It is crucial to understand that batteries are not made to store energy directly in them. The electrical power is stored in the form of chemical energy. The positive end of an AC source is connected to the positive side of the battery, and simultaneously the negative end of an AC source is connected to the battery's negative terminal. The current starts to flow from the negative terminal towards the positive end. But, as AC changes its polarity, there is no actual energy. This is because the negative half-cycle cancels out the positive half-cycle. If this process continues in the battery, it can damage the battery. Therefore, AC is not stored in batteries.