| > Normally these chargers are designed such that when the current | > goes up the voltage goes down
... of course, that's true with most real-world power sources, designed that way or not :)
| > Look at them as a constant power device. The product of current X | > voltage is relatively constant.
This is a nice rule of thumb, but it's very much an approximation, valid over a small range of voltages. Your (Red's) page on wal-warts covers it a lot better.
| The pack I'm talking about would be Ni-Mh. Is your rule of thumb | applicable?
Really, NiCd and NiMH cells are charged almost exactly the same. The only real differences are that 1) NiMH cells should not be charged quite as fast in a fast charger, and 2) the peak seen at full charge is smaller on a NiMH cell.
| I have a 9.0 volt wall that is actually putting out 13.4 volts.
... with no load. Also, the wal-wart probably only has a diode (and maybe a resistor in there) so the voltage coming out will not be a steady DC current but instead will be approximately half of a sine wave, and different voltmeters will respond to that in different ways. So it won't be emitting 13.4 volts -- it'll be emitting between 0 and X volts (with just over half the time spent at zero), and exactly how
13.4 volts compares to X depends on your voltmeter and how good it is.
(Red's page on wal-warts does cover that if I recall correctly.)
Good voltmeters will give a RMS value, but even then that's not entirely useful, because a RMS value of less 1.41 volts/cell can still fully charge your pack (because it'll go higher for a short period of each cycle.) I guess with a little math I could calculate just how high, but that's not really needed.
| Before I knew that I was attempting to charge the pack and both | transformer and pack got very hot (>130 F).
Definately not a good sign. However, once the battery is fully charged, all power goes towards heating the pack, so it's temperature rises quickly, and it's generally (past) time to stop charging once you notice it getting hot.