especially the nickel-cadmium type, should never be used until the current goes down tozero, because this can ruin them.122 Cells and batteries7-3A flat discharge curve. This is considered ideal.The area under the curve in Fig. 7-3 is the total capacity of the cell or battery inampere hours. This area is always pretty much the same for any particular type and sizeof cell or battery, regardless of the amount of current drawn while it’s in use.Common dime-store cells and batteriesThe cells you see in grocery, department, drug, and hardware stores that are popular foruse in household convenience items like flashlights and transistor radios are usually ofthe zinc-carbon or alkaline variety. These provide 1.5 V and are available in sizesknown as AAA (very small), AA (small), C (medium large), and D (large). You haveprobably seen all of these sizes hanging in packages on a pegboard. Batteries made fromthese cells are usually 6 V or 9 V.One type of cell and battery that has become available recently, the nickel-cad-mium rechargeable type, is discussed in some detail a bit later in this chapter.Zinc-carbon cellsThese cells have a fairly long shelf life. A cylindrical zinc-carbon cutaway diagram isshown at Fig. 7-4. The zinc forms the case and is the negative electrode. A carbon rodserves as the positive electrode. The electrolyte is a paste of manganese dioxide andcarbon. Zinc-carbon cells are inexpensive and are good at moderate temperatures, andin applications where the current drain is moderate to high. They are not very good inextreme cold.