Passenger side of the frunk, under the "velcro" cover. It wouldn't hurt to check it with a volt meter, just because it is living on borrowed time based on other 2014-15 owners' experiences.....
https://www.youtube.com/watch?v=lKRj8gLcsfI
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2015 BMW i3 BEV, Giga World (Lodge interior), Tech/Driving Assist Packages, 30K miles