Dear Soon Lee,

Unfortunately nobody can help you unless you say what you want.

You can not have a constant voltage and constant current at the same
time, because changing the load, at least current *or* voltage will
change too, this is pure physics based on electricity rules.

Yes, you can have a constant current generator, within limits of
voltage, when then the output current will go down upon a high load,

or,

you can have a fixed voltage generator, with limits of current, when the
output voltage will go down upon a low load.

This is quite easy to build, nothing special, but it is impossible to
help you unless you say exactly what you want.

Reading your post, I can guess you want to build a battery recharger
unit, but... you need to explain better.

Wagner.