I know the board input pins have a 3.3volt input limitation but I was hoping to measure between 11 and 13 volts (I have a voltage leak in my car electrical system and thought it would be fun to use the Imp as a regular measurement device). After a bit of googling it seems I will need some sort of voltage divider to scale the voltage so the Imp can handle it, could anyone offer any advice on something like this?
If you have a current leak in your car then you usually can find it by putting a multimeter inline with the negative battery cable and pull fuses one at a time (only have one out at a time). The current should be below ~ 50mA if things are fine. of course this depends on the car.
I have done this with cars as new as 2003 vw Jetta. When the offending circuit is disconnected there will be a drop in the current.
If your car is quite modern then you may wish to seek a car-specific forum to see if this method is still oK. I recall people saying the latest cars may not be so striaightforward.
I have found leaks at least twice with this method and the last time was just a few months back. It was quite striking when I found it.
I think you should plan for the voltage to be nominally up to 15V and you also need to have protection for the spikes. Automotive systems are famous for having the potential to have large spikes or surges of voltage.
yes you need a voltage divider to measure that signal and you also want to make sure the divider does not cause a big load. Probably want to use resistors in the 10s of thousands of ohms.
Search for automotive ADC input protection voltage divider.
Your minimum circuit will be two resistors. Many suggested circuits also have diodes to clamp the voltage so that it does not go far beyond the rails.