory effect is prevented while avoiding full charge cycles which are known to be hard on all types of dry-cell batteries, eventually resulting in a permanent decrease in battery capacity.
   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.
   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.
   Recent EVs are utilizing new variations on lithium-ion chemistry that sacrifice specific energy and specific power to provide fire resistance, environmental friendliness, rapid charging (as quickly as a few minutes), and longer lifespans. These variants (phosphates, titanates, spinels, etc.) have been shown to have a much longer lifetime, with A123 types using lithium iron phosphate lasting at least 10+ years and 7000+ charge/discharge cycles, and LG Chem expecting their lithium-manganese spinel batteries to last up to 40 years.
   The active material on the battery plates changes chemical composition on each charge and discharge cycle; active material may be lost due to physical changes of volume, further limiting the number of times the battery can be recharged. Most nickel-based batteries are partially discharged when purchased, and must be charged before first use. Newer NiMH batteries are ready to be used when purchased, and have only 15% discharge in a year.
   Pacific Gas and Electric Company (PG&E) has suggested that utilities could purchase used batteries for backup and load levelling purposes. They state that while these used batteries may be no longer usable in vehicles, their residual capacity still has significant value.
  
  Section::::Specifics.:Lifespan.
  
  Individual batteries are usually arranged into large battery packs of various voltage and ampere-hour capacity products to give the required energy capacity. Battery service life should be considered when calculating the extended cost of ownership, as all batteries eventually wear out and must be replaced. The rate at which they expire depends on a number of factors.
   It is possible however to fully discharge a battery without causing cell reversal—either by discharging each cell separately, or by allowing each cell's internal leakage to dissipate its charge over time.
  
  Even if a cell is brought to a fully discharged state without reversal, however, damage may occur over time simply due to remaining in the discharged state. An example of this is the sulfation that occurs in lead-acid batteries that are left sitting on a shelf for long periods.
   Lithium iron phosphate batteries reach, according to the manufacturer, more than 5000 cycles at respective depth of discharge of 70%.BYD, the world's largest manufacturer of lithium iron phosphate batteries, has developed a wide range of cells for deep cycle applications through precision manufacturing. Such batteries are in use in stationary storage systems. After 7500 cycles, with discharge of 85%, they still have a spare capacity of at least 80% at a rate of 1 C; which corresponds with a full cycle per day to a lifetime of min. 20.5 years.
   Lithium ion batteries are perishable to some degree; they lose some of their maximum storage capacity per year even if they are not used. Nickel metal hydride batteries lose much less capacity and are cheaper for the storage capacity they give, but have a lower total capacity initially for the same weight.
   SLI batteries are not designed for deep discharge, and their life is reduced when subjected to this.
  
  Car batteries using lead-antimony plates require regular topping-up with pure water to replace water lost due to electrolysis and evaporation. By changing the alloying element to calcium, more recent designs have reduced the rate of water loss. Modern car batteries have reduced maintenance requirements, and may not provide caps for addition of water to the cells. Such batteries include extra electrolyte above the plates to allow for losses during the battery life.
   An effect with similar symptoms to the memory effect is the so-called "voltage depression" or "lazy battery effect". This results from repeated overcharging; the symptom is that the battery appears to be fully charged but discharges quickly after only a brief period of operation. In rare cases, much of the lost capacity can be recovered by a few deep-discharge cycles, a function often provided by automatic battery chargers. However, this process may reduce the shelf life of the battery. If treated well, a Ni–Cd battery can last for 1,000 cycles or more before its capacity drops below half its original capacity. Many home chargers claim to be "smart chargers" which will shut down and not damage the battery, but this seems to be a common problem.
   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.
   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.
  