FRANKFURT, March 20 (Reuters) - Mercedes-Benz said on Wednesday it will test an 11 megawatt hour (MWh) battery at its Rastatt car factory in Germany from 2025 to help bridge the gap between swings in renewable energy output and its manufacturing requirements.

"The integration of energy storage into our production system, which is increasingly fed with renewable energy, is an important factor," production chief Joerg Burzer said on a factsheet obtained by Reuters.

Rastatt produces photovoltaic power on-site and is due to expand capacity this summer to help bring down CO2 emissions.

Harnessing variable renewables poses technical challenges when weather patterns curb supply, or provide it in big spurts.

To tackle back-up problems, Mercedes-Benz uses quick-starting Lithium-ion buffer storage at its Sindelfingen plant, borrowing the battery type from its mass electric vehicle production.

But in Rastatt, it wants to try a different approach.

It has entered into a partnership with cleantech company CMBlu Energy, which will supply what it calls an Organic SolidFlow Battery.

That is a type of battery that allows operations over long time periods and is therefore suited for smoothing power flows.

In battery jargon, it combines redox flow and solid state technologies.

The energy capacity of the 11 MWh corresponds to over 150 EQA electric compact SUVs and will be expanded if the pilot project runs well.

CMBlu Energy says its batteries improve reliability and cut costs per MWh, while the bigger space needed for them is available at industrial plants.

CMBlu says its products are made from recycled organic material and do not involve critical raw materials.

Electric carmakers are concerned about the sustainability of inputs such as rare earths, cobalt or nickel.

As part of its green drive, Mercedes-Benz has also entered into long-term purchasing agreements for solar, onshore and offshore wind power for capacity equivalent to half its German electricity demand, to lock in predictable prices. (Reporting by Vera Eckert, editing by Sharon Singleton)