Hemm, now wait minute????
Dry cell batteries are rated 1.5v each.
The battery packs has four In series,
so the rated voltage for one pack would be 4 X 1.5 volts = 6 volts (well it was when I went to school LOL)
Now what I don't know is are the two battery packs wired in series or parallel??
If the two 6 volt packs are in parallel then the total voltage would be 6 volts (rated)
If the two 6 volt packs are in series then the total voltage would be 12 volts (rated)
Seeing how both the above are max out ratings means that in either case the detector would have to be running at a lower voltage than either of them.
The reason for that it the batteries need to have a ceiling (voltage above what the unit is consuming) and the electronic regulate it down to the lower voltage. That's the only way you get any run time out of the batteries.
So if the detector has a 6 volt battery pack then the detector is probably running on either 5 volts or maybe 4 volts. The 12 volts is probably running on something like 10 Volts or 9 Volts.
I have a couple of detectors that run on two 9 volt batteries, when the batteries get to 6 volt the detector battery indicator will show Low Battery, at 5 volts it will alarm. So the the detector is running on the voltage in the battery above the 6 volt line. You can put ONE full charged 9 volt battery in the detector and it will run just fine, just not for long (two 9 volts in parallel)
I take it that the Fisher 555-D must have ran on 6 AA's (the 1200's don't)
They run on two packs of 4 AA's.
6 AA's are 9 volts,
4 AA's are 6 volts!
That means that the detector is either running on,
6 volts, or
12 volts,
with its OEM battery packs.
Now if you were to replace those with two 9 volts batteries you would have either,
9 volts, or,
18 volts
This is why Fisher told the title poster that it would run on them but it may damage the unit doing it.
Mark.