Hang On For A Second...

Before building a battery extender, there are a few things you need to do first. Let's see what they are.

Read, read, read!

Yeah, before you do anything, first read up on as many battery extender guides as you can find. You can check out the edited versions of Dennis Wolfe's excellent 'http://www.brighthand.com/' guide at Brighthand and Pocket PC Passion. Brighthand also has an article on how to build a better looking battery extender. No matter what you read on each site, check out the guides on the other sites for extra tips and information. It always pays to dig further, as you will see momentarily.

Here are the direct links to the articles I mentioned :-

Brighthand's How To Build A Battery Extender
Brighthand's How To Build The Better Battery Extender
Pocket PC Passion's How To Build A Battery Extender
Pocket PC Passion's How To Build A Deluxe Battery Extender

Check the voltages!

Next, before you even set out to buy the parts, check the voltage requirements of your Pocket PC. Also note the voltage output of the AC/DC adapter that came with your Pocket PC. This is very important. Even though most, if not all, Pocket PCs come with internal voltage regulators, you wouldn't want to turning your Pocket PC into a really expensive paper weight, would you?

To check the voltage requirement of your Pocket PC, turn it over and look for a label like this :-

This is the picture of a label on the back of my Casio E-115 Pocket PC. Look closely at the lower portion. It states that it requires a DC input of 3.7V but needs only 3V to run. The higher DC input is needed to charge the internal battery, which on closer inspection (read : on removing the battery cover) is a 3.7V Li-Ion battery. No surprise there.

But why do they use a 3.7V battery when the Pocket PC only needs 3V? Well, while the Li-Ion battery may start out with 3.7V fully charged, its voltage does not remain at 3.7V all throughout its use. The voltage actually drops as it is being used. The more current is drawn (when using CompactFlash cards or playing movies), the faster the voltage will drop. Behind the scene, the Pocket PC's internal voltage regulator converts the higher voltage to the constant 3V needed by the Pocket PC. So, by using a 3.7V battery, the battery will be able to power the Pocket PC for a certain period of time before its voltage drops below 3V.

So, what happens when the battery drops below 3V? Will it still power the Pocket PC? You will be surprised but yes, the battery will still power the Pocket PC. In fact, it will do so until its voltage drops to about 1.8V. This is because the internal voltage regulator boosts the voltage up to 3V but this draws a lot of extra current in the process. So, as the voltage keeps dropping below 3V, the current draw increases exponentially (which in turn causes the voltage to drop even more!).

Next, take a look at the AC/DC adapter that came with your Pocket PC. This adapter converts AC (alternating current) power to the DC (direct current) power which your Pocket PC needs. Check its output voltage. My E-115's adapter is able to provide up to 2A of current at 5V to the Pocket PC.