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How to Choose the right battery for portable laptop
What's the best battery for laptops? Batteries for laptops have a unique challenge - they must be small and lightweight. In fact, the laptop battery should be invisible to the user and deliver enough power to endure a five-hour flight from Toronto to Vancouver. In reality, a typical laptop battery provides only about 90 minutes of service. Many users complain of much shorter runtimes.
A battery in a laptop ages more quickly than in other applications because of heat. During use, the inside temperature of a laptop rises to 45°C (113°F). The combination of high temperature and full state-of-charge promotes cell oxidation, a condition that cannot be reversedonce present. The battery's life expectancy when operating at high temperature is half compared to running at a more moderate 20°C (68°F) or lower. Leaving the laptop in a parked car under the hot sun will also aggravate the situation. All batteries suffer permanent capacity loss as part of elevated temperatures but lithium-ion is affected more than other batteries.
How to calibrate the battery? Most laptop batteries are 'smart'; meaning that they know how much energy is left. Such a feature has definite benefits but the readings are often inaccurate. A laptop may indicate 30 minutes of remaining runtime when suddenly the screen goes dark. Here is the reason why:
With use and time, a tracking error occurs between the chemical battery and the digital sensing circuit. The most ideal use of the 'smart' battery, as far as fuel-gauge accuracy is concerned, is a full charge followed by a full discharge at a constant current. In such a case, the tracking error would be less than 1% per cycle. In real life, however, a battery may be discharged for only a few minutes and the load may vary widely. Long storage also contributes to errors because the circuit cannot accurately compensate for self-discharge. Eventually, the true capacity of the battery no longer synchronizes with the fuel gauge and a full charge and deliberate full discharge will be needed to 're-learn' or calibrate the battery.
There are no standards to tell what constitutes a fully charged and fully discharged battery. Lithium-ion packs are considered fully charged when the limiting voltage (4.20V/cell) is reached and the saturation current has decreased to 3% of the nominal value (50mA on a 1700mAh cell). Some chargers use 5% and 8% as 'ready' criteria.) A full discharge occurs when the cell reaches 3V/cell or lower. At this voltage level, the battery has a remaining capacity of 3 to 10%. Modern batteries adjust to a lower cut-off voltage on high load currents and include temperature compensation.
To calibrate a battery, a full charge and discharge is necessary. One without the other does not constitute a calibration. A problem arises if the battery is recharged after a brief use without providing the opportunity of a full discharge. A forced discharge to "Low Battery" may be needed from time to time.
What happens if no battery calibration is done? Can such a battery be used in confidence? Most 'smart' battery chargers obey the dictates of the chemical cells rather than that of the electronic circuit. In this case, the battery will fully charge regardless of the fuel gauge setting and function normally but the digital readout will become increasingly more inaccurate. If not corrected, the fuel gauge simply becomes a nuisance. Cadex Electronics manufactures 'smart' chargers and battery analyzers that are capable of calibrating a 'smart' battery.
About the Author
We specialize in substitute batteries and battery packs for laptops, camcorders, digital cameras, PDAs, mobile phones, and power tools, etc. as well as battery chargers!
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