Try this first before taking your alternator off your cars to test.
Amazon.com: Equus 3721 Battery and Charging System Monitor: Automotive
When you suspect an electrical problem, it is not easy to tell if it is the battery or the alternator. This tool can help you determine which.
I like it so much that I bought extra and give it out as gift.
If you join Amazon Prime, the 2-day shipping is free too. (There are so many "free" things AFTER you pay!!!)