Similiar looking batteries can have major internal differences.
The capacities, voltages, self-discharge rates and internal cell quantities of Nickel-Metal Hydride (NiMH) rechargeable batteries vary significantly. Voltage is the most important characteristic, because too many volts will harm an electronic device and too few will make it function partially or not at all. Battery users discern most of these differences by looking at the NiMH battery or the package it came in.
Milliamp Capacity
Greater capacity allows batteries to provide the same amount of power for a longer period of time. Larger batteries tend to have a higher capacity; a "C" battery holds many more milliamps (mA) of power than a smaller "AA" cell. At the same time, some capacity differences exist within the same size of NiMH battery. An 800 milliamp battery often has the same physical size as a 1,000 mA unit. Capacity also affects battery lifespan, charging time and cost.
Output Voltage
Differences in voltage determine how demanding a device the NiMH battery will power. An 8.4 volt battery can run a boombox or a camcorder, while a single 1.2 volt unit will only power things such as tiny flashlights and clocks. Other typical NiMH voltage levels include 3.6 and 4.8. NiMH batteries achieve most voltages other than 1.2 volt by combining multiple cells in a battery pack.
Internal Cells
While most one-time use batteries consist of a single cell internally, NiMH batteries may contain one or several separate cells. Many NiMH battery packs consist of four or more cells, combined to achieve a certain voltage. For example, a 4.8V battery pack typically contains four 1.2V cells. It produces the same amount of power as four separate 1.2V NiMH batteries, but features a plastic outer coating and uses pieces of metal to combine power input/output into a single jack.
Self-Discharge
One of the more important differences among NiMH batteries is the rate of self-discharge, which determines how long they can sit unused before completely losing their charge. It proves especially important to have batteries with low discharge rates when traveling far away from a battery charger.
Low self-discharge batteries release a tiny fraction of the energy that other NiMH batteries lose each day, according to the California Department of Resources Recycling and Recovery. Some major battery manufacturers produce low self-discharge NiMH batteries, but they remain harder to find than alkaline units or batteries with high self-discharge levels. Battery packaging advertises low self-discharge rates.
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