AA NIHM Batteries
The NiMH AA battery is a form of rechargeable cylindrical battery that is commonly used in portable devices, such as walkmans and small flashlight. The anode of the NiMH AA battery consists of a hydride absorbing alloy, while the cathode is made from nickel. This makes it a bit similar to the older NiCd (nickel-cadmium) AA battery, but since the NiMH AA battery use an hydride absorbing alloy instead of cadmium it is considered less detrimental for the environment. Cadmium is dangerous for animals as well as for the human body. Even low concentrations of cadmium are toxic and the metal will bioaccumulate in ecosystems.
We still do not know exactly how cadmium damages our bodies, but it has been showed to interfere with the action of certain zinc-containing enzymes. Cadmium is usually ingested over a long period of time, e.g. when we eat contaminated fish. If you instead inhale cadmium, it may lead to acute respiratory tract problems and severe kidney and liver damage. Inhaling cadmium is often fatal and must be avoided. The long term effects of cadmium ingestion are trickier to notice since cadmium gradually injures the body over time. Cadmium and cadmium containing compounds are carcinogenic, which means that they increase the risk of developing cancer.
Starting to use the NiMH AA battery instead of the cadmium loaded NiCd AA battery is not a hard choice to make, since the NiMH AA battery is not only better for the environment – it can also provide up to three times the capacity of the old NiCd AA battery. The so called memory effect is also less significant in the NiMH AA battery. The memory effect is also known as “lazy battery effect” and cause rechargeable batteries to hold less charge.
It should however be mentioned that the NiMH AA battery has a self-discharge rate that is somewhat higher than the self-discharge rate of NiCd AA batteries. The self-discharge for the NiMH AA battery will be around 5 – 10 percent during the first day, and then gradually stabilize at 0.5-1 percent per day. These figures are only applicable when the NiMH AA battery is kept at room temperature. The NiMH AA battery is highly temperature sensitive and the self-discharge rate will be greatly affected by the surrounding temperature.
For really high drain devices, the NiMH AA battery and the NiCd AA battery may sometimes not be enough and lithium ion based batteries can be a better choice. The volumetric energy density is higher in the lithium ion batteries and the self-discharge rate is lower. The NiMH AA battery is instead ideal for medium demanding electric appliances, such as digital cameras and portable audio players. You can also find the NiMH AA battery in certain hybrid vehicles.
Overcharging must be avoided, since this can severely damage a NiMH AA battery. Earlier, overcharging could even be dangerous, but today the NiMH AA battery variants available on the consumer market contain catalysts that will instantly neutralize any potentially dangerous gases (2 H2 + O2 ---catalyst--> 2 H2O). The NiMH AA battery can become very hot during the last stages of charging, and getting a battery charger with a fan can therefore be a good idea.
