Choosing the right battery for portable computing

Laptops are known to be tough hosts on their batteries. The host demands a stream of uninterrupted power but offers a poor working environment in return. As a result, the battery cannot provide the promised runtime and the service cuts short, often with little notice. In this paper we address the unhappy marriage between the host and battery, and examine what causes a battery to deteriorate faster than in other portable devices (Sony Vaio VGN-FZ battery).

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 (Sony VGP-BPS8 battery).

Computer manufacturers are hesitant to add a larger battery because of increased size and weight. A recent survey indicated that, given the option of larger size and more weight for longer runtimes, most users would settle for what is being offered today. For better or worse, we have learned to accept the short runtime of a laptop (Sony VGP-BPL9 battery).

The energy density of modern batteries improves by about 10% per year. However, the benefit of better battery performance is eaten up by higher power requirements of laptops. This results in the same runtime but more powerful laptops(Sony VGP-BPS9 battery).

During the last few years, batteries have improved in terms of energy density. But any benefit in better battery performance is being eaten up by the higher power requirements of the laptops. This trend is continuing and the net effect will be the same runtimes but more powerful laptops (Sony VGP-BPS11 battery).

Most laptops are powered by lithium-ion. This chemistry has a high energy density and is lightweight. There is no immediate breakthrough on the horizon of a miracle battery that would provide more power than the current electro-chemical battery (Sony VGP-BPS13 battery).

Fuel cells, when available, will offer a continued stream of power by allowing the exchange of fuel cartridges when empty. Unfortunately, commercial fuel cells for laptops and other portable devices are still several years away. Power handling, size and cost remain the biggest hurdles. The early fuel cells will function more like a portable charger than a battery replacement. The fuel cells currently in use have the difficulty in providing spontaneous high power on demand (Sony VGN-FZ150E battery).

The runtime of a laptop battery is based on the activity of the computer. The basic housekeeping, which the computer needs to stay alive, draws less power than, for example, reading, writing, computations and searching for files. Manufacturers prefer using idle time when specifying runtime (Sony Vaio VGN-FZ18M battery).

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 (Sony VGN-FZ15T battery). 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 (Sony VGN-FZ61B battery).

Some Japanese computer manufacturers have introduced a number of sub-notebooks in which the battery is mounted externally, forming part of the housing. This design improves battery life because the battery is kept at room temperature. Some models carry several different battery sizes to accommodate a range of user demands (Sony VGN-FZ180E battery).

Lithium-ion is well suited for laptop users who continually switch from fixed power to battery use. This user pattern is typical for those in the sales, service and medical field. Here is the reason why (Toshiba PA3535U-1BRS battery):

With nickel-based batteries, the charger applies a full charge each time the portable device is connected to fixed power. The battery is put on charge until a signal is received indicating that the battery is full. This signal is in form of a voltage change or rising temperature. Because of the sluggish response, permanent capacity loss occurs caused by overcharge and elevated temperature. Lithium-ion only receives charge if the voltage is low (Toshiba PA3534U-1BRS battery).

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 (Toshiba PA3399U-2BRS battery):

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 (Toshiba PA3399U-1BRS battery). 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 (Toshiba PA3285U-1BRS 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 (Toshiba PA3465U-1BRS battery).

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 (Toshiba PA3450U-1BRS battery).

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 (Toshiba PA3285U-1BRS battery).