Be aware that the battery level shown is an APPROXIMATION only. There's no real way to gauge how much energy is still left in a battery (it's not like the fuel tank of a car; there's no "liquid" to measure.)
Solution: just use the mouse until it goes dead, and never bother with the battery indicator (I've had it show a negative number for remaining power......) Then just change the batteries. To make it last a bit longer you can use the power switch at the back, but in my experience that's just a waste of effort really; my magic mouse still lasts for many weeks on one charge even with the switch left in the "on" postion the whole time. I only switch the mouse off when I'm transporting it, so it doesn't wake up my computer while still packed in the laptop bag.
Others have suggested rechargeable Eneloop batteries. I use these too; avoid using very high rating NiMH batteries in stores; these typically have very high self-discharge rates and will go flat inside of a few months even if you don't use them at all. Eneloops will hold their charge for a long time, and is thus better suited for use in low-power devices like mice and keyboards and such where you don't want to have to change batteries all the damn time. 😛
Apple's own battery charger comes with six Eneloop cells (re-badged); it's not the best charger in the world maybe (or the cheapest, lol), but it's decent enough, very small and easy to pack in a bag, and it works. I use it myself to re-charge my magic mouse's batteries.
