How can I make my voltage divider work?
This is what you  have:



simulate this circuit &ndash; Schematic created using CircuitLab
The circuit on the right will work.
The problem with your circuit is that the NPN transistor will only output the control voltage minus Vbe, which is about 0.6 or 0.7V. So it won't be related to the battery voltage at all.
The circuit on the right is a high side switch that controls the PNP transistor via the NPN transistor.