across the battery terminals should be around 14v. It sounds as if the alty internal voltage regulator/diode bridge has failed. This failure can allow excessive battery current draw when the car is off.
With car off, all doors closed, all lights off, etc - the battery current drain should be no more than ~40mA (.040 amps). With a bad alty it will read 500mA or so. Draws the battery down easily overnight since alty isn't charging it property when car is running.
A rebuilt alty can be ordered from a local car parts store. Just a quick eBay search showed a good selection from a variety of sellers. This sites parts sponsors should have them too.
