Interfacing battery to fpga
You can use a high-side switch, which is just a complementary MOSFET pair, to switch the battery connection to the resistive divider.  Then you can use lower-value resistors which will cause less ADC error, and you can have it only draw current when you're actually measuring the battery voltage.

High-side switches are available as a single component, or can be assembled from discrete parts (an N-ch MOSFET, a P-ch MOSFET and a few resistors).







simulate this circuit &ndash; Schematic created using CircuitLab