ChiefsPlanet - Electronics Great Tips for Recharging Old Batteries!

Yeah... that's a good way to have a battery explode and shower you with harmful chemicals...

This is not safe, and not recommended. And it's nearly impossible to do anyway, if the battery has been completely drained.

Here's a good explanation:

Quote:

It cannot be done safely, or with more than limited success. Your results may vary.

First, we need to understand how batteries work. Two chemicals with opposite charges are separated in a cell. When a circuit is completed from the cathode (+) to the anode (-), electromagnetism happens and the two chemicals react with each other, releasing electricity, until they are in equilibrium and the battery is dead.

In order to recharge a battery, an opposite electric charge needs to be applied to the battery in attempt to reverse the reaction and restore the two chemicals to their original state. However, this is not possible with all materials. The professor states that...

nonrechargeable, or primary, batteries can be based on irreversible chemical changes. For example, the carbon-fluoride-lithium primary batteries often used in cameras generate energy by converting (CF)n and Li metal to carbon and LiF. But the starting material at the battery's cathode, (CF)n, is not reformed when a reverse potential is applied. Instead the cell electrolyte decomposes, and eventually the fluoride is oxidized to form fluorine gas.
The professor goes on to say that...

A reversible chemical change is not the only requirement for rechargeable batteries, however. To be classified as rechargeable, the battery must be able to undergo the reverse reaction efficiently, so that hundreds or even thousands of recharging cycles are possible. In addition, there must often be provisions to ensure that the recharging process can occur safely.
(emphasis mine)

In reference to alkaline batteries specifically...

The alkaline batteries (which are generally based on the conversion of MnO2 and Zn to Mn3O4 and ZnO) offer an excellent example of this last point. Although the chemical changes at the electrodes can be reversed, until recently alkaline batteries were manufactured only to function as primary cells. Recharging one of these primary cells could allow the battery to be reused, but the possible number of recharging cycles for such a cell is very limited--it performs more poorly with each recharge. More important, recharging an old-fashioned alkaline battery is not safe. During or after a recharge, the battery might generate enough hydrogen gas to cause an explosion.

This is why there are so few products available on the market, and why the ones that do exist often get poor ratings due to battery leakage (see the comments). While the chemical reaction can technically be reversed, the basic construction of a cheap, single-use battery does not provide for handling of any hydrogen buildup (which causes leaks) or protection against the degradation of the cell's contents (which limits their rechargeability).

More info: http://www.scientificamerican.com/ar...hargeable-that