The current out of the panel is (very approximately) proportional to the light falling on it - so at STC (noon, clear blue sky, panel square to the sun) it will produce max power at ~18v and 1.72A; if it's dull, grey and overcast (say 10% light intensity) the max power point (MPP) may be more like 16.5V, 0.17A. if it's a bright noon and there are clouds around (but not blocking) the sun you can get 'cloud lensing' effects which might give you a MPP at, say, 18.5v and 1.9A.
A MPPTracker uses a uprocessor to measure power output and constantly adjusts the voltage to optimise power for the light conditions. If you want to try and approximate the max power point without calculations you would be better off regulating to a voltage setpoint rather than current, as voltage doesn't vary as much with light level.
Unless you can make exceptionally efficient dc-dc converters, your would be better of using a single stage to go from 18v to battery volts - or better yet, get a 60 cell panel (MPP ~30v) and directly connect to the 24v battery without MPPT's of dc-dc convertors IMHO.