Reduce phone charger values
Firstly you should give as much detail as possible about the battery specification.
A photo also may help.

A "1.2V" battery probably needs other than 1.2V to charge it.

Once we know what the battery needs we can advise of how a charger can be constructed using your existing power supply.  Some form of (probably very simple) linear regulator will probably do the job.  

Something from here will probably suffice. Which one depends on your battery.

Something like this with appropriate component values will quite possibly meet your requirement. (The component values shown are not at all suited to your requirement). 



From here

Something even simpler will probably work if absolute minimum cost and/or simplicity is required.