My wife's Rad City 3 worked for about a month and then had the same issue.  It was very easy to replicate as with hers it happened within the first few miles.  From a full charge if you ran throttle max it would go about three miles and then shut down with a low battery alert and then nothing.  If I  took the battery out of the the tray and put it back in,  it would reset and allow the bike to work at low power settings for a while but not past 4 bars indicated, at which point you get nothing.

Took almost 2 months to finally convince Rad yesterday it was the battery.  They are replacing it under warranty which will take a while.