constant current

i want simple one transistor constant current source to charge 3.6v dc li-ion battery.

From my understanding Li-ion batteries require complex chargers, not a simple C-C charger like NiCads.

Ken

As Ken said that likely won't work. A constant current charger may damage the Li-ion battery or even cause it to rupture.

It can be done by limiting the final voltage i.e. Run the circuit off 4.2V + Vce sat of the transistor. Cheap and dirty and prone to temperature issues. There are many single chip chargers around that cost peanuts. LTC 4054 or MAX1811 spring to mind.

You can contact these companies (Linear Tech & Maxim) and ask for samples (less than 2-3) for nothing if you can convince them you may buy more if they do the job.

what if i use pulses of 4.2vdc to prevent heat damage.

The charging of a Li ion battery is first done with initial constant current, varying voltage, then when voltage is 99% of max, it switches to constant voltage, varying current. So the charging circuit is not that straight forward