Hey Lloyd,
I finally tossed my original Minelab NiMH pack. I was only able to detect a few hours with it. I tore the pack apart to see if I could replace the batteries but no. I also have two of the alkaline packs. I filled them with high capacity NiMHs several years ago. They were good at first but eventually got to where they wouldn't hold much of a charge and self-discharged rapidly. I have a couple of high end (Maha and Lacrosse) chargers that I tried to use to revive the cells but they were just done, and from what I am reading that is common.
For the last year or so I've switched to Sony Eneloops. They have less capacity than some batteries but they do not self-discharge. I've been pretty amazed. Unless you detect and recharge every day or so they are the way to go. I recently made a trip to MN. I did lots of detecting and never had to recharge even my first pack. You will find that much of your pack running down is from self-discharge more than it is from detecting.
Chris