For larger items such as tanks, cannons or artillery pieces, equipment can be lost when they are immobilized through vehicle breakdowns or minor damages. In general, a retreating force tends to lose a lot of heavy equipment regardless of actual combat losses. The heavy maintenance demands of armored fighting vehicles are both a cause of loss and an obstacle to re-use. For example, after the Battle of Stalingrad in 1943, several hundred German Panzer III tanks and similar StuG III assault guns/tank destroyers were captured. So many were available that a significant effort was made to repair and re-use them. More than 100 were rebuilt as the SU-76i self-propelled gun, with some even serving as Soviet SG-122 self-propelled howitzer vehicle prototypes. Besides Panzer IIIs and StuG IIIs, the Soviets also used about a hundred ex-German Panzer IV medium tanks and (at least) some Panther tanks. Tiger I and II tanks seized by the Soviets were only largely used for testing rather than fighting on the frontline (no photographic evidence of any serving on the front). Artillery pieces can also be lost during retreats, when battery positions are overrun (often easily once the frontlines are punctured) or when they are immobilized during hindered road movements or maneuvers.