Hardware Is it safe using an USB cable for charging?

[ned]

Even then, the phone has to ask the charger for a higher voltage. The charger starts out providing 5 volts and only switches to 9 when a device that is plugged in tells the charger it can handle it. Also, this is out of USB spec entirely, which is why it needs to negotiate with the phone first.

I believe that's how qualcomm quick chargers work.

 

[ecko]

The regular 3DS chargers output 4.6 volts at 900mA. I have a portable powerbank that outputs 5V at 2.4A. That's almost thrice more amperes. Is it safe to use this on my 3DS?

Ampere is not the issue with these kind of things, the 3DS will only draw as much power as it needs. A higher voltage can damage a device though, but 0.4v isn't that much either.
Googling for this issue, but with laptop instead of 3DS, yields lots of results which explain exactly what you need to look for when connecting a charger that is rated differently.