Storage capacity:

Any electrochemical cell or battery has a some amount of electrical energy which can be gotten

Figure--A Weston standard cell.

from it, and this can be given in kilowatt hours. More often though it is measured in ampere hours (Ah).

A battery having a rate of 2 Ah can give 2 A for an hour, or 1 A for 2 hours. Or it can give 100 mA for 20 hours. Within reason, the product of current in amperes, and the use time in hours, may be as much as, but not over 2. The limitations are the shelf life at 1 extreme, and the maximum safe current at other. Shelf life is length of time the battery will last if it is on the shelf without being used; this may be years. The maximum safe current can be represented by lowest load resistance that the battery can work into before the voltage drops due to its own internal resistance. A battery is never used with loads which are too heavy, as it cannot supply the required current anyway, and it might burst, or blow up.

Small cells have storage capacity of some milliampere hours (mAh) up to 100 or 200 mAh. Medium-sized cells may supply 500 mAh or 1 Ah. Big automotive or truck batteries can give upwards of 50 Ah. The power capacity in watt hours is the ampere-hour capacity time's battery voltage.

When the ideal cell is used, it delivers a constant current for awhile, and then the current begins to fall off. Some types of cells and batteries approach this ideal nature, known as a flat discharge curve, and others have current that declines rapidly, almost right from the beginning. When the current that a battery can provide has tailed off to about half of its initial value, the cell is said to be weak. At this particular time, it should be replaced. If it is allowed to run all the way out, until the current goes to zero, the cell or battery is dead. Some of the rechargeable  cells and batteries, specifically the nickel-cadmium type, should be never used till the current goes down to zero, as this can ruin them.

Figure--   A flat discharge curve. This is considered  ideal.

The area under the curve in Figure given is the total capacity of cell in ampere hours. This area is pretty much the same for any particular type and size of the cell, regardless of the amount of current drawn while it is in use.