The charging chip in the Miller ML103 is tp4057.
They actually claim the tp4057 can take 4.5V-15V but its a tiny chip & pcb probably doesn’t have enough heatsinking.
I wouldn’t go much higher then 6.5 since its more its likely to drop the charging current to keep from overheating the higher you go.
Also you can’t absolutely trust the tp4057 specs, its a chinese clone of LTC4057. For one the charge curve that tp4057 claims is not true, it doesn’t match testing.
HKJ’s Review: ML-102 Charger (same tp4057 chip as ML-103)