At first, I thought this would be a simple project. But I'm quite confused and hoping I'm missing something quite obvious.

Lets say I want to have a box with a 4s3p set of 18650s in it. In that box, I of course want a BMS. This I understand. I (think) I understand what a BMS does, and that I need one.

What I do not understand is how I would charge this pack.

Of course, I could simply hook it up to my smart battery charger, and that would work fine. But that is not ideal for this project.

I simply want to be able to hook up a voltage from any power supply, to this theoretical box I am creating, and have some kind of board work with the BMS to charge all the batteries safely, limiting current and such.

Could I use one of the cheaply available buck/boost converters that has a current limit adjustment as well as a voltage adjustment and put that inside my box, feeding into the BMS? This buck boost converter, I would set to 16.4v, and limit the current to, for example, 2A. Would this be safe?