I've never liked the cheap vehicle adapters as they output unregulated voltage. Consider that a vehicle is "12 volts" when the engine is not running, but a typical voltage when the battery is charging is more in the range of 13.8 to 14.2 volts. Do the math; if you drop 12 volts to 9, you can see how excessive the voltage is when the vehicle is supplying ~14.
You must also realize that the voltage you measure will be higher when there is no load on it. In other words, you'll measure 13.8 until you connect the radio and that will cause it to drop. With regard to the scanner models you have listed, another factor is the batteries in the scanner. If they are in need of a charge, they will further drop the voltage down.
From my point of view here are the solutions to this issue:
- Purchase an adapter that has a regulated output (I'm not sure they even make them).
- Build your own adapter with a 9 volt regulated output with the required capacity (charge the batteries, operate the radio).
- Don't use a vehicle adapter.
My choice was the last one. I bought a couple sets of Eneloops NiMH AA batteries just to run my radios. Even with high volume and the display backlight on, they will run the radio for several hours. When they're dead, I switch to another set. I've set up means to separately charge the batteries in my vehicle for long trips, otherwise I just charge them at home. I also like the fact there's no extra wires to the scanner in the vehicle.
