The Yellow light on dash is lit when the 12 Volt battery is being charged from the HV battery. Perfectly normal and a good sign that the I5 is looking after itself. There is a constant drain on the 12 Volt battery to keep everything running/monitored. The drain may be small but it is a drain nevertheless so an occasional charge is needed. Of course leaving the car in accessory mode would be a bigger drain and I'm not actually sure if the maintenance charge would take place or if accessory mode would eventually turn off. I usually just go to ON mode, even if just playing around to assure 12 Volt is maintaining a charge.