What is the best type of regulator to regulate DC voltage. I need to suppress the voltage of a DC Battery Bank to 89-Volts DC or less; the Electric Motor used will not operate at 90 Volts DC or above. Basically, even if the Battery Bank charges to over 90, I just want to use 89 Volts. Thank you in advance.

DC is much harder to regulate than AC. If the battery bank is series connected, can you convert some to parallel?

For small currents it's actually very easy to regulate through a solid state device. But for large currents the price goes way up, and choices go down. You'll just need to search for one that matches your needs.