Charging 72V battery bank from 15VDC source
There are solutions to converting lower voltages to higher voltages but none (not surprisingly) are 100% in power conversion effciency.

Lets say you need 2A at 72V to charge your battery pack and run your motor. That's a power of 144W. Add-on 15% of that power to account for inefficiencies in the low voltage conversion and you get a likely input power demand of about 166W.

If your input is 12V then this will need to supply \$\frac{166}{12}\$ amps which is near enough 14A.


  How well will a 12v to 72 DC-to-DC Converter work with providing
  constant charging to the battery bank


Hopefully you have the methods to calculate whether your 12V source can supply demands. Obviously you plug in your own current (not the 2A I originally said) and redo the calculation.