Well.
Guys, I'm new here, and well blah blah. here's my two cents on rechargeable 9V's.
First of all, they're not really 9V batteries, if you didn't know this they're actually 8.4v so you're actually short about 6%
the reason they still work, and work good (except for your battery meter, which I'll explain in a second) is that your primary alkaline cell, which contains 6 cells at a nominal voltage of 1.5 a cell slowly drops to about .8V per cell, which is how your battery meter reads the voltage.
NiCd and NiMh batteries on the other hand, maintain a almost constant voltage of 1.2v cell through out the discharge until the battery is nearly dead, it's like the voltage falls off a cliff. dropping to 1V per cell, which is considered dead for the most part. this is why your battery meter may show you have half the battery remaining and within minutes it's dead.
Alkaline discharge curve
NiMh discharge curve
you can see how one holds a pretty steady voltage and the other drops.
I am aware that these charts are using different drain rates, but the discharge curve is identical weather you're discharging a .01mA as if you were drawing 1000mA
so a dead alkaline 9V is at about 4.8V where as a dead NiMh or dead NiCd battery is at 7V.
so, the meter on your detector is set to show dead at 4.8V where as when you're using a rechargeable you're actually dead at 7V
Over discharging rechargeable cuts their life in half, with NiCd's it can kill the battery over night. so, after a day of using your detector it's best to charge them, unless your charger is timer controlled, and mine is, then you have to worry about over charging. the charger you want is a "computer controlled smart charger" or any charger that says it uses the ΔV charging method, unfortunately they're hard to find for 9V batteries, for AAA-D cells they're pretty common.
I hope this wasn't too much information and helps at least someone understand what's going on.
I was honestly concerned about using rechargeable NiCd's that I bought from batteryspace in my bounty hunter since i recall the instruction manual saying use alkaline only.