Why Use A Battery Extender?

First of all, the cost of acquiring several Li-Ion battery packs is extremely prohibitive. Common batteries like alkaline or NiCad batteries are much cheaper than buying Li-Ion packs from Casio. And as far as I know, there are no other companies offering battery packs that fit the Casio Pocket PCs. So, you will have to get them from Casio, at Casio prices. :)

Next, if you choose the battery swap method, you are limited to the number of battery packs you purchase. That means if you purchase two extra battery packs, then the most you will last is 18 hours for the E-115, 21 hours for the EM-500 and 24 hours for the E-125. So, if you really have critical work to do when the battery runs out, you are dead meat if all your extra battery packs are drained. But if you are using a battery extender and it runs a little low, you can still run the Pocket PC using your main battery (which is still fully charged) while you look for a store selling AA-sized batteries.

As you can see, using a battery extender offers an additional degree of safety in that your main battery remains fully charged until the battery extender's voltage drops below that of the main battery. Once that happens, the main battery and the battery extender will together feed the Pocket PC. While your main battery will start draining at that point, its battery life at that point will be much longer than usual because the battery extender is still supplementing it with power. This gives you some extra time while you look for an alternative source of power or load the battery extender with fresh batteries.

In addition, I mentioned earlier that you will need to power down the Casio Pocket PC before you can swap the batteries. With a battery extender, you don't have to do that. While you are swapping the AA batteries, the main battery takes over and powers the Pocket PC so you don't have to turn it off. This is definitely less troublesome and risky than removing the battery cover and swapping the main battery.

Finally, the biggest problem, IMHO, in the using extra battery packs to extend the life of the Casio Pocket PCs is the recharging of the batteries. As far as I know, Casio does not sell separate chargers for their battery packs. You will have to recharge them using your Pocket PC. That means the Pocket PC will have to remain in the cradle for a very long time. The recharging time has an approximate 1:1 ratio to battery life, which means if you have completely drained two spare battery packs in addition to the main battery, it will take you about 24 hours to recharge them all! In that case, we might have to call your Pocket PC a Cradle PC instead. :)

As for the battery extender, if you are using ordinary dry cell or alkaline batteries, there's no recharging to worry about. Even if you are using rechargeable NiCad or NiMH batteries, you can easily recharge a bunch of them at a go, using multiple chargers. In the meantime, you can still use your Pocket PC (with or without the battery extender) and it will remain where it belongs - in your pocket.