A cheap voltage meter from radio shack will be needed to diagnose your problem.
Brake and battery light come on at the same time by any chance?
I'd start with disconnecting the positive side of your battery and checking for a current drain with the ignition off. If it is registering a drain start pulling fuses one at a time to see if you can isolate it easily.
If nothing registers move to battery condition.
Still good check alternator out put, think it's 14.4V or more at idle when good. Most major auto parts stores will check your battery and alternator for free. I had a simlar condition when by alternator brushes wore down to almost nothing. Thought my battery was dead or something was drianing it but it turned out to be a $14 brush set.