** Ballard Power Systems shares up 11.5 pct, 3rd-biggest pct gainer in U.S. premarket trading
** The Canadian hydrogen fuel cell maker says it has signed a 3.5-year extension to its contract with Audi
** Ballard Power says the HyMotion program extended to Aug. 2022, with the extension valued at C$80 mln-C$130 mln
** The previous six-year contract was until March 2019
** After a 167 pct surge in 2017, BLDP is down ~35 pct so far this year, hurt in part by short sellers (Reporting by Savio D’Souza)