Posting back with test results. I let the Garmin 66i run out of power (it was about the 35 hours the advertise!) and the NP10000 fully recharged it in about 2 hours, with charge left over. I could probably get nearly 2 charges out of this backup. With proper battery saving measures, I could get several days out of this setup. Nice! but a bit heavier than my 62st and extra AA batteries. I can live with that considering the extra features I now have. For a map geek - this is perfect!
Sorry about the delayed reply! Yes I did but it is not a Garmin map. It is a map of West Virginia that I got from GPS file Depot quite some time ago. I have added the trails in the areas I hike in and that's primarily what I use the GPS for.
As for new maps I'm generating them in caltopo converting them to Garmin format and then installing those.. My understanding is Garmin is pushing out updates for their units that disabled turn-by-turn routing on anything except for their Garmin maps. If it affects my unit it'll go on eBay and I'll never buy another one.