I always charge in an external charger because it is more convenient for me to do so. Plus I believe it is safer as I don't want to damage my radio if something went wrong with one of the batteries. But many people have great success and always charge from inside the radio. So it is a materr of personal choice and what works for you. I have several different scanners, so an external charger works best for me. The most important thing is to buy a quality set of batteries to start with.
I also have a 246. I used to charge inside the scanner and never had any problems doing so. When you have several handheld scanners, it is easier to use a smart charger which allows you to choose the charge speed. I have been using a LaCrosse BC 900. Many others also use the Maha chargers.
I also use the external LaCrosse BC 900 charger. I always keep freshly charged batteries ready to go in the scanners. I have heard that using the scanner while charging the batteries burn out the batteries faster.
I charge mine in the scanner (396XT). I have been running the scanner pretty much 24-7 for about a year and have only had to change batteries once so far. I plug it in to run and charge overnight, then bring it to work the next day. It will run on battery power until early afternoon. I then plug it in until I go home at 5. It again runs on battery power until about 8-9. The it gets plugged in until about 6:30am.