Running arduino from 50volt DC

I have an underwater scooter that I have repowered with a (12s x 4.2v=) 50.4 volt Li Ion pack.

I'd also like to run an arduino from the same pack. Or more to the point I'd like to run an atmega328p programmed but removed from the arduino board ( and on board regulator). And suggestions on what is best to use?

The atmega328p will only be running a few LED's and perhaps a buzzer and relay.

Could maybe select from a range here: ....click here.....

Or search DC buck converter

There's a whole raft of DC-DC converters that take an input of 18--72V, nominally 48V in, which are what you need. If you need 12V as well get a 48->12 converter and a 12->5V converter (the smaller the voltage ratio the more efficient DC-DC converters are). Figure out your maximum power draw and size the converter conservatively.

Or more to the point I'd like to run an atmega328p programmed but removed from the arduino board ( and on board regulator).

Unrelated to your question, FYI, it is not necessary to program the ATmega328 chip in your UNO. You can buy an FTDI USB to TTL Serial converter and use that to communicate with ATmega328 chips that are in breadboarded circuits. Moreover, you can buy ATmega328 UNO chips with the OPTIBOOT BOOTLOADER pre-installed.

When you receive your FTDI USB-to-TTL Serial converter you need to install the correct USB driver. With Win 7, the OS will usually do this automatically as soon as you connect it. There are a variety of types so you need to let the OS tell you which one you have and then "search for the correct driver online".

For the less than $5 that it costs to buy the FTDI converter, you can program as many breadboarded ATmega328 chips as you want , or you can make your own programmer

Search for “LM2596HVS” on ebay.
HV is the High Voltage version of that chip.

Isn’t it easier to use a $2 Nano.
Set the buck regulator to ~7volt, and feed it into V-in.
Leo…

Pick off 4.2V from the lowest battery and use that. Arduino power usage is next to nothing compared to the motor current. Or maybe use 8.4V and run it thru a 5V regulator if you need 5V for Aref.