Going on a double century next week and purchased an external battery for my Garmin 705 which uses 2 AA NiMH batteries. I suppose it will work fine if I tape the battery door shut before I mount it to my bike.
It occurred to me that I have an extra Lithium ion battery for my Dinotte lights which is in a nicely sealed container that is easily attached to my bike (and, I believe, with larger capacity than 2 AA batteries). Has anyone used such a battery with their Garmin GPS? Maybe not for this ride but it seems to me that the Dinotte battery might be a better solution.