For those who haven't seen the movie: the gist of the tank battle is that the Germans put a crew of POWs into a T-34 tank to test weapons on them, against all odds the crew defeats the Germans tanks and escapes.
The movie T-34 is based on a much older Soviet movie called Zhavoronok, which came out in 1965. The premise is largely the same, although the tank is different (the original crew had a T-34 with a 76 mm gun and a hexagonal turret, the new one has a T-34-85). That movie itself was based on a 1963 article in the Pravda newspaper called "Podvig kapitana" (The Captain's Feat), in which a similar scenario took place: a Soviet test subject takes his tank and crushes the guns at the proving grounds, after which he was captured and shot. The article is based on an eyewitness account. A later investigation later revealed a nurse who looked after German soldiers who were injured in an incident at the Ohrdruf proving grounds and a second hand account of a gunner who was allegedly one of the crewmen initially present.
Another similar story was told by TASS correspondent Colonel P.L. Pavlovtsev. According to him, a Soviet tanker reached the Sandomierz foothold after escaping in a tank and walking the rest of the way, during which time the rest of the crew died of exposure. The escaped tanker himself died soon after. In the resulting investigation Pavlovtsev found just one old man who stated that a group of Soviet tankers escaped in 1943 (a year prior).
The only more or less well documented instance of any similar event is described in Popel's memoirs "Vperedi - Berlin!" (Berlin Ahead!) He tells of the capture of the Kummersdorf proving grounds, where an IS-1 tank and an ISU-152 SPG were found with corpses inside.
Based on this, all one can say for sure is that the Germans used POWs and captured tanks to test the effectiveness of their weapons. There is no record of filling these tanks with fuel, let alone ammunition, like depicted in the movie.
Sources:
N. K. Popel, Vpederi - Berlin!
A. Statsenko, Legenda o sbezhavshey tridtsatchetverke