Well this is just off the top of my head you see, but it's a hack idea I've considered for some time. There's all that empty space, and the "Intelligent" (hah) batteries in the 5x0 series are almost always beyond recovery. AA rechargeable batteries are the cheapest per mAh, because that's a standard size. So one idea would be to bypass the internal battery connections altogether and wire straight to the DC input socket on the back, and have enough cells in series to match the voltage of the DC adapter. You'd have to figure out some way of charging them.
Beyond that, if you need it explained, it's probably not a good idea to try it
