As we all know, the capacitor is capable of holding amount of charge. However, the charge of the capacitor is limited, no matter whether the capacitor is small or big, which will be consumed in a very short time after connecting the load;

In contrast, the charge of the battery is almost infinite. In other words, even if the original charge of the battery is consumed after connecting the load. At the meantime, some of new charges which moves continuously to the positive and negative poles under the action of chemical reaction.

That’s why we can ’t use big capacitors to replace batteries.