question: Why does your batteries performance drop if you leave it on charge while it is already fully charged?
comment: It doesn't. Not with modern batteries/chargers, anyway. The charger or the battery itself will have a circuit that prevents overvoltage/overcharging.
passages: 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.
   BULLET::::- Charging systems may attempt to gauge battery string capacity by measuring overall voltage. Due to the overall string voltage depletion due to the dead cells, the charging system may detect this as a state of discharge, and will continuously attempt to charge the series-parallel strings, which leads to continuous overcharging and damage to all the cells in the degraded series string containing the damaged battery.
   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.
   In order to prevent cell damage, fast chargers must terminate their charge cycle before overcharging occurs. One method is to monitor the change of voltage with time. When the battery is fully charged, the voltage across its terminals drops slightly. The charger can detect this and stop charging. This method is often used with nickel–cadmium cells, which display a large voltage drop at full charge. However, the voltage drop is much less pronounced for NiMH and can be non-existent at low charge rates, which can make the approach unreliable.
   Voltage depression is caused by repeated over-charging of a battery, which causes the formation of small crystals of electrolyte on the plates. These can clog the plates, increasing resistance and lowering the voltage of some individual cells in the battery. This causes the battery as a whole to seem to discharge rapidly as those individual cells discharge quickly and the voltage of the battery as a whole suddenly falls. This effect is very common, as consumer trickle chargers typically overcharge.
  
  Section::::Other problems perceived as memory effect.:Temporary effects.:Voltage depression due to long-term over-charging.:Repair.
   Memory effect
  
  Memory effect, also known as battery effect, lazy battery effect, or battery memory, is an effect observed in nickel-cadmium and nickel–metal hydride rechargeable batteries that causes them to hold less charge. It describes the situation in which nickel-cadmium batteries gradually lose their maximum energy capacity if they are repeatedly recharged after being only partially discharged. The battery appears to "remember" the smaller capacity.
  
  Section::::True memory effect.
   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.
   When recharged, the old cells recharge more rapidly, leading to a rapid rise of voltage to near the fully charged state, but before the new cells with more capacity have fully recharged. The charge controller detects the high voltage of a nearly fully charged string and reduces current flow. The new cells with more capacity now charge very slowly, so slowly that the chemicals may begin to crystallize before reaching the fully charged state, reducing new cell capacity over several charge/discharge cycles until their capacity more closely matches the old cells in the series string.
   Recovery effect
  
  The recovery effect is a phenomenon observed in battery usage where the available energy is less than the difference between energy charged and energy consumed. Intuitively, this is because the energy has been consumed from the edge of the battery and the charge has not yet diffused evenly around the battery. 
  
  When power is extracted continuously voltage decreases in a smooth curve, but the recovery effect can result in the voltage partially increasing if the current is interrupted.
   Cell reversal can occur to a weakly charged cell even before it is fully discharged. If the battery drain current is high enough, the cell's internal resistance can create a resistive voltage drop that is greater than the cell's forward emf. This results in the reversal of the cell's polarity while the current is flowing. The higher the required discharge rate of a battery, the better matched the cells should be, both in the type of cell and state of charge, in order to reduce the chances of cell reversal.
   A sulfated battery has higher electrical resistance than an unsulfated battery of identical construction. As related by Ohm's law, current is the ratio of voltage to resistance, so a sulfated battery will have lower current flow. As the charging process continues, such a battery will reach the charger's preset cut-off more rapidly, long before it has had time to accept a complete charge. In this case the battery charger indicates the charge cycle is complete, but the battery actually holds very little energy. To the user, it appears that the battery is dying.
  
  Section::::Regeneration.
   Battery users may attempt to avoid the memory effect proper by fully discharging their battery packs. This practice is likely to cause more damage as one of the cells will be deep discharged. The damage is focused on the weakest cell, so that each additional full discharge will cause more and more damage to that cell.
  
  Section::::Other problems perceived as memory effect.:Permanent loss of capacity.:Age and use—normal end-of-life.
   The problem is, the magnitude of "delta-V" can become very small or even non-existent if (very) high capacity rechargeable batteries are recharged. This can cause even an intelligent battery charger to not sense that the batteries are actually already fully charged, and to continue charging. Overcharging of the batteries will result in some cases. However, many so called intelligent chargers employ a combination of cut off systems, which are intended to prevent overcharging in the vast majority of cases.
   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.
   Conventional battery chargers use a one-, two-, or three-stage process to recharge the battery, with switched-mode power supply include more stages in order to fill the battery more rapidly and completely. Common to almost all chargers, including non-switched models, is the middle stage, normally known as "absorption". In this mode the charger holds a steady voltage slightly above that of a filled battery, in order to push current into the cells. As the battery fills, its internal voltage rises towards the fixed voltage being supplied to it, and the rate of current flow slows. Eventually the charger will turn off when the current drops below a pre-set threshold.
   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.
   Often a timer charger and set of batteries could be bought as a bundle and the charger time was set to suit those batteries. If batteries of lower capacity were charged then they would be overcharged, and if batteries of higher capacity were charged they would be only partly charged. With the trend for battery technology to increase capacity year on year, an old timer charger would only partly charge the newer batteries.
   However, they then require a longer time spent in the current-tapering off intermediate absorption charge stage after the initial bulk charge, when the LA battery charge acceptance rate gradually reduces, and the battery will not accept a higher C-rate. 
  
  When the absorption stage voltage setpoint is reached (and charge current has tapered off), the charger switches to a float voltage setpoint at a very low C-rate to maintain the battery's fully charged state indefinitely (the float stage offsets the battery's normal self-discharge over time). 
   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.
   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.
   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.
   All electrochemical cell batteries deliver a changing voltage as their chemistry changes from "charged" to "discharged". A nominal output voltage and a cutoff voltage are typically specified for a battery by its manufacturer. The output voltage falls to the cutoff voltage when the battery becomes "discharged". The nominal output voltage is always less than the open-circuit voltage produced when the battery is "charged". The temperature of a battery can affect the power it can deliver, where lower temperatures reduce power. Total energy delivered from a single charge cycle is affected by both the battery temperature and the power it delivers. If the temperature lowers or the power demand increases, the total energy delivered at the point of "discharge" is also reduced.
   For this reason it is often recommended to charge a battery that is intended to remain in storage, and to maintain its charge level by periodically recharging it.
  
  Since damage may also occur if the battery is overcharged, the optimal level of charge during storage is typically around 30% to 70%.
  
  Section::::Charging and discharging.:Depth of discharge.
   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.
   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.
  
presuppositions: Battery performance drops if left on charge while it is fully charged. A battery's performance drops if left on charge when it's already fully charged.
corrections: Battery chargers or the battery itself has circuits that prevent overvoltage/overcharging. A battery's performance doesn't drop if left on charge when it's already fully charged.
raw_presuppositions: Battery performance drops if left on charge while it is fully charged. A battery's performance drops if left on charge when it's already fully charged.
raw_corrections: Battery chargers or the battery itself has circuits that prevent overvoltage/overcharging. A battery's performance doesn't drop if left on charge when it's already fully charged.