Is it? I want to leave my Japan 3DS turned on and plugged in for mostly 24/7. Is this a bad idea? I'm not to much of a technician, so hardware inst my field of expertise.
The manual to the 3DS says that the system can be played while charging, so nothing should happen. I do that all the time and my 3DS still keeps battery life for a long time.
Consulting Nintendo is the best bet; this can vary hugely by device and battery type. I don't think when they say in the manual that it can be played while charging they mean that you can plug it in all the time and expect nothing to come of that.
For one thing I would certainly expect more rapid battery degradation with that; with everything that inherently needs charging frequently (smartphones, laptops/netbooks, handheld consoles) I notice that over time if I don't do the commonly advised thing and simply use it until low battery, then charge it nonstop until it's full again, battery life worsens severely more than naturally expected over time. It's tempting to leave things on all the time for a variety of reasons so charging them (or, if you just use them a lot, topping them up) seems the obvious thing to do, but with everything I have used that strategy with too often I end up with a battery that barely holds a charge a few months down the line.
The bill would get higher, but I don't see why not. BUT you should not take in and out. If you charge your 3DS from blinking red to maximum 500 times, the battery will drop pretty quickly. Then you need to buy a new one, which are fairly cheap.
I can confirm the battery thing. I have a normal 3DS that I tend to leave on for days at a time while plugged in, and I've completely shot my battery.
My battery will stay alive roughly for say, an hour an a half, without being plugged in and having the brightness up all the way. It stays alive for a good few hours if it's on and I have it shut and in sleep mode.
Yep, it is a terrible idea to leave it plugged in. My recommendation is to leave it in sleep mode when not using it and ONLY plug it in when the battery gets low, then unplug it when you are done charging. When you leave it plugged in, the metals in the battery crystallize and thus don't hold a charge.