Failing battery or failing motors
Ok, so the stock setup with 4 C batteries runs on 6V but has a capacity of 8,000 mAh because its in series.
Your &quot;security battery&quot; only has 1,300 mAh. This means that it will die far quicker 6.15 times quicker than the stock setup.
The other people with there 10 AA batteries in series have 2,500 mAh of capacity. So their set up should die 3.2 times quicker than the stock setup.
So long story short you need a bigger battery (more capacity). Dead Battery Good luck, sounds like a fun project!