Here is how i just soldered it to a Alkaline battery.
Step 5: Adjust your output voltage
Now, connect the NiCd to your dc-dc converter and adjust it to output 1.5 to 1.8 volts. Then power up your MP3 and fine tune it to get the full battery scale. This circuit will be able to charge your MP3 battery and at the same time power up your MP3.