how to step down both voltage and current for high power
If you want to get power for you Arduino from the 48 Volt battery bank, you just need a buck converter or DC-DC converter to convert the 48 volts to 5 volts.  The converter just needs to be able to supply at least the maximum current that the Arduino and any other 5 volt loads require.
If you select a DC-DC converter that is only rated to deliver 100 mA, that will be the maximum current you can draw from it, regardless of the original battery's voltage and current rating.
The 5 volt loads will only draw whatever current they require – a converter rated for 5 V 100 mA output will not force 100 mA into the loads.
Consider the electric outlets in your home – you can plug a 1000 Watt heater or a 15 Watt night light into the same outlet with no special precautions – each device will only draw the power or current that it requires.