Nobody has given you the best answer yet. Do it like this, with two transistors. Put a PFET on the top of the divider (between battery + and voltage divider). PFET source is connected to battery. Drain is connected to divider. PFET has pull-up from gate to source. Pullup can be around 100k or even more if you want. Connect NFET drain to PFET gate. Connect NFET source to GND. Connect NFET gate to processor VCC, or to a processor GPIO. When NFET gate is high, divider will be in operation. When NFET gate is low, divider will be disconnected from battery.
Personally, I think it is a good idea to make sure the battery sense divider does not drain the battery when the device is powered off, even if it is just 10s or 100 uA. Furthermore, the battery voltage should not be applied to the ADC input when VCC is not present, not even through a large resistor (unless this is a highly specialized input pin). So I would argue that you MUST disconnect the battery from the ADC any time VCC is not present.
If you cannot visualize what I typed, let me know and I will draw it for you.
Edit: Use BSS138 for NMOS and BSS84 for PMOS. Just a recommendation. These parts are very low cost in volume and are readily available.