# What would happen if you let a full battery charge an empty

• stargazer193857

#### stargazer193857

Suppose you have a full battery and an empty one, both the same type, and you hook up positive to positive and negative to negative, and for this particular battery chemistry, the difference in voltage is great enough to commence charging of the negative one. How much current would likely travel between them? Wire melting / battery cooking current, or a more controlled rate of discharge / recharge?

Well, mrBigGuy told me by PM that a similar set up would go ka-bam.
So I will not try that myself.
But I would really like to see a you-tube video of someone else doing it. I know there has to be someone who tried it, given all the skateboard crashes on you tube. But what keywords to use.
Anyone want to link me to such a video of one battery charging another and exploding?

Never mind. No You-Tube video is needed. I now fully believe how dangerous it is. Probably a much safer way to do it is to put a resister between the batteries scaled to the voltage difference, and put a heat sink on the resister. I thought of that idea before, but I abandoned it because of a silly math error. Suppose the voltage difference is 1.5v and a 3 Ohm resister is used to get 0.5 amps. I then calculated the voltage drop across the resister: V = IR = 0.5 x 3 = 1.5v. I incorrectly concluded that the 1.5v drop across the resister would consume the whole 1.5v difference between them, preventing any charging. After thinking about that some more, and now realize the 1.5v drop is the same drop that was there before, and current would still be 0.5A (and decreasing). If the current were somehow zero, the resister voltage drop would decrease to zero, contradicting the assumption.

Of course I'd never play with something like this. I'm just trying to understand the physics.