Technically you could hook everything up to charge the trolling motor battery off the main motor but in simple terms the trolling motor sucks more "juice" than the main motor can supply. And with lithium batteries you can make 3 or 4 trips before they need recharging and actually they recommend running the lithium batteries down before you recharge them. Added to which you can get an app for your phone that will tell you how much juice you have left in your lithium batteries. I made the switch from lead-acid to lithium a year ago and am delighted. I did a trip 2 days ago where I started with fully charged batteries. I trolled for 8 hours between 2 and 5 setting on the Terrova and still have 75% charge left.