If the driving impedance is small then the 10msec settling time should be adequate.

You can use single rail CMOS operational amplifiers to buffer your reference...these amplifiers have very low quiescent current -less than 1mA but can provide low impedance output.

A simpler way is to use an emitter follower with the collector connected to the Vcc (same supply as your MCU); but remember that the output will be about 0.66volts less than your voltage divider...

A third method would be to use a zener in series to drop the battery voltage to below Vcc (9.1volt zener will give you 4.5volt max at 13.6volt) and take your reference from, say, 2.2k connected with the zener to ground. This will limit the current to ~2mA.

_warrier