You'll need a free programme called hwinfo and enable on screen display in that programme. There are options for what you want to display on the screen and you can select the option to display the charge level. Note that in order for this feature to work, you need the RivaTuner Statistics Server service running in the background to intercepy the draw calls. This is also available for free download all over the internet.
Most people use MSI afterburner to display fps etc to screen but i find hwinfo to be much more lightweight and customisable at the same time