How to decrease 83v 17ah DC battery to 72v?
As mentioned it the comments, the voltage across a Li-Ion based battery varies a lot between discharged and charged state. For laptop batteries (also Li-Ion based) most manufacturers use 3.6 V per cell as the nominal voltage but this will increase to 4.2 V per cell when fully charged.

So 72 V / 3.6 V = 20 cells

fully charged that could be upto:

20 x 4.2 V = 84 V

So 83 V for fully charged looks perfectly normal to me :-)

If you still have doubts, then fully charge the original battery and measure it's voltage, I bet that it will also reach at least 83 V !