[Pblais]

I think you need to consider the above plus consider having someone help you review the entire electrical system. You really won't want to be replacing batteries all the time if something about your system is killing them. I wouldn't connect any new batteries until you can be sure the charging system is working properly and not the cause of you death batteries. A dead alternator seems like it might be a sign of something else. Charging power needs to be regulated for optimal performance and long battery life. The cost of doing this correctly is quickly recovered in longer battery life and adds the joy of trouble free power.