It's a really good thing that the MixAmp Rechargeable Battery Pack
is a drop-in-and-forget-it solution. In addition to the convenience factor, it's not very easy to actually remove the battery. For that matter, it's not overly easy to simply "drop-it-in."
The battery holder in the MixAmp is designed so that it can hold four 'AA' batteries (normal or rechargeable), or it can hold a battery pack that is basically the equivalent of four batteries combined into one package. There are metal spring clips that hold the batteries in place - on the 'AA' batteries, these actually make contact and conduct the electricity. With the Rechargeable Battery Pack, however, these merely hold the pack in place; the actual connections are made by some connectors in the middle of the back of the MixAmp, battery compartment. The spring clips, however, have to sort of accommodate two different sorts of things. The 'AA' batteries seemed to fit a bit loosely (along one axis), in that they weren't snugly fit against each other. However, this didn't affect the score of the MixAmp, since nothing I did - including some fairly rough shaking - would cause the batteries to lose connection.
While the 'AA' batteries were a bit loose in the compartment, the MixAmp Rechargeable Battery Pack was more than just a little bit tight in the compartment. If you start by setting the bottom in place, it takes just too much pressure to wedge the top into place, and vice-versa. The best way I found to put the battery in is to center the battery in place and press it in, top and bottom, at the same time.
What goes in, must come out... at some point, and with the battery clips as tight as they are, getting the battery out can be difficult as well. Here's an install tip to make that day easier on you: take a small piece of ribbon and lay it in the bottom on the battery compartment so that it doesn't block the contacts, then pres the battery into place on top of the cord. Carefully lay the cord over the battery and close 'er up. When you need to remove the battery, carefully grab both ends of the cord and use it to help pull the battery out of the compartment.
At any rate, that's mainly forward thinking. The Rechargeable Battery Pack is a drop-in-and-forget solution, and offers a very convenient way to keep your MixAmppowered and, at a retail price of around $20 USD, it's a great option. I would highly suggest picking up the Rechargeable Battery Pack to add "always ready" and "easy to recharge" to the USB MixAmp's list of finer qualities.