FRANKFURT (Reuters) - German automotive supplier Bosch [ROBG.UL] and utility EnBW on Friday started a storage battery at EnBW's coal-fired Heilbronn plant that will supply the balancing power market at times when demand outstrips supply.

The two firms set up a joint venture last year to construct the battery, which over a year can provide electricity roughly equivalent to that consumed by 400 small households.

They added it was just the start of further forays into storage technology as power markets move away from fossil fuels to green energy.

"We are combining for the first time a battery with a large-scale generation plant," said Ralf Klein, managing director of the joint venture. "You can make money with that."

The battery is small, with a maximum performance of just 5 megawatts (MW). The Heilbronn site's most modern unit has 750 MW of electric capacity and 550 MW heat.

However, it can absorb and release one fifth of the volume a big power plant normally releases into the balancing market at advantageous costs, because it can use existing staff, operations and grid access, Klein said.

The so-called primary balancing market, which is organised by grid operators, sets and pays service fees to providers in an auction system. Batteries help stabilise grid swings within seconds, potentially helping to resolve major problems brought on by volatile wind and solar energy.

Batteries are becoming more important across industries. Bosch and EnBW want to sell their expertise to utilities and electric vehicle companies that are redesigning their material flows and work patterns.

Car companies expect a boom in mobile batteries for electric vehicles (EVs). EVs are expected to ultimately replace cars run on petrol or diesel, however EV providers cannot sell their cars unless charging stations can be built and reliably operated.

These are areas that EnBW and other utilities such as E.ON and Innogy have begun to embrace.

(Reporting by Vera Eckert, editing by Alexandra Hudson)

By Vera Eckert