It doesn't wiggle, jiggle, or fall off (with a good cage that is). Also, its mostly protected from the elements.
When I did 24 hr races the promoter required bottle mounted batteries have an auxiliary strap or tape to guarantee they would not pop out of a cage. Seems they had a bad OTB crash from a battery popping out and getting caught in between the spokes. Of course batteries were very heavy then compared to now.
Here is mine. The strap from Backcountry Research works KILLER for this application...v. secure. I have a GeoMan battery and the base is contoured to fit round top tubes; mine is more square and even then, it doesn't move. Before I was using the Backcountry strap, the battery came with a generic velcro strap that allowed the battery to rotate around the tube; the Backcountry strap has a rubberized backing that prevents movement and grips the tube securely.
Geologist by trade...bicycle mechanic (former) by the grace of God!