08. Choosing the Right Battery 2

GUIDE: Batteries in a portable world. 8. Choosing the Right Battery 2

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8.2 What’s the best battery for two-way radios?

The two-way radio market uses mostly NiCd batteries. In the last few years, environmental agencies have been attempting to discourage the use of NiCd, especially in Europe. NiMH have been tried and tested in two-way radios for a number of years but the results are mixed. Shorter cycle life compared to NiCd is the major drawback.

The reasons for the relatively short life of NiMH are multi-fold. NiMH is less robust than NiCd and has a cycle life expectancy that is half or one third that of the standard NiCd. In addition, NiMH prefers a moderate discharge current of 0.5C or less. A two-way radio, on the other hand, draws a discharge current of about 1.5A when transmitting at 4W of power. High discharge loads shorten the life of the NiMH battery considerably.

NiCd has the advantage of maintaining a low and steady internal resistance throughout most of its service life. Although low when new, NiMH increases the resistance with advanced cycle count. A battery with high internal resistance causes the voltage to drop when a load is applied. Even though energy may still be present, the battery cannot deliver the high current flow required during transmit mode. This results in a drop in voltage, which triggers the ‘low battery’ condition and the radio cuts off. This happens mostly during transmission.

The Li-ion has been tested for use with two-way radios but has not been able to provide the ultimate answer. Higher replacement costs, restrictions posed by the safety circuit and aging pose limitations on this battery system.

8.3 What’s the best battery for laptops?

Batteries for laptops have a unique challenge because they must be small and lightweight. Text Box: For better or worse, we have learned to accept the short runtime of a laptop.In fact, the laptop battery should be invisible to the user and deliver enough power to last for a five-hour flight from Toronto to Vancouver. In reality, a typical laptop battery provides only about 90 minutes of service.

 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 to obtain 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.

During the last few years, batteries have improved in terms of energy density. Any benefit in better battery performance, however, is being eaten up by the higher power requirements of the laptops. It is predicted that the even more power-hungry PC’s of the future will counteract any improvements in battery technology, as marginal as they might be. The net effect will result in the same runtimes but faster and more powerful computers.

The length of time the battery can be used will get shorter as the battery ages. A battery residing in a laptop ages more quickly than when used in other applications. After a warm-up, the official operating temperature inside a laptop computer is 45°C (113°F). Such a high ambient temperature drastically lowers the battery’s life expectation. At a temperature of 45°C, for example, the life expectancy of a NiMH battery is less than 50 percent as compared to running it at the ideal operating temperature of 20°C (68°F).

The Li-ion does not fare much better. At this high ambient temperature, the wear-down effect of the battery is primarily governed by temperature as opposed to cycle count. The situation is worsened by the fact that the battery resides in a high SoC most of the time. The combination of heat and high SoC promotes cell oxidation, a condition that cannot be reversed once afflicted.

A fully charged Li-ion battery that is stored at 45°C suffers a capacity loss from 100 percent to about 70 percent in as little as six months. If this condition persists, the capacity degrades further to 50 percent in twelve months. In reality, the battery in a laptop is exposed to elevated temperatures just during use and the battery is in a full charge state only part of the time. But leaving the laptop in a parked car under the hot sun will aggravate the situation.

Some Japanese computer manufacturers have introduced a number of sub-notebooks in which the battery is mounted externally, forming part of the hinge. This design improves battery life because the battery is kept at room temperature. Some models carry several size batteries to accommodate different user patterns.

What then is the best battery for a laptop? The choices are limited. The NiCd has virtually disappeared from the mobile computer scene and the NiMH is loosing steam, paving the way for the Li-ion. Eventually, very slim geometry will also demand thin batteries, and this is possible with the prismatic Li-ion polymer.

Besides providing reliable performance for general portable use, the Li-ion battery also offers superior service for laptop users who must continually switch from fixed power to battery use, as is the case for many sales people. Many biomedical and industrial applications follow this pattern also. Here is the reason why such use can be hard on some batteries:

On a nickel-based charging system, unless smart, the charger applies a full charge each time the portable device is connected to fixed power. In many cases, the battery is already fully charged and the cells go almost immediately into overcharge. The battery heats up, only to be detected by a sluggish thermal charge control, which finally terminates the fast charge. Permanent capacity loss caused by overcharge and elevated temperature is the result.

Among the nickel-based batteries, NiMH is least capable of tolerating a recharge on top of a charge. Adding elevated ambient temperatures to the charging irregularities, a NiMH battery can be made inoperable in as little as six months. In severe cases, the NiMH is known to last only 2 to 3 months.

For mixed battery and utility power use, the Li-ion system is a better choice. If a fully charged Li-ion is placed on charge, no charge current is applied. The battery only receives a recharge once the terminal voltage has dropped to a set threshold. Neither is there a concern if the device is connected to fixed power for long periods of time. No overcharge can occur and there is no memory to worry about.

NiMH is the preferred choice for a user who runs the laptop mostly on fixed power and removes the battery when not needed. This way, the battery is only engaged if the device is used in portable mode. The NiMH battery can thus be kept fresh while sitting on the shelf. NiMH ages well if kept cool and only partially charged.

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