The

active materials are held in nickel-plated steel tubes or perforated pockets. The nominal

cell voltage is 1.2V. It is a very robust battery which is tolerant of abuse, (overcharge,

overdischarge, short-circuiting and thermal shock) and can have very long life even if so

treated. It is often used in backup situations where it can be continuously charged and can

last for more than 20 years. Its use has declined due to low specific energy, poor charge

retention, and poor low-temperature performance, and its high cost of manufacture

compared with the lead-acid battery.

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nickel-iron battery "how to make"

http://www.fieldlines.com/story/2006/12/5/16551/2063