Using External Battery Pack on Optio S40

To preserve the charge in my internal batteries, I have created an external battery pack that I plug into the external 3v socket. I use 2xD(L20) NiCad batteries. The camera works fine in Playback mode (useful for reviewing the day's takes), but when in Capture mode, the zoom continualy cycles out and in, and the display is blank. This makes it impossible to take pictures using the external pack. Has anyone else used the external 3v socket successfully?