I recently started having severely shortened battery life on my Magic Trackpad. It turns out the fix is pretty easy.

When researching the problem, I found the standard fix reported over the last few years was to “update service” in OS X’s bluetooth preferences. Looking for that option in Mavericks proved fruitless. Finally, out of desperation, I selected the option to disconnect the trackpad, then re-associated it. Immediately, the day-old batteries showed a 100% charge instead of the prior 10% and I’ve had no problems since.

Source: Mac OSX Hints