The down tube battery I'm using for my Ebike is $300 it's 36v 13Amp hour I'm sure the cells a Samsung.
Anyhow all i can say is don't skimp on the fuel cell.
I'm actually looking into how to make em you know that DIY bug in me...
Here's a cool clip I was looking at today.
My thoughts exactly this is probably more appropriate for Americans than me as I think this blokes based over there somewhere.But batteries to store solar power, that would be neat.
Thanks I'll check them out .look through this guys vids, great for beginners, has a website and a forum
https://www.youtube.com/c/WillProwse/videos