Low Self-discharge Ni MH Battery - Comparison With Other Battery Types

Comparison With Other Battery Types

NiMH cells and chargers are readily available in retail stores in the common sizes AAA and AA. Adapter sleeves are available to use the more common AA size in C and D applications. The sizes C and D cells are somewhat available, but are often just a AA core hidden in an outer shell, with a rating of about 2500 mA·h, much less than ordinary alkaline C and D batteries. Real NiMH C and D batteries are expensive (and the chargers are uncommon); they should be rated at least 5000 mA·h for C and 10,000 mA·h for D sizes.

PP3 (nine volt) NiMH batteries are available; these usually have an output voltage of 8.4 V (1.2 × 7) and a capacity of roughly 200 mA·h. Also available are eight-cell nine volt batteries with a nominal output voltage of 9.6 V (1.2 × 8).

NiMH cells are not expensive, and the voltage and performance is similar to primary alkaline cells in those sizes; they can be substituted for most purposes. Although alkaline cells are rated at 1.5 volts and NiMH cells at 1.2 volts, during discharge the alkaline voltage eventually drops below that of NiMH. This is particularly true for high drain applications, where the voltage of even a fresh alkaline battery can be lower than a NiMH battery while under a load. Furthermore, NiMH batteries offer a flatter discharge curve, particularly at higher current draw.

NiMH cells are often used in digital cameras and other high drain devices, where over the duration of single charge use they outperform primary (such as alkaline) batteries. Applications that require frequent replacement of the battery, such as toys or video game controllers, also benefit from use of rechargeable batteries. With the development of low self-discharge NiMHs (see section above), many occasional-use and very low-power applications are now candidates for NiMH cells.

NiMH cells are particularly advantageous for high current drain applications, due in large part to their low internal resistance. Alkaline batteries, which might have approximately 3000 mA·h capacity at low current demand (200 mA), will have about 700 mA·h capacity with a 1000 mA load. Digital cameras with LCDs and flashlights can draw over 1000 mA, quickly depleting alkaline batteries. NiMH cells can deliver these current levels and maintain their full capacity.

Certain devices that were designed to operate using primary alkaline chemistry (or zinc–carbon/chloride) cells will not function when one uses NiMH cells as substitutes. However, this is rare, as most devices compensate for the voltage drop of an alkaline as it discharges down to about 1 volt. Low internal resistance allows NiMH cells to deliver a near-constant voltage until they are almost completely discharged. This will cause a battery level indicator to overstate the remaining charge if it was designed to read only the voltage curve of alkaline cells. The voltage of alkaline cells decreases steadily during most of the discharge cycle.

Lithium-ion batteries have a higher specific energy than nickel–metal hydride batteries, but they are significantly more expensive to produce. In October 2009, ECD Ovonics announced that their next-generation NiMH batteries will provide specific energy and power that are comparable to those of lithium ion batteries at a cost that is significantly lower than the cost of lithium ion batteries.