They vary from miniature devices to rotating utility generators producing megawatts. They range from intermittent, generating a single pulse at each mechanical motion, short term as in plunger type explosive detonators, to steady state power plants with continuous rotation. 60 Hz power out of the wall comes from such generators. Any two metals, or a metal and graphite, generate a voltage between them when they are joined by an electrolyte, typically a water solution of an ionized acid. Typical voltage is 1.5 volts per cell and it is common practice to connect several cells in series.
Dry cells are such wet cells with a minimum amount of water sealed in.
One of the metals goes into solution as the battery is discharged. Secondary, or storage, batteries. These resemble primary batteries except that the electrodes are of the same material to begin with. The battery becomes a source of voltage and current until it is discharged and returns to its original state. Automobile batteries are storage batteries, typically producing 12 volts.