Due to battery acid, batteries were never intended to be in weapon reps. That was an example weight for comparison to give an idea for the maximum weight of the rep for safety purposes. Do not put acid in your weapons, please.
Javelins are a heavy thrown weapon between 24 to 40 inches and need to follow the thrown weapon rules, including no hard materials or core (page 82 of the Alliance Rule Book) and must be hand-thrown for safety reasons (no propulsion devices or methods).
I made mine out of 36 inch long pvc foam tubes and semi-circle shaped pieces of open cell foam for the heads, and that has worked pretty well with good accuracy to about 15 yards. I can throw them about 30 yards maximum, but the accuracy at that range is not as good.