The system-monitor extension ( https://extensions.gnome.org/extension/120/system-monitor/ ) has a option to enable/disable the battery icon.
This extension also shows the CPU/memory/network status which I miss as in Gnome 2. Great extension!
The upower service may fail to start if an iPhone is connected to the laptop, which cause the battery icon in Gnome-shell disappear.
Simply removing the iPhone before booting the Linux may make upower start successfully.