The Low Cost BMS is an open-source design for a battery management system. You can build this BMS yourself to monitor cells within your homemade battery. It will take you some time to do, and you may have to learn a bit along the way, but it will be cheap!
What is cell balancing and why should I do it?
A beginner’s guide to cell balancing in a home-made battery
The Selectronic SP PRO Inverter/Charger
How I’ve configured what is probably the most advanced and flexible battery inverter/charger on the market.
How I made my DIY Home Battery
This page describes my homemade home storage battery (DIY Powerwall). It is a grid-connect battery, it charges from my solar array and is built around some windfall lithium cells.
When Lithium falls off the back of a Bus
Introduction to my home battery