Why does the second sentence use 个?
It’s vernacular language.


  Charge your phone every day, and don’t forget to also give your speaking [skills] a charge!


個 is a generic counter for an event or session in this case, and will commonly be heard in a casual spoken style.

You can get similar constructions e.g.


吃個飯 (have a meal)
旅個遊 (go on a travel)