PDA Batteries

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here are two main categories of battery choice for PDAs - replaceable batteries, usually specified as alkaline AAAs, and internal rechargeable batteries, usually of the Lithium Ion Polymer type.

Almost modern PDAs use internal rechargeable batteries, whereas as you look at older devices, you can often have a choice between the two types.

Furthermore, many users use rechargeable replaceable batteries - I do this myself with the Palm IIIxe and NiMH batteries.

Replaceable Batteries

The original Palm Pilot models used replaceable batteries, as do all Palm III models with the exception of the Palm IIIc, which uses an internal rechargeable battery. Most PalmOS devices with replaceable batteries use two AAA cells, although there are a few exceptions (such as the Handera 330).

Replaceable batteries offer the advantage that if you ever suffer from “battery fade” problems (by using rechargeable batteries), you can simply replace the cells.

Alkaline batteries are also less susceptible to extremes of heat or cold, which may be useful if you either live in a very cold climate, or perhaps you have an ambition to drive to the Arctic Circle /cough/.

Alkaline AAA batteries are readily available all over the world, and given that most PalmOS devices that use AAAs have an effective battery life measured in weeks of use rather than hours, planning for a trip away is usually no more complicated than buying another pair of AAA batteries.

Replaceable alkaline batteries do offer some disadvantages, too. Firstly, although unlikely if one uses a quality brand name, sometimes batteries can leak their contents and this may well ruin the Palm. Secondly, by their very nature alkaline batteries are more efficient when used in low load conditions and they become relatively less efficient when used in higher power drain scenarios, for example when the backlight is used. They are also relatively bulky and are rated at either 1·5 volts (alkaline) or 1·2 volts (most rechargeables). The higher the voltage supplied to the PDA components, the more efficient the device is at handling power.

You do also need to consider the additional cost of having to buy alkaline AAAs every fortnight – this gets expensive over the course of a year. A charger and some rechargeable AAAs does add to the initial outlay of the device, but is usually cheaper in the long run.

Finally, some companies offer a NiMH battery pack that replaces the replaceable AAAs and allows in-cradle charging.

Internal Rechargeable Batteries

The first PalmOS handheld I encountered that used an internal rechargeable battery is the Palm V, simply because AA batteries are relatively bulky compared to the Palm V!

Rechargeable batteries offer a number of other advantages over AA and AAA batteries. First off, they can be made to fit the available space within a certain device. This is especially important for the smaller handhelds.

Secondly, they can be engineered to provide a higher voltage than replaceable batteries, which enhances the efficiency of the power conversion process within the device. This in conjunction with the chemistry of rechargeable batteries means that they are more efficient in higher load conditions - such as a faster processor, more memory to maintain, a backlight, and perhaps onboard radio communications.

Against the above, internal batteries (or their charge circuits) can and do wear out. Most devices were originally built to be “sealed for life,” that is, the battery should outlive the unit. Of course, sometimes a user won’t want to replace the entire PDA if the battery or charge circuit has died, and will instead look for a battery replacement. Thus, an entire cottage industry has grown up, providing replacement batteries for all manner of PDAs.

Rechargeable batteries also require a means of charging, which can necessitate the user carrying with them a power supply, or perhaps a cradle, especially for longer trips away from home. This can be an inconvenience, especially if the user has a habit of travelling light.

Finally, a small number of devices are now designed with a replaceable rechargeable battery and there is a certain niche of users who refuse to buy a replacement PDA unless it has an easily replaceable rechargeable battery.

PDA Processor Internal Power Efficiency

As an example, the Dragonball EZ processor requires a core voltage of 3·3±0·3 and the Dragonball VZ requires 3·0 volts. This means that the voltage provided by the power supply needs to be adjusted to fit this level. Replaceable batteries typically provide under 3·0 volts, so the voltage has to be stepped up, whereas rechargeable Lithium Ion batteries usually supply a voltage greater than 3·0 volts, so the voltage has to be stepped down. As the voltage is stepped down, so for a given resistance, the current provided is increased - thus the current taken from the battery can be reduced. The reverse is true when the voltage is stepped up - the current taken from the batteries has to be increased.

Although the above is an overly simplistic explanation (for one, we are making the assumption that the resistance does not change), it illustrates the point that for a given battery capacity, the higher the voltage supplied, the more energy the battery contains.