The AC adapter that comes with the 770 (in the US anyway - AC-4) has a rated output of 5 Volts and can source 890 mA of current. That's a maximum of 4.45 Watts. While the actual current draw of the 770 while charging the battery is likely less than 890 mA, it's very likely to be more than the 230 mA available from a 1.38 Watt, 6V source.

To minimize the current the 770 is using in excess of what's needed to charge the battery, try removing the memory card and also charge it while it's powered on so the screen will blank. The LCD screen and it's backlight along with the memory card draw a significant amount of current.

You could also wire 2 or 3 of them in parallel.