A battery, such as a D battery, is a device that generates electrical energy. Batteries are convenient portable sources of energy. Unlike other energy-conversion devices, batteries contain no moving parts. Batteries convert chemical energy into electrical energy by means of a chemical reaction.
Amount of Current
A standard D-size carbon-zinc battery has an Ah (amp-hour) capacity of approximately 4.5 to 8 Ah (4500-8000 mAh). This means that a D battery could supply 6.25 amps of current for about one hour, more or less. This can also be calculated as the D battery supplying a current of 1 amp for about 6 hours, or any other combination with this same formula.
Just to permit a comparison of the different types of the same D size batteries, an Alkaline battery of the same size is rated at between 12000 to 18000 mAh, NiCd is rated at about 2000 to 5500 mAh, and NiMH at about 22000 to 120000 mAh.
You should keep in mind though, that this is only a guideline. If you are running the D battery at a maximum current of 4.5 amps, the amount of heat would cause the battery to lose part of its chemical reaction, causing it to be fully discharged before the one hour span. The same is true for an extremely light load as well. For example, if the battery was connected to a load of just 1 milliamp, it would give current for 4,500 hours. But, more than likely, evaporation of the chemicals needed for the reaction would also cause the battery to lose charge before the 4,500-hour mark.