Today I learned about....batteries

I'm not an industry expert, but the vast majority of impurities in used lead are probably smelted out. I found one difference between a 12 month and a 72 month battery is the distance between the bottom of the electrodes and the bottom of the battery. The more expensive batteries have more space between the plates and the floor.

The whole process of extracting electricity from a chemical lead acid battery involves forming lead sulfate on the surface of the plates. Since both plates get plated, the dissimilarity between them diminishes -- as does the charge. Recharging involves dissolving the lead sulfate back into sulfuric acid and water. Ultimately, some of the lead sulfate crystalizes and falls off to the bottom. When there's enough to start shorting between the plates, the battery is bad.

I well kept secret fully explained in 1922 text books is to occasionally charge the battery to about 16.2 volts to fully de-sulfate the plates. There is a sequence to get it perfect, but just an occasional conditioning can extend battery life by 3X or more.