My apologies if I have misunderstood the question, but connecting the output of a secondary battery to the charge input of the primary would not work "in the field" most, if not all such batteries include a BMS (battery management system) this limits the charge current, often not more than 2A (to avoid damage and fire etc). The primary battery output (when running the motor) will be in the region of 10A (36A 10A will run a 360W motor). In short, you'll be filling a bucket with a thimble.