question: why cant we store electricity indefinitely in batteries?
comment: Batteries work by storing electrons between an electrolyte layer. That layer prevents the cathode from making contact with the anode, which would result in a short circuit. When a load is introduced to the battery, it pulls electrons from one side to the other, creating an electric current through the attached device. What causes batteries to lose charge overtime is that the electrolyte is not perfect. Electrons will still slip past it and move to the anode layer, which means the batteries capacity is lowered. When batteries are connected to electronics, they could also be powering part of the device, even if it's turned off. IE: In phones, the battery will keep the real-time clock powered on, and possibly other components, while it's turned off.
passages: 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.
   Battery life (and its synonym battery lifetime) has two meanings for rechargeable batteries but only one for non-chargeables. For rechargeables, it can mean either the length of time a device can run on a fully charged battery or the number of charge/discharge cycles possible before the cells fail to operate satisfactorily. For a non-rechargeable these two lives are equal since the cells last for only one cycle by definition. (The term shelf life is used to describe how long a battery will retain its performance between manufacture and use.) Available capacity of all batteries drops with decreasing temperature. In contrast to most of today's batteries, the Zamboni pile, invented in 1812, offers a very long service life without refurbishment or recharge, although it supplies current only in the nanoamp range. The Oxford Electric Bell has been ringing almost continuously since 1840 on its original pair of batteries, thought to be Zamboni piles.
   As a mixed string of new and old batteries is depleted, the string voltage will drop, and when the old batteries are exhausted the new batteries still have charge available. The newer cells may continue to discharge through the rest of the string, but due to the low voltage this energy flow may not be useful, and may be wasted in the old cells as resistance heating.
   Virtually all devices that operate on electricity are adversely affected by the sudden removal of their power supply. Solutions such as UPS (uninterruptible power supplies) or backup generators are available, but these are expensive. Efficient methods of power storage would allow for devices to have a built-in backup for power cuts, and also reduce the impact of a failure in a generating station. Examples of this are currently available using fuel cells and flywheels.
  
  Section::::See also.
  
  BULLET::::- Battery electric vehicles
  
  BULLET::::- Battery-to-grid
  
  BULLET::::- Cost of electricity by source
  
  BULLET::::- Distributed generation
  
  BULLET::::- Energy demand management
  
  BULLET::::- Energy storage
   BULLET::::- "Primary" batteries are designed to be used until exhausted of energy then discarded. Their chemical reactions are generally not reversible, so they cannot be recharged. When the supply of reactants in the battery is exhausted, the battery stops producing current and is useless.
  
  BULLET::::- "Secondary" batteries can be recharged; that is, they can have their chemical reactions reversed by applying electric current to the cell. This regenerates the original chemical reactants, so they can be used, recharged, and used again multiple times.
   A voltaic pile can be made from two coins (such as a nickel and a penny) and a piece of paper towel dipped in salt water. Such a pile generates a very low voltage but, when many are stacked in series, they can replace normal batteries for a short time.
  
  Sony has developed a biological battery that generates electricity from sugar in a way that is similar to the processes observed in living organisms. The battery generates electricity through the use of enzymes that break down carbohydrates.
   The effect can be overcome by subjecting each cell of the battery to one or more deep charge/discharge cycles. This must be done to the individual cells, not a multi-cell battery; in a battery, some cells may discharge before others, resulting in those cells being subjected to a reverse charging current by the remaining cells, potentially leading to irreversible damage.
  
  Section::::Other problems perceived as memory effect.:Temporary effects.:High temperatures.
  
  High temperatures can also reduce the charged voltage and the charge accepted by the cells.
  
  Section::::Other problems perceived as memory effect.:Temporary effects.:Other causes.
  
  BULLET::::- Operation below 32 °F (0 °C)
   Some rechargeable batteries can be damaged by repeated deep discharge. Batteries are composed of multiple similar, but not identical, cells. Each cell has its own charge capacity. As the battery as a whole is being deeply discharged, the cell with the smallest capacity may reach zero charge and will "reverse charge" as the other cells continue to force current through it. The resulting loss of capacity is often ascribed to the memory effect.
   Another electricity storage method is to compress and cool air, turning it into liquid air, which can be stored, and expanded when needed, turning a turbine, generating electricity, with a storage efficiency of up to 70%.
  
  Section::::Forms.:Batteries.
   This is the area of greatest success for current energy storage technologies. Single-use and rechargeable batteries are ubiquitous, and provide power for devices with demands as varied as digital watches and cars. Advances in battery technology have generally been slow, however, with much of the advance in battery life that consumers see being attributable to efficient power management rather than increased storage capacity. Portable consumer electronics have benefited greatly from size and power reductions associated with Moore's law. Unfortunately, Moore's law does not apply to hauling people and freight; the underlying energy requirements for transportation remain much higher than for information and entertainment applications. Battery capacity has become an issue as pressure grows for alternatives to internal combustion engines in cars, trucks, buses, trains, ships, and aeroplanes. These uses require far more energy density (the amount of energy stored in a given volume or weight) than current battery technology can deliver. Liquid hydrocarbon fuel (such as gasoline/petrol and diesel), as well as alcohols (methanol, ethanol, and butanol) and lipids (straight vegetable oil, biodiesel) have much higher energy densities.
   Watch batteries (strictly speaking cells, as a battery is composed of multiple cells) are specially designed for their purpose. They are very small and provide tiny amounts of power continuously for very long periods (several years or more). In some cases, replacing the battery requires a trip to a watch-repair shop or watch dealer. Rechargeable batteries are used in some solar-powered watches.
   It is possible to store energy and produce the electricity at a later time like in Pumped-storage hydroelectricity, Thermal energy storage, Flywheel energy storage, Battery storage power station and so on.
  
  Section::::Storage power stations.:Pumped storage.
   Because the transport sector uses the energy from petroleum very inefficiently, replacing petroleum with electricity for mobile energy will not require very large investments over many years.
  
  Section::::Economics.:Reliability.
   Section::::Newer approaches.
  
  Flash and EEPROM's limited write-cycles are a serious problem for any real RAM-like role, however. In addition, the high power needed to write the cells is a problem in low-power roles, where NVRAM is often used. The power also needs time to be "built up" in a device known as a charge pump, which makes writing dramatically slower than reading, often as much as 1,000 times. A number of new memory devices have been proposed to address these shortcomings.
  
  Section::::Newer approaches.:Ferroelectric RAM.
   The battery survives thousands of charges/discharges cycles. Also it is possible to lower the memory effect by discharging the battery completely about once a month. This way apparently the battery does not "remember" the point in its charge cycle.
   The term "memory" came from an aerospace nickel-cadmium application in which the cells were repeatedly discharged to 25% of available capacity (plus or minus 1%) by exacting computer control, then recharged to 100% capacity without overcharge. This long-term, repetitive cycle régime, with no provision for overcharge, resulted in a loss of capacity beyond the 25% discharge point. True memory cannot exist if any one of the following conditions holds:
  
  BULLET::::- batteries achieve full overcharge.
  
  BULLET::::- discharge is not exactly the same each cycle, within plus or minus 3%
  
  BULLET::::- discharge is to less than 1.0 volt per cell
   An alternative to rechargeable batteries for electricity storage is supercapacitors, now being used in some devices such as the mechanically powered flashlight shown here. Devices that store the energy mechanically, rather than electrically, include clockwork radios with a mainspring, which is wound up by a crank and turns a generator to power the radio.
   One important classification for batteries is by their life cycle. "Primary" batteries can produce current as soon as assembled, but once the active elements are consumed, they cannot be electrically recharged. The development of the lead-acid battery and subsequent "secondary" or "rechargeable" types allowed energy to be restored to the cell, extending the life of permanently assembled cells. The introduction of nickel and lithium based batteries in the latter 20th century made the development of innumerable portable electronic devices feasible, from powerful flashlights to mobile phones. Very large stationary batteries find some applications in grid energy storage, helping to stabilize electric power distribution networks.
   Public transport systems like trams and trolleybuses require electricity, but due to their variability in movement, a steady supply of electricity via renewable energy is challenging. Photovoltaic systems installed on the roofs of buildings can be used to power public transportation systems during periods in which there is increased demand for electricity and access to other forms of energy are not readily available.
  
  Section::::Applications.:Electronics.
   Some systems, operating at high temperature (Na-S) or using corrosive components are subject to failure even if they are not used (calendar ageing). Other technologies suffer from deterioration caused by charge-discharge cycles (cycle ageing), especially at high charging rates. These two types of ageing cause a loss of performance (capacity or voltage decrease), overheating and may eventually lead to critical failure (electrolyte leaks, fire, explosion). Some batteries can be maintained to prevent loss of performance due to ageing. For example, non-sealed lead-acid batteries produce hydrogen and oxygen from the aqueous electrolyte when overcharged. The water has to be refilled regularly to avoid damage to the battery and the inflammable gases have to be vented out to avoid explosion risks. However, this maintenance has a cost and recent batteries, such as Li-Ion, are designed to have a long lifespan without maintenance. Therefore, most of the current systems are composed of securely sealed battery packs which are electronically monitored and replaced once their performance falls below a given threshold. Sometimes battery storage power stations are built with flywheel storage power systems in order to conserve battery power. Flywheels may handle rapid fluctuations better than older battery plants.
   In the latter case, the problem occurs due to the different cells in a battery having slightly different capacities. When one cell reaches discharge level ahead of the rest, the remaining cells will force the current through the discharged cell.
  
  Many battery-operated devices have a low-voltage cutoff that prevents deep discharges from occurring that might cause cell reversal. A smart battery has voltage monitoring circuitry built inside.
   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
   Non-volatile memory retains the stored information even if not constantly supplied with electric power. It is suitable for long-term storage of information. Volatile memory requires constant power to maintain the stored information. The fastest memory technologies are volatile ones, although that is not a universal rule. Since the primary storage is required to be very fast, it predominantly uses volatile memory.
   Even just a single string of batteries wired in series can have adverse interactions if new batteries are mixed with old batteries. Older batteries tend to have reduced storage capacity, and so will both discharge faster than new batteries and also charge to their maximum capacity more rapidly than new batteries.
   In the 6th century BC, the Greek philosopher Thales of Miletus experimented with amber rods and these experiments were the first studies into the production of electrical energy. While this method, now known as the triboelectric effect, can lift light objects and generate sparks, it is extremely inefficient. It was not until the invention of the voltaic pile in the eighteenth century that a viable source of electricity became available. The voltaic pile, and its modern descendant, the electrical battery, store energy chemically and make it available on demand in the form of electrical energy. The battery is a versatile and very common power source which is ideally suited to many applications, but its energy storage is finite, and once discharged it must be disposed of or recharged. For large electrical demands electrical energy must be generated and transmitted continuously over conductive transmission lines.
  