Google has gone through several versions of location sharing over the years, all the way back to Google Latitude. That feature was inside Maps, but then location sharing moved to Google+. It came back to Maps last year, and an interesting new feature has rolled out. Now, sharing your location also shares your battery level.
We first became aware of the battery sharing feature early this year when some hints popped up in an APK Teardown. At the time, it looked like Google was going to give approximate battery levels in plain language—e.g. "Bob's battery is between 50 and 75 percent." However, the feature seems to be live for more people after appearing intermittently for a few weeks, and it's more precise. When checking a contact's location, there's now a battery icon right next to the distance. There's a battery percentage, and the icon indicates charging status. At least one Redditor also noticed the change recently.
I've tested this feature out and can confirm the battery level is accurate—it's exactly the battery percentage on your friend's phone when their location was last updated. This just happens automatically, and there doesn't appear to be any way to turn it off when sharing your location. So, you might want to think twice before you tell someone you didn't answer their call because your battery was almost dead. They might know the truth.