(how it works in a resistor's v/i curve)

I was just giving the basic formula, but yeah. In practice almost everything has tolerances.

Well, yes and no. That's true for a purely resistive load, but in many situations and circuits, a too high voltage can be harmful (for batteries), or even a too low voltage (for switching power supplies.)

Current (noted as I) is equal to Voltage divided by resistance.

I = V/R; V = I*R

That's how voltage works!

Okay yeah 125 is just on the cable. I'm new to voltage I guess.

Yeah, if your mains is 120V, then you're 100% fine. If the mains voltage is exceptionally low then you might find the adapter having issues outputting enough power but aside from that you're safe.

Where do you get the 125 V figure from?

Edit: The cable is just a cable. The voltage printed on it is the maximum voltage it's rated for, apart from that it will just carry the voltage from wall socket. There's nothing magic in it that will make it output 125 V if connected to a 120 V socket.

ahh okay i thought the "~" meant "give or take".

Also it's giving 125 to a charger that accepts 120, not the other way around.

Wait a minute. I wonder why the adapter is marked with 125 V. USA is 120 V and Japan is 100 V. Maybe it's rated for 125 V to account for tolerances. (120 V actually means 114 V and below 126 V.) But under normal conditions, your wall socket will give you a lower voltage than that transformer is rated for. This is just a regular transformer (not a switching power supply) so the output will vary with the input. If the tranformer was designed for 125 V and you give it 120 V, the output voltage will actually be 4% lower than the normal output voltage. This won't damage anything but maybe the battery won't be fully charged. My guess is that 125 V is just overprovisioning, though.

I'm also unsure what "So I've got a few that are straight 125V, but one of them says 125~V." means. ~ in this case wouldn't mean "approximately" but AC. Even if the ~ is missing on the other packs, it's all the same, just different markings.

The battery packs were made around the same time, right? It's the same issue/non-issue. Technically, the power regulation circuits should be tolerant of that small a difference (since mains power can fluctuate by that much and more in some areas) but since it's old it might not like it much.

At the risk of being blamed for your battery pack dying, I think you're probably safe.

This is the voltage going into a gameboy battery pack, not the game boy itself.

Actually, in my experience that small a difference is within tolerance with almost any electronic devices. However, given the aged nature of something like a Game Boy it's hard to make any guarantees. I've been running my Fami and SuFami in the same way and neither has died or exploded, but that doesn't mean it's not putting a little bit of extra strain on them.

sweeeeet!

I haven't burn anything down yet!

it probably isn't a good idea.. it could work, and if you are willing to take a chance then go ahead, but it is sometimes hard to know where the line of too much voltage is for a device. at least that is the experience i have had with things..