How to tell if my battery is bad

[soundstream15]

All connections are fine and solid.

My kitneik hc2400 was reading 12.6 with car off so it was out of my car for a week.

Put it back in and it was reading 11.5? So it may need charged. So my optima yellow top is reading 11.9 with the car on. That is going off of what advanced said. I found my DMM but have yet to use it. If my yellow top took a shit could it be why my rear batt is not charging? Bc idk why it's only reading 11.9 I know when my rear battery was out, system wasn't hooked up, both windows would go up very slowly. I posted something like this in mechman, but there's a good chance my alt is fine. I hear ppl say that advanced doesn't work on HO Alts and says its not working properly. I also did the test where, the car is running and you disconnect the front pos on battery. Well my car shut off so does that mean alt. just need some pointers on how to check the battery here

[tacomaguy1]

take it out and charge it, then have it load tested. Could even take them out or isolate them, charge them, than see what the resting voltage is after 24hrs or so

[inferno1985]

if 1 battery is bad, it can very well affect the others also. have a local autozone test it. it should be free