The Duo batteries are NiMH. They are similar to NiCds, except for the way they behave near full charge. A NiCd's voltage just maxes out as you keep charging it. A NiMH's voltage actually peaks, then droops. Charging logic is tailored to the chemistry, so if you change chemistries, you can run into some trouble. In this case, you'll run the risk of grossly overcharging NiCds if you use it in place of the NiMHs that the charging logic probably assumes, because it may continue to charge while it waits for a voltage dip that never shows up.