What do you mean here by "backup"? Please describe the use case, target loads, current rates et.c.
> So I wired them all up in series (1S12P; 3.7v, 120Ah
There are no series connections there, all are paralleled.
It would be much more efficient to go to 2S and step down to 5V.
I am confused by your description of your charging problem.
Your charging of these cells should not have anything to do with the DC converter on the output side, whether buck or boost.
Maybe some photos of your components laid out on a bench showing how they are connected
and/or drawings might help.
> So I wired them all up in series (1S12P; 3.7v, 120Ah
There are no series connections there, all are paralleled.
It would be much more efficient to go to 2S and step down to 5V.
I am confused by your description of your charging problem.
Your charging of these cells should not have anything to do with the DC converter on the output side, whether buck or boost.
Maybe some photos of your components laid out on a bench showing how they are connected
and/or drawings might help.
