Some fairly recent update (last month or so) to the T-Mobile app caused my V10 battery to drain in about 4 hours.  When I looked at battery usage, I saw the T-Mobile app at the top, typically at about 30% usage.  This is ridiculous since I don't use this app at all.  I read somewhere else that they pushed an update that is using your phone to send network quality information up.  It's a clever idea - use the huge network of devices to determine in real-time what network quality is, but they apparently didn't do it in a way that is very respectful of battery usage.
So... I found and launched the T-Mobile app. I disabled every opt-in setting there.  I then chose to Uninstall Updates - I think you do this from the battery saving page.  That restored it to the factory default.
Problem solved!  I know am getting closer to 10 hours per day out of my battery.
Bad T-Mobile!  Bad!