Two batteries connected to a resistor
You can think of this as the source V2 charging the battery represented by V1. It is absorbing energy, and V2 is providing it. The current circulates just as you said, into the + terminal of V1 and out the - terminal. 

If you do an energy balance sheet, the resistor will dissipate I^2*R = 90W but V2 is providing 3A * 150V = 450W. The remainder (360W) is going into the V1 source. If V1 is a rechargable battery it may be able to absorb that for a while, but at some point it will become fully charged and the battery will get very hot and may be damaged from overcharging.