Essentially the charger is operating without a load with the batteries pulled. Without the proper load, it is now supplying power to the radio without adequate filtering in a trick configuration.
Unless you need alpha tags, pl and the like, I'd seriously consider the BC370CRS or newer 345CRS as your emergency scanner for conventional use if you need casual AA backup.
If you lose AC power, the 3 internal AA's will actually operate the scanner for about 6-8 hours. Of course a BCT-15x would be nicer, but it also pulls much more power and you'd need a large backup 12v battery to get you through a prolonged ac outage.
If you had a stash of AA's handy, the 370CRS or the newer 345CRS might be an easier implementation - AND you get to use your 125AT portable, or if worse came to worse at an emergency shelter. However, I have even used my old 370CRS in a semi-portable fashion from AA only, but you just can't hang it off your belt.