Johanna Quandt, the billionaire widow whose husband turned Germany's Bayerische Motoren Werke AG into the largest maker of luxury cars, has died. She was 89.

She died Aug. 3 at her home in Bad Homburg, Germany, according to an emailed statement from the Johanna Quandt Foundation. No cause of death was given.

The third wife and onetime secretary of Herbert Quandt, Johanna Quandt inherited a 16.7 percent stake in Munich-based BMW when her husband died in 1982. She also owned a stake in Datacard Group, a closely held Minnetonka, Minnesota-based credit-card and passport maker, and held shares in Gemalto, a publicly traded security-software designer based in Amsterdam.

Johanna Quandt's net worth of $11.5 billion ranked 98th in the Bloomberg Billionaires Index and eighth within Germany. BMW has a market value of $65.3 billion.

Johanna Quandt and her two children held a combined 46.8 percent of the company.

Quandt, who rarely spoke to the media while supporting journalists through her foundation, remained on the company's supervisory board until she stepped down in 1997.

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"I appeal to the people in my company, and not to the general public," she was cited as saying in the Frankenpost newspaper in April 2012.

Johanna Bruhn was born on June 21, 1926, in Berlin. Her parents were art historians.

After attending school in Potsdam and Berlin, she began an apprenticeship in medical technology, but her training was interrupted by the war. She worked as a banker's secretary in Cologne before joining Herbert Quandt's office in Bad Homburg near Frankfurt, in the mid-1950s.

Within a few years, Johanna Bruhn became Herbert's personal assistant, with an increasing influence over his business decisions. They married in 1960 and had two children, Susanne and Stefan.

In 1995, Johanna Quandt set up her own foundation, which supports young people training to become business journalists and awards a media prize each year.