What causes a MOSFET to short?
As HandyHowie told you, you put the Mosfet across the battery.

Look on Fig 1 and 2.
http://www.irf.com/product-info/datasheets/data/irfr4105.pdf

11 V as Vds and 10V as Vgs and the Mosfet will allow ~ 100A,
assuming you battery is capable, a nice 1kW.

The devices will blow up in ~ 100 us. (Zth ~ 0.2 C/W)