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Battery Technologies in handheld PCs

Here's how to pick the right one for your HPC (June 98)

This article is about primary battery power for your HPC. The more you know about batteries, the better—especially if you go wireless or have one of those power hungry modems that can reduce your battery life to as little as 20 minutes.

Battery classes
Batteries are available as non-rechargeable or rechargeable as follows:

Rechargeable Non-Rechargeable

o NiCad o Alkaline
o NiMH o Lithium
o Lithium Ion
o Renewable Alkaline

Battery metrics
There are two relevant measurements, voltage and milliamp hours (mAh). Voltage is the power a battery can deliver. mAh is the time that power can be delivered. That means if you have two 1.5 Volt batteries, and one is rated at 2000 mAh and the other is rated at 2500 mAh, the 2500 mAh battery lasts longer.

That doesn’t mean the higher capacity cell performs better. Batteries respond to loads differently. Memory cards and modems are massive loads to batteries. You may experience low voltage conditions with brand new alkaline batteries because they do not respond to power surges as well as Lithium, Nicad, and NiMH batteries. What works for you may not work for your friend.

Milliampere Hours (mAh)
Most HPCs run at about 200 mAh. HPCs are 3v devices, so two AA batteries in series equal 3 Volts. Divide the mAh of the batteries (2600 for two AA alkalines) and you can figure the run time. When you add components like a PCMCIA card, add that mAh rating and usage time to the mix. There is a caveat for this formula for Alkalines. They are rated to their cutoff point, but the HPC stops working long before that.

Voltage is like horsepower in a motor. Consider a 4-cylinder 120 HP car and a 250 HP truck pulling a heavy boat and trailer. While both do reasonably well on a flat surface, the small car labors when it comes to a hill. Both vehicles must exert the same force to pull the trailer up the hill at the same speed. But one has a better capacity for doing so. The same applies for batteries. A 5v source is better able to deliver power to PCMCIA cards because the cards require 5v. A 3v source must first convert amps into a higher voltage. It works harder and burns out sooner under these conditions.

Alkaline Batteries
Alkaline batteries are predictable. Brand new batteries deliver 1.5 Volts at 2600 mAh. While they have a long shelf life (90% after five years), they begin degrading when you start using them. So while they start at 1.5v, this drops through usage. Since the degradation rate is constant and known, you can measure the voltage and determine how much life is left in the batteries. This is how CE power management software operates.

Alkaline batteries have cutoff voltages around 0.9v. Your HPC won’t work with voltage that low. So, you are going to get less than the rated capacity out of alkaline batteries. How low can the voltage be? Somewhere around 1.1v. On a positive note, you can take the "dead" batteries from your HPC and still use them in something that doesn’t demand so much power.

The batteries from different manufacturers are different and they respond to surges differently. If your brand new alkalines give you a power warning when you hook up a modem or memory board, they probably don’t perform well under heavy loads. Before you curse your HPC, try different batteries. This is especially true of some renewables which are rated at only 1.2v.

Eveready manufactures Lithium batteries. These batteries have a long shelf life (90% at 10 years). Lithium batteries maintain a constant voltage over the life of the battery and perform well under heavy loads. Like alkalines, they have a high capacity (2600 mAh), and their starting voltage is 1.5v. Lithium battery voltage does not degrade until the battery is nearly discharged, so you get full advantage of the battery’s rated capacity.
When these batteries die, they die quickly. Because their degradation characteristics are not known, CE Power Manager does not work well with these batteries. It tells you if the batteries are still good. By the time it tells you they are weak, you are close to going on backup. Depending on your HPC, this should not be a problem. At most, you may lose active data, but you should not lose data already stored on the HPC.

NiCads (Nickel Cadmium) are the most used rechargeable batteries today. They have many good features, and last a long time. You can buy NiCads and a charger off-the-shelf and use them instead of AAs.

You will find them considerably more expensive than Lithium or Alkalines. NiCads perform the same way as Lithium batteries but they don’t die as quickly. Their starting voltage is 1.25v and they hold their charge for the life of the battery. They are very good at responding to power surges, which means they provide reliable power for many applications. There are various brands available, with Radio Shack offering 650 and 850mAh batteries

NiCads have several disadvantages. The capacity is much less than Lithium and alkalines, and they dissipate just sitting on the shelf – 10% in the first 24 hours. It’s use it or lose it with NiCads. Also, to get full potential from the batteries you have to let them drain completely. They are infamous for their "memory" effect—crystals forming on the cell plates. As the crystals build up, the battery’s maximum charge potential degrades. (Fully discharging the batteries breaks down the crystals.) Finally, if you are environmentally conscious, NiCads contain heavy metal cadmium, which is a targeted environmental pollutant.

The next most popular rechargeable battery is Nickel Metal Hydride (NiMH). Designers find NiMH attractive because its energy density is considerably greater than NiCad. That means the NiMH battery holds 100% or more charge over a NiCad that is the same size. Like NiCads, NiMH batteries hold their charge for most of their rated capacity and then drop off very quickly. NiMH batteries do not respond to high power draws as well as NiCads. The self-discharge rate of NiMH is about 1.5 times that of NiCads, the cycle life of recharges is smaller (about 500), and they generate more heat during recharging.
Despite these drawbacks, NiMH will continue to gain market share. People using the batteries are not concerned with shelf life (although self-discharge is crucial in charging). They are concerned about capacity and getting power. Unlike NiCads and their memory effect, NiMH batteries perform better when charged frequently. Finally, they contain no heavy metals.

Lithium Ion
After a tough (explosive) start in the commercial market, Lithium Ion is gaining popularity. Lithium Ion combines the best of NiCad and NiMH. It has a low self-discharge rate, it is environmentally safe, has no memory, and holds a high number of recharge cycles. In addition, it responds well to high current draw. Its composition makes it the lightest of all the batteries, and it has the greatest energy density.
Since its composition is unstable, Lithium Ion is difficult to work with. The batteries require circuitry to prevent thermal runaway during charging. The circuitry also limits the maximum discharge. This makes Lithium Ion the most expensive of all the batteries for HPCs.

Chargers are a lot more complex than most people realize. First, the charger must provide the proper current for optimal charging. Second, the charger must limit voltage to avoid overcharging. Third, the charger must balance the charging rate against the heat generated during charging. Too much heat (thermal runaway) or gas (venting) in a battery is catastrophic. Last, the charger must match the battery’s self-discharge at full charge. This is because NiCads and NiMH batteries lose 10-15% of their charge within 24 hours of full charge. To obtain a full charge, the charger must offset that discharge. Because battery chemistry changes from battery to battery, it is very important that the charger is built for the battery’s particular characteristics. This is the reason you have chargers for all your rechargeable batteries – phone, computer, HPC, etc. You risk damage or even an explosion, by using chargers not designed for the battery.

Battery drainers
PCMCIA modems are notorious for killing HPC batteries. That’s because they require up to 5.8v for operation with a 5.2v minimum. Why? Telephone companies use 3-6v on the standard phone to generate the analog phone signal. To get lower power requirements in PC Card modems would require telephone companies worldwide to lower their power requirements. Don’t hold your breath for that to happen. And by the way: even software modems must still power that phone signal.

Powering wireless devices is a massive drain on batteries. The modem requirements aside, dialing a cellular phone requires 4.8v or more. Don’t expect your HPC’s batteries to power a wireless modem.

Then there is color. All CE 1.0 units had grayscale screens. HP was the first on market with a color unit, Sharp followed, and now most of the manufacturers follow suit. As a rule of thumb, running color is like running your unit with the backlight constantly on. For this reason, most color devices use NiMH or Li-Ion batteries instead of alkalines. -

Gene Quickel is VP Marketing of Viatek Corporation, a company providing a variety of services and products for HPCs.

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