A capacitor is used to expel all its energy very quickly...
That gives me an idea, what if the lower tier batteries fan only output so much in a movement. I.e. You have a structure that takes 50 power units to move, but the basic batteries are only capable of outputting 20 per movement. That means you would need three batteries to make the move. What the capacitor would do would be able to output as much as needed while being supplied by one battery. The trade off is less material needed, but longer waits between moves. I think this would make it more accessible start game especially if the cost to make batteries is high.