Nickel-Iron rechargable batteries have been around for over 125 years, and are sometimes called “Edison batteries” because they were produced by Thomas Edison’s battery company.

The active material of the negative plates is iron oxide (i.e. rust). They could more accurately be called iron-oxide/nickel-hydride batteries.

They have some disadvantages especially for portability (liquid electrolyte and poor weight to charge ratio) and require regular topping up electrolyte like older models of car batteries. However, they are tolerant of frequent cycling, undercharging and overcharging, and can last 20-30 years in continuous use.