Consider a battery of EMF $E $ and label its positive terminal $P $ and Negative terminal $N $.
Consider current flowing through the battery from $N $ to $P $. Then work is done by the battery. This makes sense because the electrostatic charge on the battery tries to oppose the current. So the battery needs to oppose that force for the current to go ahead and thus it does work.
But consider the opposite case. Current flows from $P $ to $N $ in the battery. Then we say work is done on the battery. Well, this statement doesn't seem intuitively right to me. What does it really mean for the work to be done on the battery? I mean the electrostatic charges are trying to support the current to go ahead. So why is work done on the battery?
My TA says it means charging the battery. But that doesn't give me a sense of satisfaction.
One way I imagine it is that we are doing the reverse of what we did first. So if in the first case work was done by the battery in the second case it will be done on the battery. Any other way to do that?