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.

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.

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.

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.

Figure 1: Net runtime. The energy density of modern batteries increases by about 10% per year. This gain is compensated by the demand for better laptop performance. The runtime remains the same.

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.

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.

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.

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.

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.

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 size batteries to accommodate different user demands.

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:

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.

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 deliberate full charge and discharge is needed to 're-learn' or calibrate the 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 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.

At what point of a discharge does the reset occur? Lithium-ion is considered fully discharged between 2.5 and 3.0 volts per cell. The electronic circuit in the battery is designed to reset between these voltage levels. A problem occurs if the laptop cuts off before this low voltage can be reached. In such a case, an external charger with discharge function may be used. Cadex Electronics manufactures 'smart' chargers and battery analyzers, both of which can be used to test and calibrate the 'smart' battery.