AC Power With Alkaline Batteries
I think the key is to select Alkaline setting. If I understand correctly, the AC adapter will power the scanner and not attempt to charge the batteries as long as the Alkaline setting is selected.
http://info.uniden.com/twiki/bin/view/UnidenMan4/SettingUpTheHardware#Install_the_batteries
Install the batteries
BCD346XT
Your scanner can use either regular alkaline batteries or rechargeable nickel-metal hydryde (NiMH) batteries.
The batteries that came with your scanner are rechargeable NiMH.
To install the batteries,
Remove the cover from the battery compartment.
Make sure that the charge switch is set to the correct battery type:
Use the Alkaline setting for any regular or non-rechargeable battery
Use the NiMH setting for any rechargeable battery
Make sure the polarity (plus and minus) of each battery matches the diagram inside the compartment.
Replace the battery compartment cover.