While the Power Pak + completely replaced the internal battery of the 3DS for one with a bigger capacity, the 3DS Powerplus is an extra battery that slides around the bottom of your 3DS and plugs into the device's own charging port. In short, the Powerplus extends your existing battery's life by giving it, more or less, a second fuel tank.
Another way to put it is that the 3DS Powerplus is a portable charging station that will let you recharge your internal battery even if you aren't near a plug. Basically, you use your 3DS like always. You play as much as you can until the battery light starts to flash. Then, with a little flick of a switch on the back of the Powerplus, your system's Charging Light, the one with the little plug symbol next to it, comes on and you are playing just like you are plugged into the wall.
The Powerplus has its own charge indicator on the bottom of the black, non-slip surface. This is a series of four LED indicators that will light up when you press the button next to them. The more lights, the more reserve charge you have.
The only other detail worth mentioning about this device's design, besides how easily is slides around your 3DS and snaps into place, is the fact that it charges off of a standard USB mini connection. You know, the same one a PS3 controller plugs into. As a result, the 3DS Powerplus makes an attempt to standardize the Nintendo handheld's charging connection. Unfortunately, there are some odd issues involving the actual charging of the battery and 3DS that I observed and will talk about later in this review.