There is a specific thing to use as a weight to avoid this question 

Look on bricklink for 73090A or 73090B 

It looks like the 73090B is cheaper. 

Tell your team good job on innovation. 

On 2013-10-28 12:11, Scott Rakestraw wrote: 

> My Code Crackers teams wants to add counter weight to the back of their robot to keep it from tipping due to a front heavy attachment. They asked if they could use a spare NXT battery since it is manufactured by Lego. In looking at rule 4 - Equipment, it depends on how batteries are classified. To clarify if this is within the rules, are Lego batteries considered electric or non-electric? 
> 
> "REGULAR ELEMENTS - You may use as many non-electric LEGO elements as you like, including pneumatics, rubber bands, 
> 
> and string, and they may be from any source or set (MINDSTORMS(R)/TECHNIC/DUPLO(R)/BIONICLE(tm)/STAR WARS(tm)/ 
> 
> HARRY POTTER(tm)/etc.). Exception: Factory-made wind-up/pull-back "motors" are not allowed. 
> 
> Scott Rakestraw 
> 
> www.fllstartingpoint.com 
> 
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