Interesting, but. If the charger is rated for 200mA output @ 15V and the cell count is five or 7.2V static. Presuming the combined cells would consume 7.2V under charge is unrealistic, IMO. The math or formula is accurate, but the V rate at which the battery pack will be charged is unknown. Most likely is it will be much higher than 7.2V as the charger is capable and rated to deliver 15V. It knows not what's at the other end. A simple exercise with a DVM will reveal the actual V under load.

I am not convinced that the charger will charge at a higher mA output simply because the V is well below the output rating. There is no load other than the cells IR...

As Jazz mentioned math and theory are often worlds apart from reality.