transistor aplication

Hi, i'm doing a small projects that is trickle charger. my charger have 2 parts , normal charge and trickle charge...i'm using power mosfet in buck converter as normal charge (voltage-controlled).
Then, is tat i can using a transistor (BJT) for my trickle part that to produce a low current but the voltage still the same?...

Assuming lead-acid battery, the difference between trickle and normal is the voltage applied to the battery. Switching between charge modes requires changing the output voltage of the converter.

Are you making the switchover automatic, or manual?

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My project is looking for charging the li-ion battery... i need to making the switchover automatic, is that possible?...

Charging lithium cells is tricky.
Have you checked out www.batteryuniversity.com ?

A lithium battery must never be trickle charged. When its charging current drops to a low amount then it is fully charged and the charger should shut off.
A lithium battery holds its charge for years.