I used an external battery on my Garmon 705 this weekend. It worked very well.
The battery I used contains two NiMH AA batteries which allowed me to carry 2 AA alkalines as backup. The backup was not needed so I think, for me, the Duracell Li-Ion battery will be a better choice because it is much more compact. http://www.duracelldirect.com/dr7000li.aspx


Are you sure you have no battery level indicator? On my 705 I believe it appears as an icon when I select "menu".