Help with battery voltage indicator

My battery indicator has percentage and voltage options. On the back it is a function button and a set button. When I turn on the board is showing battery percentage and I press function button to switch to voltage and then when I turn off and on again it goes back to battery percentage… Anyone familiar with it? There is anyway to stay always on voltage?

Where did you buy it and do you have some kind of datasheet?

Bought on ebay. Ia this one