Please note that many prefabricated models of FIRST-specific parts made available over the internet are not accurately modeled and require some modification such that they accurately reflect their true weight. The battery model available at
www.firstcadlibrary.com, for example, weighs 5 lbs. as per Inventor.
To correct these errors, the easiest thing to do is learn the component's weight and manually manipulate the density of the model so as to make it match its real counterpart.
Further, please understand that both wiring and fasteners can quickly become a significant portion of your robot where its weight is concerned -- sometime as much as 15%. Modeling these things is both difficult and resource intensive, so many people leave them out.