The new batteries are rated at 3000 mAH, so expect more than twice the run time than the original pack. You may wonder -- will the higher capacity hurt the tool? No: the important thing here is that the voltage is the same, but you may end up working the tool harder with the increased capacity, so don't over-do it. I've been using it for about a year and a half, and it's been excellent.
Since my old charger is expecting to charge 1300MAH NiCads, it will require a much longer charging time. The spec sheet that comes with your new batteries should help you here. Also checkout batteryuniversity.com
for more important detailed info on charging different types of batteries. A better charging method may increase your battery life.
Why do the batter packs die? I've been analyzing the old cells, and I've noticed that several of the cells are bad, but many of them are ok - it only takes one dud to render a battery pack useless.
Since the drill came with two battery packs, I can combine the good ones to make a second working pack. The old cells can be tested as shown below - the important part here is the load. I've used a flashlight bulb as a load which also server as a visual indicator of the battery's charge. Testing without the load will give useless results. I've seen some cells read 1.2volts without a load -- which seems good, but drop to 0.3 volts when the load is added -- clearly a dud.
A good mod should be economical too! Sears wants $40.00 for a new battery pack - if you can find it (I paid about $75 for the tool with two batteries - so why is a replacement so expensive?) So with this mod, I get a better battery for $24.00
Like many of the projects here on instructables, there are potential safety problems if you don't know what you are doing -- so if you don't know your series circuits from your parallel circuits STOP! don't attempt to perform this mod - it can be potentially dangerous if the cells are mis-wired or short circuited. Also beware any leaking chemicals from bad cells. Also make sure your soldering skills are sufficient before working on this project. A reader from Makezine's blog noted that one of the solder tabs is thinner than the others - this acts as a fuse - make sure you include the fuse in your completed pack.
dispose used NiCad cells properly. Your new HiMh batteries are more environmentally friendly though some still recommend re-cycling.