An 'iron-air' battery has been developed in the US that can store electricity from wind or solar power stations for days at a time, slowly discharging it to the grid. 

It will help tackle climate change by reducing the need for fossil fuel power plants, according to technology startup Form Energy, from Massachusetts, US.

The Iron-Air battery is a 'new class of cost-effective, multi-day energy storage system,' that can feed electricity for 100 hours at 1/10th the cost of lithium-Ion, the 'holy grail' in terms of renewable energy technology. 

It is made using iron, one of the most common elements on Earth, and works by breathing in oxygen, converting iron to rust, and turning rust back to iron.

As it takes in the oxygen and converts the iron back and forth it is charging and discharging the battery, a process that keeps the energy stored for a longer period.  

The batteries are too heavy for use in electric cars, according to the firm, who say they are designed to meet the challenge of keeping a constant power supply.

If development continues at pace, Form Energy hope the first batteries will be working to supply the grid by 2025. 

This will solve one of the most elusive problems facing renewable energy, that is how to cheaply store large amounts of electricity and supply it to power grids when the sun isn't shining for solar panels or wind isn't blowing for turbines.

Solar and wind resources are the lowest marginal cost sources of electricity in most of the world, but they don't provide a constant supply like fossil fuel power plants.