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