I have the circuit shown here:
where I'm using a switching regulator (not the linear 78xx shown) to provide me with 5V to power an Arduino and a little more circuitry that the Arduino is monitoring. The Arduino is measuring (and logging) the voltage at A - ie I'm logging the input power supply voltage (which varies ~12.7 > 14V according to the solar charging the 12V batteries).
Without the op amp (ie measuring the voltage directly from the potential divider [and with resistor values ~1/2 of that shown]) the voltage logged is as expected. I want the unity gain op amp so as I can increase the resistor values to reduce the current drawn.
However with the op amp in place the output (A) is a static ~3.74V. I measured this directly at a time when the voltage to the +ve op amp input was ~3.85V, and I also know from reviewing the logging that the voltage on the output of the op amp was static throughout the previous day*.
I'm probably making some rookie mistakes, I'm new to the idea of an op-amp as a buffer - in this case I'm using an LM358 (datasheet pdf) as it's what I have available - I'm very open to suggestions of a more appropriate amplifier. While I could get away without it in this case, I do want to minimise the current drawn here, and I will also be logging the solar voltage (~70V) so will definitely want > 10k resistors in that potential divider and will therefore need a buffer.
*the voltage is logged whenever the scaled measured voltage varies by 0.05V (ie battery voltage goes 13.1 -> 13.15). Without the op amp this logged ~1000 entries in a day. With the op amp it logged half a dozen, and these varied only within ~12.4-12.5V (despite the solar MPPT charger reporting the day max of 14V).