Battery

A Battery stores electrical energy (DC).
A wet cell (car battery) contains sulfuric acid mixed with water for the electrolyte.  In a dry cell (flashlight battery) the electrolyte is held in an absorbent pasty material. Electrolyte is caustic.  Chemical action within the battery accumulates excess electrons on the negative plate, then when a load like a lamp is connected to the battery the electrons released from the battery's negative electrode flow through the lamp, illuminating it, and back to the battery's positive electrode.

The battery's voltage depends on the chemicals used and the number of individual cells connected in series to produce the rated output.

The current depends on the resistance in the total circuit (including the battery—and also on electrode size).

Batteries may be connected in series (the positive electrode of one to the negative electrode of the next), to increase total voltage, or in parallel (positive to positive and negative to negative), to increase total current.

Automobile batteries, cell phone batteries and some special types of batteries are rechargeable.

The standard AAA, AA, C and D cell batteries that are sold for one time use are not rechargeable. They were given an initial charge by the manufacturer’s special machinery and were not designed to be recharged.  Attempting to recharge a battery that was not designed to be recharged may cause the battery case to either break open or explode.  The cases of these batteries are sealed to protect you from coming in contact with the caustic electrolyte. 

Excessive internal overheating during the charging or discharging of a battery can cause it to explode.  News reports have told of cell phones that had exploded due to a battery problem.  The batteries used in the small hand held cell phones store a lot of energy in a very small package.  A short circuit in a cell phone can cause rapid internal overheating of the battery which causes rapid internal gassing within the battery. 

Safety tips:  Protect your eyes when working around wet cell batteries, wear protective glasses.  Never short out the battery terminals, as the battery just might explode due to very rapid internal overheating.