AAA NiMH Batteries
There exist several different types of AAA batteries, including the common alkaline AAA battery, the lithium AAA battery and the NiMH battery. The AAA NiMH battery is very popular in digital cameras, MP3 players and other consumer appliances. In really high demanding devices, the lithium based batteries are more common than the NiMH AAA battery. For low-demanding gadgets, the alkaline AAA battery is still very popular.
The abbreviation NiMH in NiMH AAA battery stands for Nickel Metal Hydride. The NiMH AAA battery is a rechargeable battery.
Earlier, most rechargeable AAA batteries where NiCd batteries consisting of nickel and cadmium, but today the new NiMH AAA battery has grow extremely popular since it poses a lesser danger to the environment than nickel and cadmium based batteries. The technology behind the NiMH AAA battery was developed in Michigan by a division of the company ECD Ovonics named Ovonic Battery. The first NiMH batteries appeared on the market in 1983.
One important difference between the NiMH AAA battery and the NiCd AAA battery is that the former one has an anode consisting of a hydride absorbing alloy instead of cadmium. The cathode consists of nickel in both the NiMH AAA battery and the NiCd AAA battery.
Another feature that makes the choice between the NiCd and the NiMH AAA battery easy is the fact the NiMH batteries can have two to three times the capacity of NiCd batteries of the same size. The NiMH AAA battery is less prone to corrosion than the NiCd AAA battery, which means that if you load your little flashlight with a NiMH AAA battery and leave it in the garage for a year it is much less likely to form excessive corroded content than if you would have used a NiCd battery.
The NiCd AAA battery has a lower internal resistance and is therefore still popular in very high current drain devices, e.g. RC cars and cordless power tools. Compared to the lithium batteries, the NiMH AAA battery has a lower volumetric energy density. The self-discharge is also lower in NiCd batteries and lithium batteries than in the NiMH AAA battery. The self-discharge for a NiMH AAA battery will typically vary from five to ten percent during the first day after being charged, and then stabilize to a lower number – usually 0.5 to 1 percent per day. These numbers assume that you store your batteries at room temperature. Even small temperature changes can have a significant effect on the self-discharge rate for the NiMH AAA battery.
So, what is really going on inside the NiMH AAA battery and how does it work? Well, as mentioned above the abbreviation NiMH stands for nickel metal hydride. The cathode in the NiMH AAA battery is made of nickel, while the anode consists of a hydride absorbing alloy. The metal in the NiMH AAA battery is not one single metal; it is an intermetallic compound. Most NiMH batteries use one of several intermetallic compound referred to as AB5. A stands for a titanium and/or a mixture made from rare earth metals, while B stands for aluminium, cobalt, manganese and/or nickel.
The chemical reaction that takes place in your NiMH AAA battery is H2O + Mm + 2e- ↔ OH- + 0.5H2 (stored as Mm-Hx). Water (H2O) reacts with Mischmetal (Mm) and two electrons. Hydrogen is formed and stored as Mm-Hx; metal hydride. Mischmetal is a German word used for alloys of rare earth elements in various naturally-occurring proportions.
The NiMH AAA battery is charged in the right direction of the above equation and will discharge in the left direction. Note how no hydrogen gas is formed inside the NiMH AAA battery, since the hydrogen is stored in the form of metal hydride.
