Hi Jess ,
What your planning on doing for your home gasifier , is what I have been doing here for the past few years so if I may just give you a heads up that may or may not help .
Please before you start buying new batteries work out what it is that you want to power up at home , and add them up on a daily usage as to how much power you will need , that way you will know what size engine you will need to run what size generator/alternator , you always need to size the amount of power you will drain from the batteries as undercharging them is as bad as over charging .
I would also wait and buy the batteries all at the same time , get hold of a few second hand batteries just to start with while you save up and buy a new set all at once , that way you will have a perfectly balanced new set , while you have tested running on old ones
How often and for how long do you plan to run your charcoal generator ?