It is based on the MAX712 integrated circuit. Even if this model has been recently replaced by newer components like MAX846A, it is still possible to purchase is on the web. I chosen it due to the easy way to configure the number and capacity of cells to be recharged, and because it is possible to have it in a friendly PDIP package. The MAX712 provides also a trickly current charge at the end of the main Fast Charge phase, in order to maintain the battery pack always at top of its charge. The load can be directly applied to the battery pack without to disconnect the charger. 

To have a full project description, refer to the "Circuit Special 2024" edition available at the following link: https://www.elektormagazine.com/circuit-special-2024