All Macbooks (save the new Retina Macbook which uses a USB-C charging port) use nominally 20V power supplies (the magsafe chargers, 20V will be marked on the bottom of them). In actuality the peak output voltage is under 19V (18.5V as you mentioned is normal), the same as almost all other laptops. However, note that this is peak. The actual supply voltage depends on the draw required (which depends on charge state of battery and workload) and will be either approximately 14.5V, 16.5V or 18.5V. So 16.6V is normal.
Hope that helps!