"4 Days Out"
Season 2, Episode 9

Walt and Jesse head out to the desert for a long weekend in the Winnebago to handle business. After cooking more than 42 pounds of meth that will earn them $672,000, their celebration is cut short after they discover that the keys Jesse left in the ignition had drained the battery. After flipping his lid, Walt simmered down and put his brain to work.

He cuts a container in two with a sponge soaked in potassium hydroxide, filling one side with mercuric oxide and graphite from the Winnebago's brake pads (not bad, eh?) and the other side with coins from his pockets. He connects the two sides with a copper wire: from there, the brake pads create a negative charge, the zinc from the coins create a positive charge, and the sponge acts as an electrolyte that keeps them separated. He connects this make shift battery to the RV, and after hitting the ignition and sending sparks flying, the Winnebago comes back to life. 

Later in the episode, Walt finds out that his tumor has shrunk by 80-percent. Then, while in the bathroom looking at his reflection in a metal towel dispenser, he pounds it out of anger.