I'm getting ready to replace my batteries. Right now I have six group 31, 900 cranking, 205 reserve. Thinking about Optima dual purpose yellow top. Anybody use these batteries? What works best for you?
If you are talking about 'house' batteries, you want true deep cycle batteries. Anything which has a cranking rating is designed to give a lot of current for a short period of time and then be immediately recharged. These don't work as well or as long in 'house' applications where you want a smaller, steady amount of current for a long time. True deep cycle batteries will have 'Amp-Hour' ratings only.
You can get 6 12 volt batteries and hook them in parallel, but the best tend to be 6 volt batteries, so you will need them in pairs of 2 in series to get 12 volts. You can put several of these pairs in parallel to get longer run times.
Note that for longest battery life, it is best not to discharge the batteries more than 50% before recharging.