tammy2211 - There are no simple answers to your battery question. I assume you are referring to 'primary cells', batteries that are not rechargeable, like traditional alkaline or Leclanche cells we associate with flashlights, portable consumer appliances, toys etc. In most cases, the designer of the appliance tries to best match the battery with the application need. One of the key variables is the current drain of the system. A familiar AAA vs AA vs C vs D cell are basically all 1.5 volt devices that produce a range of output currents.
Here is a few lines from a battery Wikipedia that summarizes some of the issues:
Capacity of an alkaline battery is greater than an equal size Leclanché cell or zinc-chloride cell because the manganese dioxide is purer and denser, and space taken up by internal components such as electrodes is less. An alkaline cell can provide between three and five times capacity.
The capacity of an alkaline battery is strongly dependent on the load. An AA-sized alkaline battery might have an effective capacity of 3000 mAh at low drain, but at a load of 1 ampere, which is common for digital cameras, the capacity could be as little as 700 mAh. The voltage of the battery declines steadily during use, so the total usable capacity depends on the cut-off voltage of the application. Unlike Leclanche cells, the alkaline cell delivers about as much capacity on intermittent or continuous light loads. On a heavy load, capacity is reduced on continuous discharge compared with intermittent discharge, but the reduction is less than for Leclanche cells.
The nominal voltage of a fresh alkaline cell is 1.5 V. Multiple voltages may be achieved with series of cells. The effective zero-load voltage of a non discharged alkaline battery varies from 1.50 to 1.65 V, depending on the purity of the manganese dioxide used and the contents of zinc oxide in the electrolyte. The average voltage under load depends on discharge and varies from 1.1 to 1.3 V. The fully discharged cell has a remaining voltage in the range of 0.8 to 1.0 V.
The amount of current an alkaline battery can deliver is roughly proportional to its physical size. This is a result of decreasing internal resistance as the internal surface area of the cell increases. A general rule of thumb is that an AA alkaline battery can deliver 700 mA without any significant heating. Larger cells, such as C and D cells, can deliver more current. Applications requiring currents of several amperes, such as powerful flashlights and portable stereos, will require D-sized cells to handle the increased load.[/i]
A flash light would provide a reasonable load for the physical size batteries it was designed to use, but you need some ability to measure a consistent 'cut-off' point to determine which batteries had the best capacity. Typically a voltmeter would be used to make that measurement. Your eyes would not be a good measurement device as the bulb grew dimmer, your vision is too subjective. This test might be best used to evaluate several different manufacturer's batteries of the same size to evaluate their capacities. As each cell starts with about 1.55 volts each (3.1 volts for two batteries in series), you might set your arbitrary measurement criteria at 0.8 volts/cell for the test. At that voltage the flashlight will still be giving off light, but it is quickly diminishing.
Hope this helps. There is a lot of material available on the web found by searching battery capacity and similar terms.