BQ24650 or BQ24620 for LiFePO battery charger | Solar powered + Grid powered
BQ24650 is a MPPT controller/charger for solar panel. While the BQ24620 is designed for adapter charger. 

I would try the MPPT, because it is suited for panel, I guess when you will connect the DC voltage on the input, the MPPT point is trivial and it shouldn't make troubles. Anyway you can experiment with different DC voltages - higher or lower than 18V if the circuit will have weird behaviour.  

It seems no problem to operate the BQ24650 from adapter:
https://www.youtube.com/watch?v=GN7At6CLAyE