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!
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.
Introduction to my home battery