So how about something simpler. NO FAN! aka passive ventilation 
Check out this guy's box. He has 14 batteries
I'm still trying to understand how it works. But it apparently does.
Here is a video by another guy using the same design of concentric pipes. Another video using PVC pipes.
I read the below in a book
I will have a two battery series setup, yours will be four batteries. Guess what if we use 200AH batteries, both our systems have the same capacity for Ah.
