> I'm interested in learning how to build my own battery management system
There are 3 good books the subject:
Expect to spend ~ 10 K$ and 1 year of your (free) time in the process.
(I know, I have been doing this for 10 years.)
> they're outrageously priced
They're expensive, yes, but not outrageously so: they are far cheaper that it will cost you to develop your own.