1. The problem statement, all variables and given/known data This isn't a homework problem per say but I am struggling with what we are doing in class at the moment which is to do with circuits with two sources of Emf. More specifically, how one source can be used to re-charge a battery. I have looked at the following website which i will refer to below (https://www.boundless.com/physics/t...battery-emfs-in-series-and-parallel-536-5597/) 2. Relevant equations Kirchhoff's laws. 3. The attempt at a solution I get that when one the positive side of one source of Emf is wired to the positive side of the other source then the combined Emf is the difference between the two. What I don't get is why this charges a battery. Imagine the source Emf (to charge the battery) is 12V and it is charging a battery which is running low at 4V (compared to its normal voltage of 10V). A diagram of this set up that I have seen has nothing else in the circuit just these two Emf sources. How does Kirchoff's voltage loop rule apply? If I gain 12V through the source voltage then drop 4V I am not at zero volts? What am I missing? Many thanks to any help offered!