I have a question that I believe requires knowledge on the graduate level, but I can't necessarily express it in terms more complicated than plain English. If this is not so, moderator, please adjust to your taste, and thank you! If it helps to know this, I'm an audio/video systems designer and installer with electrical and electronic experience including electronic design to the board level (PC Board) of analog circuitry. I've worked with electronics for decades. If I have it right, when one charges a battery, energy is required from the charger to undo the chemical reaction that discharged the battery. This is accomplished by having, at its simplest, a source of voltage higher than the battery will be when charged, with some kind of current limiting typically set to a value that maintains the best battery life. But someone posed one of those it-can't-happen questions and I've found an interesting detail. First, let's say this is a physics problem where everybody promises not to blow up while we discuss things. If one were to connect two batteries of identical design, one charged and one depleted, positive to positive and negative to negative, the charged battery would charge the discharged battery, and as it did so its charge and ultimately its voltage would lower. This differs from standard battery charging in that here, the "supply" is lowering in voltage and energy as the charging battery charges. Here's the question: do the two batteries come to equilibrium of voltage? I propose not, since, speaking inexactly, electrons are being pushed around, and in electricity, pushing is voltage. Does the supply battery have to be some particular voltage higher than the charging battery in order to keep charging it? Does the process stop when the supply battery is exactly that voltage higher than the charging battery? Let me describe a roughly analogous situation: If one were to do this with a silicon diode between the batteries, we know that the supply battery would not go below 0.7 volts above the charging battery, because of the forward voltage drop of the diode. But is there a similar forward voltage drop of the charge process itself? Why? How? Thanks.