It's kind of a mopar...2017 Cherokee starting system issue

Daughters car, 2017 Jeep Cherokee. She lent it to a friend, so excuse if I don't have all the information. Apparently it would not start for him. He tried jumping it, and apparently was hooking the cables to the small start/stop battery under the pax seat instead of the jump start terminals under the hood. Both batteries were then replaced. The smaller battery is now not receiving a charge from the alternator. Main battery is. If I put a charger on the smaller on it will start and run for a short time...the smaller one runs down very quick. No idea where to look, but shouldn't the two batteries be connected together somewhere to get the alternator current? Fuse? Fusible link? Ideas and location of where to look?
Thanks!