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The limitations of notebook battery life can be a pain, although the situation's better than it used to be. Over the years, the running time you can expect on a single charge has crept up. With a Centrino laptop you can now get up to five hours, a huge improvement on the hour or two you could manage a few years ago. But batteries still need looking after, and there are things you can do both to improve the time you get out of them per charge and increase their overall lifespan.

Although we live in a super-accurate digital world, batteries are resolutely non-digital devices. A battery converts chemical energy into electrical energy. It is primarily designed and developed by chemists and is very much analogue in nature. Like most analogue devices, its performance can fluctuate and deteriorate over time.

All batteries work in basically the same way: they're filled with a chemical compound known as an electrolyte, combined with plates of a different material. When in contact with each other, the plates and electrolyte react to produce electrical charge and a voltage between the anode and cathode, which are the negative and positive electrodes of the battery. This is why batteries tend to have double-barrelled compound names such as lead-acid and nickel-cadmium.

Memory loss

When notebook computers started appearing in the late 1980s, they were powered by nickel-cadmium (NiCd) batteries. NiCd technology has been around since the early 20th century. NiCd batteries have the advantage of being able to supply huge amounts

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of power if required and can be recharged up to 500 times. But they have one big drawback, known as the memory effect. If a NiCd battery isn't 'fully cycled' - completely drained before recharging again - it remembers the partially discharged state it was in before charging and subsequently refuses to deliver a full charge. This means less energy from the battery and shorter life between charges. Also, the cadmium used in NiCds is a toxic heavy metal that the world's landfills can really do without.

Then came nickel metal-hydride (NiMh), which did away with the cadmium and, along with it, some of the memory effect. NiMh cells tend to have a higher energy capacity for a given size, too, and can be recharged up to 1,000 times. The memory effect is still able to make its presence felt, though, unless the user is careful about fully charging and discharging now and then.

And so we come to the present day, and lithium-ion (li-ion) batteries. The basic technology has been around since the 1960s, but it was enormously expensive. It was used only in such applications as the military and space missions until the 1990s. One problem for many years stemmed from the fact that li-ion cells need far more carefully controlled charging and discharging than other types of cell. Fail to properly control li-ion cells, and they have a nasty habit of catching fire or exploding. This happened on several occasions to Apple's ill-fated Powerbook 5300 in the mid-1990s.

The solution was to embed highly sophisticated current and voltage-sensing and tracking circuitry into the batteries. Li-ion battery packs are invariably 'smart' and consequently expensive; that is, they were until recently, when the costs of the circuitry dropped and huge economies of scale kicked in.

Time LIne

The one thing you can guarantee is that a li-ion notebook battery will die eventually. From the time they are manufactured, they inevitably deteriorate. Even with the best care, you'll be lucky to get more than five years' use out of one. But if you follow some of the care tips here, you'll find you can as much as double a battery's lifespan.