I used a similar approach to make a high-current smart charger for my electric golf cart. However, since the battery voltage goes all the way to 48V, I couldn't use the Arduino's on-board regulator directly. I made a "pre-regulator" which drops 48V to ~10V to power the Arduino mini...
...Interesting! Do you remember the components and/or design of this? I'd like to learn more about how that's done...
The schematic is on another computer across the parking lot...
The problem is that most IC regulators (or cheap Chinese Buck Regulators) will not tolerate an input voltage as high as 48V, besides I like to build things from parts I have laying around without waiting for the Brown truck. I had some NPN power transistors which would dissipate >5W and had a Vce(max) of ~100V. I also have a collection of 1W Zener diodes, so I threw together some thing like this using a 12V Zener:
I ran a simulation to help explain how it works. AFAICR, the Arduino and its associated stuff draws about 50mA from its Vin pin. The simulation shows what happens when the battery voltage is 30V (fully discharged, GREEN trace) and again when the battery voltage is 48V (peak under charge, YELLOW trace) as the simulated Arduino current varies from 20mA to 70mA (x-axis).
I show two sets of plots. The upper pane shows what the Arduino's V(in) will be as a function of battery voltage and the Arduino's current demand. About all we can say is that at any given Arduino current, the Arduino's input voltage will be between the green and yellow trace. However, this is well within the allowed range, and the fact that it is not perfectly regulated doesn't matter because the Arduino's on-board regulator will take care of that.
The lower plot pane shows the power dissipation in the NPN transistor Q1. Note that when the battery voltage is 48V (Yellow Trace) and the Arduino current is 70mA, the power dissipation in the NPN Q1 is 2.5W. This is the range where the NPN has to be bolted to a heatsink. AFAICR, in my Golf Cart charger, I bolted it to the sheet steel enclosure using an insulated mounting kit. R1 should be a 1/2W resistor.