LONDON: Anglo-Dutch oil giant Royal Dutch Shell agreed Friday to buy German rechargeable battery maker Sonnen, as the sector eyes growing demand for cleaner energy.

Shell, which already invested in the German start-up in May 2018, revealed in a statement that it will buy 100 percent of Sonnen for an undisclosed amount.

Sonnen, which makes lithium-ion batteries for storing wind and solar power, was founded in 2010 and has since grown rapidly to become a dominant player in Europe.

"Sonnen is one of the global leaders in smart, distributed energy storage systems and has a track record of customer-focused innovation," said Mark Gainsborough, executive vice president of Shell's New Energies division.

"Full ownership of Sonnen will allow us to offer more choice to customers seeking reliable, affordable and cleaner energy.

"Together, we can accelerate the building of a customer-focused energy system in support of Shell's strategy to offer more and cleaner energy solutions to customers," he added in the statement.

Friday's announcement shows how the traditional oil sector is seeking to diversify further into greener energy, analysts say.

The announcement is "100 percent a sign that oil majors are positioning for green energy", said Neil Wilson at Markets.com.

In June 2018, Shell's main British rival BP bought Britain's largest electric vehicle charging firm Chargemaster, in a bet on booming demand for greener transport.