You can use slightly over voltage (normal lithium charging is [constant current > constant voltage]). What you can’t do is “over capacity” it much.
That’s a very low rate of charge but since you are using a NiXX charger you really don’t know WHAT it’s putting out. NiXX over volts the cells to achieve charge, Lithium do not.
Is that 6400mAh known good cells or Chinese claimed capacity? If not verified in all probability the actual capacity is less to WAY less. Do the math: if the charger is 1000 and it charges in 3 hours that either indicates it was still half charged or it’s a 3000mAh cell.