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Early notebook computers came with nickel-cadmium batteries. NiCd batteries (sometimes called Ni-Cad, a registered trademark of SAFT Corporation) have largely been superseded by improved technologies.
Nickel-cadmium batteries are particularly prone to a condition usually called the memory effect. There are two different causes, but the symptoms are the same. A fresh nickel-cadmium battery will maintain approximately 1.25 volts until late in the discharge cycle when the voltage quickly drops to near zero volts.[1] If a nickel-cadmium battery is repeatedly partially discharged and then fully charged, it will begin to lose significant voltage near the end of the discharge cycle. It will maintain this lower voltage until the normal end of the discharge cycle is reached. Over time, the first voltage drop occurs earlier and earlier in the discharge cycle until the battery becomes useless. The same happens if the battery is repeatedly partially charged and fully discharged. Nickel-cadmium batteries should always be fully charged before use and then fully discharged before recharging.
A nickel-cadmium battery suffering from the memory effect may be revived by completely discharging[2] and completely recharging the battery several times.
Old nickel-cadmium batteries may become shorted because electrolyte crystals grow between the metal plates. Such batteries may be revived by placing a significant voltage (12 volts limited to 1 ampere) across the terminals—positive to positive and negative to negative—for a very short time. This may vaporize the crystals and give the battery some more functional life.
Nickel-metal hydride batteries are a direct replacement for nickel-cadmium batteries. They come with greater capacities and are less prone to the memory effect. Most rechargeable AA and AAA batteries are nickel-metal hydride. They are rarely used in notebook computers these days.
Lithium-ion and related batteries are currently the most common choice for notebook computer batteries. They have a much higher power density (high capacity in a small package) than other current technologies and are not prone to the memory effect. However, lithium-ion batteries used in notebook computers tend to have a useful calendar life of only two to six years. This short life may be due to the common (accidental) practice of completely discharging the batteries, which can damage the copper current collector. Charging the batteries to only 80% of capacity and discharging them to only 20% of capacity is said to extend the life.
Most lithium-ion batteries consist of several 18650 cells. Batteries that have gone beyond their useful life may be rebuilt with replacement cells. However, some charge controllers in battery packs are programmed to stop working after a certain number of charge-discharge cycles. It may be possible to buy third-party charge controllers, but those may be incompatible with the computer or power supply/charger.
Lithium-ion polymer (aka lithium polymer) batteries use a polymer (gel) electrolyte instead of a liquid electrolyte. They have a higher energy density than regular lithium-ion batteries and are the most common battery in cell phones. Lithium-ion polymer batteries have recently become the most common battery in notebook computers.
Lithium-ion polymer batteries are usually made in a flat package instead of the round cells used for standard lithium-ion batteries.
Lithium-ion polymer batteries may be more prone to some of the safety issues of lithium-ion batteries. All current lithium-ion batteries have a flammable electrolyte. Lithium-ion batteries can catch fire under conditions that can occur in any part of the charge-discharge cycle. There have been several incidents of notebook computers catching fire on commercial airliners. Batteries in electric vehicles also catch fire occasionally.
Lithium-ion polymer batteries, having no metal casing, often inflate when damaged by overcharging, over-discharging, heat, or other issues.
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These bloated batteries contain flammable and toxic gasses. The terminals should be taped over, and the battery should be taken to an e-waste collection site if possible (do not mail damaged batteries). Some say to discharge them first by putting a 12-volt, 20-watt lightbulb across the terminals, but that is debated. Others suggest dropping such batteries in a bucket of salt water, which is a terrible idea since lithium reacts violently with water.
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